- Email: [email protected]
Design and implementation of an integrated, platform independent 3D visualization of complex process data
12th IFAC Symposium on Analysis, Design, and Evaluation of Human-Machine Systems August 11-15, 2013. Las Vegas, NV, USA
'HVLJQ DQG LPSOHPHQWDWLRQ RI DQ LQWHJUDWHG SODWIRUP LQGHSHQGHQW ' YLVXDOL]DWLRQ RI FRPSOH[ SURFHVV GDWD Felix Mayer, Dorothea Pantförder, Birgit Vogel-Heuser Technische Universität München Faculty of Mechanical Engineering Institute of Automation and Information Systems e-mail: [email protected], [email protected] Abstract: Most of the current visualizations for process data are two-dimensional, even if the process itself is a three-dimensional process. The benefits of three-dimensional visualizations, for example for error detection, have already been evaluated, but especially for complex problems the use of multiple independent diagrams for different process values of the same plant yields problems. The integration of multiple process values into a single diagram promises better results. This paper presents a concept for an integrated visualization of two process values of a continuous fiber board plant into a single 3D diagram using HTML5 as a frontend. Keywords: Automation, Information integration, Operators, Process control because of powerful new hardware, even smartphones are able to display 3D data.
1. INTRODUCTION Today every industrial process uses different means of visuDOL]DWLRQ 7KLV YLVXDOL]DWLRQ LV WKH RSHUDWRU¶V JDWH WR WKH SUR cess and thus enables the operator to easily control and monitor the process, even from a remote location. Over the years, the processes got larger and more complex and nowadays a large quantity of sensors and actuators are built into every single machine to allow for this complexity. Along with this trend, the visualizations got more complex, to a point where only skilled and trained personnel can operate a process. Handling this complexity is challenging, increases operator¶V ZRUNORDG and leads to more operating errors and thus higher production costs. Current process data visualizations are still mostly twodimensional, i.e. even if the process has 3D properties and the third dimension is important for the process, the visualization is still in 2D. Interrelated 3D process data is often split up into multiple independent 2D diagrams to be able to display it on a screen. Consequently, an operator has to keep all those diagrams in sight and must not confuse them to make no operating errors. A further trend are cyber physical systems to optimize production and maintenance. In this context, visualization of process data on mobile devices comes into play. Especially in complex distributed production plants, there is a need for additional visualizations on mobile devices to support the operators and the maintenance personnel directly at the plant. For both, the important data can be shown with great flexibility and adjusted to their current needs. For example, in case of an error, the cause can be tracked down more easily. This additional support can reduce downtimes and thus save costs. Today, displaying something in 3D does not represent a problem, as well as displaying data on mobile devices. 3D technology is part of many consumer products, for example within games. During the last years, the 3D technology made great progress in the entertainment industry and nowadays,
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Being able to visualize process data in 3D and thus supporting the operator is a new approach. To optimize the visualization of all the process data, the final result should consist only of a single 3D diagram that contains all relevant process data. According to Wickens¶ (1995) Proximity Compatibility Principle (PCP), such an integrated approach promises a better support to the operator in process control. This paper first describes Wickens PCP and the associated goals as well as the current state of the art. Afterwards the application example and the requirements for the new concept are presented, followed by the new concept for the 3D process data visualization. At the end the current results are shown. 2. THE PROXIMITY COMPATIBILITY PRINCIPLE In his Proximity Compatibility Principle (PCP), Wickens states that information that belongs together should also be displayed close to each other (mentally and spatially) in order WR PLQLPL]H WKH RSHUDWRU¶V HIIRUW QHHGHG WR JDWKHU DOO UHOHYDQW information. Amongst others Wickens distinguishes between spatial proximity and mental proximity. The fewer information sources there are and the better they are mentally and spatially integrated, the lower the information access costs are. Creating spatial proximity is comparatively easy. Information that belongs together, or should be considered together, should also be displayed together. Values for bar graphs for example should be displayed in such a way, that the correlation between bars and (exact) values can easily be done. In general spatial proximity can be archived by giving items similar colors or shape and by placing them close to each other on the screen. Spatial proximity can be maximized by using only one diagram and combining all values into just this one diagram. 317
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the displayed data in different ways, depending on the application used. Every application has its own way of interaction between the user and the application and especially in the field of virtual reality there are some very special approaches, mainly because the user experience itself is one of the key features of such an application. With none of the recent 3D applications the operation of a process plant is a design goal. Also, the visualization of process data in general is nothing new. Until now, process data is almost always displayed in 2D. The old control elements used in analog times are still present as digital equivalents used in current process visualizations. Sometimes some parts of a plant are shown in ³ 5D´ meaning the usage of features like shading, shadows or some kind of fixed perspective views, but showing process data in real 3D views of plant elements is not common when visualizing process data.
This integration allows for very easy reading of a complex visualization, because all relevant information can be read at once and does not have to be assembled in mind. Wickens states that if the information that should be displayed is close in regard to content and has a high spatial proximity, the integration is efficient. Mental proximity means that all relevant data is displayed in such a way that an operator does not have to create an overall image in his mind, but in such a way that gathering and understanding the information is preferably easy. This leads to less frustration and less workload. For a visualization of process data, this means for example that certain data is already combined and conditioned when it is displayed on a screen. It also means that all analysis that can be done automatically should be done prior to displaying it on screen. The PCP states that integrative tasks, meaning the gathering of information by combining data, e.g. set different data in relation to each other, benefits from a 3D representation. Tasks requiring a high level of attention, e.g. reading a value over a longer period of time, benefit from a 2D representation (Wickens et al. 1994). Wickens states that even the integration of undetailed information already produces better results. John et al. (2001) describe that the advantage of a 3D visualization depends on the task. In accordance to Wickens, he could show in several experiments that the identification of specific data is difficult in 3D. In contrast, if integration of various data is required for specific tasks, 3D facilitates performance and reduces cognitive demand. Consequently, the proximity compatibility principle delivers an approach to reduce the complexity and requirements for process visualizations. Through the integration of relevant information, the operator is relived and the probability of handling errors is reduced. In addition, the operator can react faster and more systematic to problems, because the time to extract the relevant information is reduced.
4.1. 2D process data visualization Almost every machine/plant built today has some kind of process data visualization. SCADA tools allow designing a 2D process visualization in a simple way. Those visualizations are typically in 2D, mainly because creating a 3D visualization is a complex task and requires more computing power than displaying simple 2D objects. The use of dedicated programming languages is also not common. Nowadays this kind of SCADA software is widely used to visualize almost every industrial process and is often already bundled with the Programmable Logic Controller (PLC), machine or plant by the respective manufacturer. However, today common 2D concepts how to support current SCADA tools in HMI, may not support the operator sufficiently. The use of 2D visualizations for 3D processes leads to very high demands on operators, because the final (3D) image has to be FUHDWHG LQ WKH RSHUDWRU¶V PLQG DV RSSRVHG WR DQ LQWHJUDWHG ' visualization. Additionally, every process value is often displayed in a separate diagram, so it is difficult to estimate the criticality of a situation, because all the diagrams have to be considered simultaneously. Unfortunately the manufacture specific visualization software does often not permit the use of third party plugins or additional software as add-ons. Often the only way to get process data from the PLC is via OPC or sometimes interfaces like ActiveX.
3. GOALS The goal of this work is to develop a platform independent concept for visualizing 3D process data in a common web browser ± on PC workstations and mobile devices. For the user, it should be possible to interact with the visualization and to make well-founded decisions based on what is displayed on the screen. The focus lies on developing a website which visualizes the process data in different integrated ways, following the PCP and evaluating those different approaches concerning the ability to relieve the operator. After implementing the integrated process data representation, the operator should be able to decide faster and more accurately.
4.2. 3D process data visualization Nowadays, 3D visualizations are rarely used, mainly because the creation of 3D views and models is quite time-consuming and cost-intensive and there is little experience how such a visualization performs. In addition little research has been conducted on the question how a 3D visualization should be created. In either case, the views and 3D models have to be created anew for each problem that should be visualized. This process is significantly more complex than building a 2D representation. To research different possibilities on how to display data in 3D, Pantförder et al. (2005) developed and evaluated a prototypical 3D object library of multiple so called 3D patterns. The analyzed patterns include the 3D histogram, the surface plot, the helix, the cone tree and the 3D line plot. These 3D
4. STATE OF THE ART IN PROCESS VISUALIZATION AND VISUALIZATION TECHNIQUES As stated before, visualization of data in 3D in general is nothing new. Many different applications, for example in the field of CAD or CAM, use 3D visualizations to present 3D data to a user. In contrast to this work those existing applications do not use online process data, but more or less invariable 3D object data. This data is often rendered for the purpose of presenting and is not intended as process data visualization, but more as a method to better visualize a 3D object and WR VXSSRUW WKH XVHU¶V LPDJLQDWLRQ The user can interact with 318
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In order to be able to have just one global visualization for all kinds of personnel, another approach is needed. By now HTML is a standard that can be displayed by many different devices and that can easily be programmed. HTTP and HTML are old standards that were almost exclusively used to display static content to a user so far. Because HTTP is a stateless web protocol, there is no possibility to exchange data after the initial query. This last point was addressed by using background services and by the new HTML5 standard. During the last years, there have been major efforts to update the old HTML standard, which originates from 1998, to better reflect the requirements of the modern internet. The new W3C HTML5 standard (2013), which is currently available online as a draft, should ensure a more dynamic and more flexible internet, with easier integration of complex data. One of the major innovations is the support of accelerated 3D content via WebGL, which is a slimmed-down OpenGL implementation that also runs on embedded devices, since more and more chip manufacturers integrate a graphic accelerator into their chips. Therefor WebGL content can be displayed on any modern device, independently of its size and origin, as long as a recent web browser is installed, to fulfill the need for a so called canvas element, required by the current HTML5 draft. Since the HTML5 standard is very new, not final and still evolving, there are currently no 3D process data visualizations that use HTML5 and WebGL. Some Siemens PLCs permit the creation and display of HTML webpages, e.g. the CP343-1 IT series. This series has a build-in webserver that can communicate directly with the CPU module of the PLC and can thus display process data. The webpage itself consists of a HTML page and a java applet, both programmed in a Siemens specific way. In contrast the new concept in this work enables the operator to view the visualization on any device he wishes, as long as it is reasonably old.
patterns use standard interfaces like OPC or ActiveX and can be integrated in most state-of-the art SCADA systems as easy as 2D visualization objects. This approach simplified the use of a 3D visualization for the application engineer significantly. A 3D visualization should help the operator building a realistic mental representation of the process, to ensure an easy, fast and secure operation. The correlation between mental model, visualization and reality should be bidirectional, so that identified problems can easily be located, either from the visualization to the process or the other way around. There are numerous ways of rendering 3D data and many ways of interacting with the 3D visualization, but the amount of research done concerning 3D process data visualizations is comparatively low. The benefit of using integrated 3D visualizations and the portability of Wickens PCP for process data was shown in some projects. Beuthel (1997) and Hoppe et al. (2004) showed, based on the application example of a coal-fired power plant and a highvoltage energy storage system that the visualization of relevant process data in 3D on top of a 2D schematic leads to faster intervention by the operator. Also, the benefit of using just one 3D diagram over multiple 2D diagrams could be shown. Zeipelt & Vogel-Heuser (2003) developed a 3D visualization of an ethylene reactor. The old visualization sensor data was displayed in tables on four separate displays. 7KH RSHUDWRU¶V WDVN LV WR SUHYHQW KRW VSRWV within the reactor as this can lead to an explosion of the reactor. He had to check every single display in order to identify possible critical situations. $FFRUGLQJ WR :LFNHQV¶ 3&3 with a new 3D approach, the demands on the operator got significantly lower. The sensor values are now combined in a single 3D diagram and visualized together with further information. Pantförder et al. (2009) evaluated the benefit of a 3D visualization in a process monitoring task of a continuous hydraulic press. 70 subjects were randomly divided into five groups. Each group got a different training prior to the evaluation and had to use either a 2D or a 3D representation of a given process. During the monitoring task, the subjects had to find different kinds of problem situations in four diagrams divided into two groups: simple and complex problem situations. The analysis of error rates showed significantly better results in the 3D group, but only if the subjects had the possibility to interact with the 3D Scene.
5. APPLICATION EXAMPLE The data that is displayed in the 3D process data visualization is gathered from a continuous thermo-hydraulic press used in the timber industry. The hydraulic press is used to produce different kinds of fiber boards. The data was recorded during the initial operation of the plant and was saved in a MySQL database. A glued material mat runs into the press and is pressed between two moving steel belts. The heat that is necessary for the technological process, is transferred by roller rods from the heating plate to the steel belt. Hydraulic cylinders generate the pressure to press the material mat to set values of distance. The cylinders are located equally spaced along the whole length and width of the press. The thickness of the mat is measured by means of distance transducers and is controlled by increasing or reducing the pressure. Thickness, temperature and pressure are continuously measured and displayed for the operator in the control room (Vogel-Heuser 2008). Without a process model, the operator has to manually adjust various parameters of the plant from time to time, in order to compensate for specific issues, e.g. the moisture of the wood,
In the described experiment, the PCP was implemented by the spatial proximity of the visualization. The process values pressure and distance were further represented in two diagrams, but with a close spatial relationship to the process. However, both process values have a direct relationship to each other. The following concepts describe the integrated visualization of both process values, pressure and distance, in just one diagram. The achieved spatial and mental proximity supports the operator to gather the actual system state and reduce the demand and the RSHUDWRUV¶ ZRUNORDG 4.3. HTTP, HTML & WebGL Almost all visualizations are manufacturer specific solutions and thus cannot be used widely by other projects or devices. 319
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its temperature, or thickness variations of the material at the beginning of the press. For high-quality fiber boards it is important that the operator recognizes potentially problematic situations very early. It is also important WKDW WKH RSHUDWRU¶V DFWLRQV DUH IDVW DQG HIIL cient.
Saturation or by additional charts which are integrated into the surface plot. The following charts were considered in this work: Bar charts Pie charts Line charts Bubble chart Radar chart Ternary plot
6. REQUIREMENTS FOR THE VISUALIZATION As stated before, the data that was recorded during the initial operation of continuous thermo-hydraulic press and saved in a MySQL database. There are two data sets, each for a time of about a few minutes. The outline of the press, i.e. the frames, should be visible in the visualization to ensure the bidirectional correlation between visualization and real plant. Furthermore, the material within the press should be visualized in 3D (surface plot), together with all other relevant process data, i.e. distance, pressure and temperature at every cylinder of every frame. The final visualization should be clearly and easily readable (clarity), so that the operator does not confuse different process values (unambiguousness). To further support the operator the difference between set pressure and current pressure, as well as the difference between set distance and current distance should be displayed if needed. To further relieve the operator, the criticality of the current situation at a given frame should also be calculated and displayed. All this data should be integrated in one 3D diagram to maximize mental and spatial proximity (integration), without cluttering the display and without the need to constantly manipulate the view (operating effort). This should ensure a better and faster reaction by the operator. In order to be able to evaluate different approaches and styles, it should also be possible to easily switch between those variants in the final demonstrator. In order to be able to evaluate the final visualization on multiple devices without customization, the visualizations should be done in HTML5 and WebGL. In summary the following aspects have to be bore in mind:
To allow for a maximum of spatial and mental proximity, the charts should be integrated into the 3D surface plot. Apart from bar, pie and bubble charts, every other chart cannot be read properly when placed on top of the plot, especially when viewed from the distance. Therefor only those three diagram types were investigated. To simplify the initial development, in a first step only the values for distance and pressure were used. Together with the 3D parameters (color, transparency, texture, luminance, saturation), the following 13 different variants in table 1 were chosen and considered: Position 1 2 3 4 5 6 7 8 9 10 11 12 13
XY
Distance Z + RGB-Color Z + RGB-Color Z + RGB-Color Z + RGB-Color Z + RGB-Color Z + RGB-Color Z + RGB-Color Z + RGB Frames Z + RGB-Color Z + Red-Channel Z + Planes Z + Contour Colored Bars
Pressure Transparency Saturation Luminance Texture / Pattern Colored Bars Colored Bubbles Colored Frames RGB-Color Texture  Transp. Blue-Channel RGB-Color RGB-Color Colored Bars
Table 1: Overview of the 13 possible visualization variants
Usage of MySQL as database Display of the frames 3D surface plot for the fiber board Integrated visualization of pressure, temperature and distance Integrated visualization of the criticality Multiple variants for evaluation
The position is always displayed on the x- and y-axis, whereas the distance is in all but the last variant displayed on the (vertical) z-axis. The first four variants are relatively close to each other. The distance of the material is displayed in vertical direction. Additionally, the plot has a color gradient from green to red, dependent on the distance variance comparison. Pressure is displayed as transparency, saturation, luminance or texture of the colored surface plot. The next two variants (five and six) differ in the way the pressure is displayed. It is not displayed as a surface plot component, but as a chart. For each variant a different chart is used that is placed on top of the surface plot. In variant seven and eight, the frames around the surface plot are colored according to the distance or pressure variance comparison. The ninth variant displays the distance in vertical direction and the plot gets redder the greater the difference between set distance and current distance gets. The pressure is displayed as an overlay texture that becomes more and more visible and
7. CONCEPT OF AN INTEGRATED 3D VISUALIZATION As stated before, the distance of the material, measured at three positions across each frame as well as the pressure and the temperature at those cylinders should be displayed. To achieve this, there are numerous possible diagram types and graphic renditions. The distance of the material and its expansion are represented by the 3D surface plot. The current pressure and temperature at a given position can be visualized either by additional parameters of the surface plot, i.e. Color Transparency Texture Luminance 320
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dense the greater the difference between set pressure and current pressure gets. The next variant (ten) splits the color components. The red channel is used to further visualize the distance ± the surface plot gets redder, whereas the blue channel is used to visualize the pressure ± the surface gets bluer. In conclusion, very critical areas (areas where distance and pressure are critical) are purple through the combination of red and blue. Variants eleven and twelve have additional elements to emphasize the distance of the material. For variant eleven there are two planes that represent the minimum and the maximum allowed thickness. That means the distance of the material should always stay between the two planes. In the 13th variant there are contour lines to better show the width. The pressure is displayed as a color gradient. The last variant does not show the distance of the material in vertical direction. The third dimension is only used for bar charts that display the distance and pressure.
10 11 12 13
Detection of critical situations
Operating effort
++ ++ ++ + 0 + 0 0 +
++ 0 0 0
+4 ±0 ±0 -1
1 4
Position XY XY
Distance Z + RGB Z + RGB
5 7 9
XY XY XY
10
XY
Z + RGB Z + RGB Z + RGBColor Z + RedChannel
Pressure Transparency Texture / Pattern Colored Bars Colored Frames Texture / Transparency Blue-Channel
Sum +3 +3 +3 +3 +3 +4
Table 3: Selection of variants to implement 8. IMPLEMENTATION To enable a richer and easier internet, the old HTML standard was recently revised. Many new features were introduced and the new standard for the next generation HTML is almost finished. The majority of modern browsers already support the new HTML5 standard, at least a sufficient subset. HTML5 is especially interesting for mobile devices, because it enables them to bring a whole new experience to users without the need for completely rewritten applications or websites. Nowadays, mobile devices are also powerful enough to handle lots of data and also to display this data in 3D. HTML5 makes it possible to render this 3D data in a web browser at real time. Those new standards and possibilities enable the creation of just one visualization that runs on many different devices. The creation of this visualization is presumably easy, because no manufacture specific tools or languages are used. There is also a vast amount of libraries and templates available.
Sum
Unambiguousness
0 0 0 0 0 0 0 0
Integration
Clarity
+ ++ 0 + 0
0 0 0
In summary, table 3 shows the implemented variants.
The 13 different variants were evaluated by experts to narrow down the number of variants to a number suitable for implementation. Tthe requirements from the last section were used as criteria.
+ + + + + + + + +
+ -+
As seen in Table 2 there are quite a lot notable variants that should be further evaluated. However, variants one, two and three are very similar and thus it was sufficient to only implement one of them. Likewise it is possible to implement bar charts (variant five) and colored frames (variant seven) independently from other variants, because they do not interfere with the other variants. In conclusion, the variants one, four, nine and ten were implemented separately. In addition, and to further assist the operator the variants five and seven were implemented as add-ons.
The most important thing for the operator is to reliably identify critical situations, i.e. areas with very high pressure and high distance deviation. It should also be possible to read the diagram easily and quickly. Critical situations should be visible by the operator without turning and zooming the diagram. Because the values are integrated into one diagram there is a higher risk of confusing them. As this is to be avoided, values should be easily assigned to the correct real world values. It is not desired that the operator has to manipulate the displayed diagram constantly in order to view every important detail. The relevant information should be easily recognizable. Finally, to achieve a high spatial and mental proximity, as required by Wickens¶ proximity compatibility principle, the displayed values should be displayed in a highly integrated way.
+ + + 0 0 0 + +
0 ++ + -
Table 2: Rating of the 13 variants
7.1. Rating and Selection
1 2 3 4 5 6 7 8 9
+ 0 0 -
+3 +3 +3 +3 +3 +2 +3 -2 +3
The final application consists of the following parts which are illustrated in Figure 1: A MySQL database for all the data that is required by the frontend A PHP application on a web server that gets the data from the database by request and preprocesses it, so that the client can handle it A HTML5 website that displays the data 321
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REFERENCES
as can be seen in Figure 5. At the same time non-critical areas are still fully visible. The critical situation at the entry of the plant is very easy to spot with this variant. It is also visible, that the area around the end of the press has very high pressure (very blue) but little distance (no red content).
Beuthel, C.M. (1997). Dreidimensionale Prozessvisualisierung zur Führung technischer Anlagen am Beispiel eines Kohlekraftwerks. PhD Thesis University of Clausthal-Zellerfeld. Papierflieger, Clausthal-Zellerfeld. Friedrich, D., Soeren, M., Pantförder, D., Vogel-Heuser, B. (2005). 3D-Patterns for the data analysis and operation of process plants. International Association of Science and Technology for Development - Human Computer Interaction (IASTED-HCI). Phoenix. Hoppe, S.M., Essenberg, G.R., Wiegmann, D., Overbye, T.J. (2004). Three-Dimensional displays as an effective visualization technique for power systems. Technical Report. University of Illinois.
Fig. 5: Example for variant ten
Pantförder, D., Vogel-Heuser, B., Schweizer, K. (2009). Benefit and evaluation of interactive 3D process data visualization for the presentation of complex problems. 13th International Conference on Human-Computer Interaction (HCI). San Diego.
9.2. Bar charts In the next figure the optional bar charts for the difference between set pressure and current pressure are show. Together with the colored frames, the bar charts help the operator to better evaluate the current situation and estimate the remaining scope of action.
St. John, M., Cowen, M.B., Smallman H.S., Oonk, H.M. (2001). The use of 2D and 3D displays for shapeunderstanding versus relative-position task. Human Factors, 43 (1), 79-98. Vogel-Heuser, B., Schweizer, K., v. Burgeler, A., Fuchs, J., Pantförder D. (2007). Auswirkungen einer dreidimensionalen Prozessdatenvisualisierung auf die Fehlererkennung. Zeitschrift für Arbeitswissenschaften, 61 (1), 2334.
Fig. 6: Example for optional bar charts
Vogel-Heuser, B. (2008): Automation in wood and paper industry. In Nof, S.Y. (ed.), Handbook of Automation, 1015-1026. Springer, New York.
The bar charts can be customized to show the absolute pressure or the difference between set pressure and current pressure. Additionally they can be turned off if they interfere with the surface plot. 10.
Vogel-Heuser, B., Zeipelt, R. (2003). Nutzen der 3DProzessvisualisierung in der industriellen Prozessführung. Automatisierungstechnische Praxis (atp). 45 (3), 45-50.
SUMMERY AND OUTLOOK
Operating a plant is a complex task that requires highly attentive operators. Managing multiple independent diagrams with process data makes this task even more difficult. This paper presents a concept for an integrated view of process data in a single diagram. Multiple variants to achieve this task were developed and inspected afterwards. Four of the variants promised good results and were therefor implemented for evaluation. In order to be able to give a final rating for the four variants an empiric evaluation has to be done in the future. To achieve this, more variants have to be implemented and some minor problems with the already implemented variants have to be solved, e.g. mainly aliasing effects in some variants and the invisibility of certain areas of the surface plot in one variant. After solving those issues, an empiric evaluation with already existing state-of-the-art semi-3D and 2.5D visualizations in comparison to the newly developed integrated visualization can be conducted. This task will be part of future work. In case of the final evaluation being positive, the integration of a third process data ± the temperature ± can be done, in order to demonstrate and evaluate further possibilities.
W3C HTML5.1 Draft (2013). HTML5.1 1LJKWO\ (GLWRU¶V Draft 26 March 2013. Online: http://www.w3.org/html/ wg/drafts/html/master/single-page.html Wickens, C.D., Merwin, D.H., Lin, E.L. (1994). Implications of graphics enhancements for the visualization of scientific data: Dimensional integrity, stereopsis, motion, and mesh. Human Factors, 36 (1), 44-61. Wickens, C.D., Carswell, C.M. (1995). The Proximity Compatibility Principle: Its psychological foundation and relevance to display design. Human Factors, 37 (3), 473-494.
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'HVLJQ DQG LPSOHPHQWDWLRQ RI DQ LQWHJUDWHG SODWIRUP LQGHSHQGHQW ' YLVXDOL]DWLRQ RI FRPSOH[ SURFHVV GDWD Felix Mayer, Dorothea Pantförder, Birgit Vogel-Heuser Technische Universität München Faculty of Mechanical Engineering Institute of Automation and Information Systems e-mail: [email protected], [email protected] Abstract: Most of the current visualizations for process data are two-dimensional, even if the process itself is a three-dimensional process. The benefits of three-dimensional visualizations, for example for error detection, have already been evaluated, but especially for complex problems the use of multiple independent diagrams for different process values of the same plant yields problems. The integration of multiple process values into a single diagram promises better results. This paper presents a concept for an integrated visualization of two process values of a continuous fiber board plant into a single 3D diagram using HTML5 as a frontend. Keywords: Automation, Information integration, Operators, Process control because of powerful new hardware, even smartphones are able to display 3D data.
1. INTRODUCTION Today every industrial process uses different means of visuDOL]DWLRQ 7KLV YLVXDOL]DWLRQ LV WKH RSHUDWRU¶V JDWH WR WKH SUR cess and thus enables the operator to easily control and monitor the process, even from a remote location. Over the years, the processes got larger and more complex and nowadays a large quantity of sensors and actuators are built into every single machine to allow for this complexity. Along with this trend, the visualizations got more complex, to a point where only skilled and trained personnel can operate a process. Handling this complexity is challenging, increases operator¶V ZRUNORDG and leads to more operating errors and thus higher production costs. Current process data visualizations are still mostly twodimensional, i.e. even if the process has 3D properties and the third dimension is important for the process, the visualization is still in 2D. Interrelated 3D process data is often split up into multiple independent 2D diagrams to be able to display it on a screen. Consequently, an operator has to keep all those diagrams in sight and must not confuse them to make no operating errors. A further trend are cyber physical systems to optimize production and maintenance. In this context, visualization of process data on mobile devices comes into play. Especially in complex distributed production plants, there is a need for additional visualizations on mobile devices to support the operators and the maintenance personnel directly at the plant. For both, the important data can be shown with great flexibility and adjusted to their current needs. For example, in case of an error, the cause can be tracked down more easily. This additional support can reduce downtimes and thus save costs. Today, displaying something in 3D does not represent a problem, as well as displaying data on mobile devices. 3D technology is part of many consumer products, for example within games. During the last years, the 3D technology made great progress in the entertainment industry and nowadays,
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Being able to visualize process data in 3D and thus supporting the operator is a new approach. To optimize the visualization of all the process data, the final result should consist only of a single 3D diagram that contains all relevant process data. According to Wickens¶ (1995) Proximity Compatibility Principle (PCP), such an integrated approach promises a better support to the operator in process control. This paper first describes Wickens PCP and the associated goals as well as the current state of the art. Afterwards the application example and the requirements for the new concept are presented, followed by the new concept for the 3D process data visualization. At the end the current results are shown. 2. THE PROXIMITY COMPATIBILITY PRINCIPLE In his Proximity Compatibility Principle (PCP), Wickens states that information that belongs together should also be displayed close to each other (mentally and spatially) in order WR PLQLPL]H WKH RSHUDWRU¶V HIIRUW QHHGHG WR JDWKHU DOO UHOHYDQW information. Amongst others Wickens distinguishes between spatial proximity and mental proximity. The fewer information sources there are and the better they are mentally and spatially integrated, the lower the information access costs are. Creating spatial proximity is comparatively easy. Information that belongs together, or should be considered together, should also be displayed together. Values for bar graphs for example should be displayed in such a way, that the correlation between bars and (exact) values can easily be done. In general spatial proximity can be archived by giving items similar colors or shape and by placing them close to each other on the screen. Spatial proximity can be maximized by using only one diagram and combining all values into just this one diagram. 317
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the displayed data in different ways, depending on the application used. Every application has its own way of interaction between the user and the application and especially in the field of virtual reality there are some very special approaches, mainly because the user experience itself is one of the key features of such an application. With none of the recent 3D applications the operation of a process plant is a design goal. Also, the visualization of process data in general is nothing new. Until now, process data is almost always displayed in 2D. The old control elements used in analog times are still present as digital equivalents used in current process visualizations. Sometimes some parts of a plant are shown in ³ 5D´ meaning the usage of features like shading, shadows or some kind of fixed perspective views, but showing process data in real 3D views of plant elements is not common when visualizing process data.
This integration allows for very easy reading of a complex visualization, because all relevant information can be read at once and does not have to be assembled in mind. Wickens states that if the information that should be displayed is close in regard to content and has a high spatial proximity, the integration is efficient. Mental proximity means that all relevant data is displayed in such a way that an operator does not have to create an overall image in his mind, but in such a way that gathering and understanding the information is preferably easy. This leads to less frustration and less workload. For a visualization of process data, this means for example that certain data is already combined and conditioned when it is displayed on a screen. It also means that all analysis that can be done automatically should be done prior to displaying it on screen. The PCP states that integrative tasks, meaning the gathering of information by combining data, e.g. set different data in relation to each other, benefits from a 3D representation. Tasks requiring a high level of attention, e.g. reading a value over a longer period of time, benefit from a 2D representation (Wickens et al. 1994). Wickens states that even the integration of undetailed information already produces better results. John et al. (2001) describe that the advantage of a 3D visualization depends on the task. In accordance to Wickens, he could show in several experiments that the identification of specific data is difficult in 3D. In contrast, if integration of various data is required for specific tasks, 3D facilitates performance and reduces cognitive demand. Consequently, the proximity compatibility principle delivers an approach to reduce the complexity and requirements for process visualizations. Through the integration of relevant information, the operator is relived and the probability of handling errors is reduced. In addition, the operator can react faster and more systematic to problems, because the time to extract the relevant information is reduced.
4.1. 2D process data visualization Almost every machine/plant built today has some kind of process data visualization. SCADA tools allow designing a 2D process visualization in a simple way. Those visualizations are typically in 2D, mainly because creating a 3D visualization is a complex task and requires more computing power than displaying simple 2D objects. The use of dedicated programming languages is also not common. Nowadays this kind of SCADA software is widely used to visualize almost every industrial process and is often already bundled with the Programmable Logic Controller (PLC), machine or plant by the respective manufacturer. However, today common 2D concepts how to support current SCADA tools in HMI, may not support the operator sufficiently. The use of 2D visualizations for 3D processes leads to very high demands on operators, because the final (3D) image has to be FUHDWHG LQ WKH RSHUDWRU¶V PLQG DV RSSRVHG WR DQ LQWHJUDWHG ' visualization. Additionally, every process value is often displayed in a separate diagram, so it is difficult to estimate the criticality of a situation, because all the diagrams have to be considered simultaneously. Unfortunately the manufacture specific visualization software does often not permit the use of third party plugins or additional software as add-ons. Often the only way to get process data from the PLC is via OPC or sometimes interfaces like ActiveX.
3. GOALS The goal of this work is to develop a platform independent concept for visualizing 3D process data in a common web browser ± on PC workstations and mobile devices. For the user, it should be possible to interact with the visualization and to make well-founded decisions based on what is displayed on the screen. The focus lies on developing a website which visualizes the process data in different integrated ways, following the PCP and evaluating those different approaches concerning the ability to relieve the operator. After implementing the integrated process data representation, the operator should be able to decide faster and more accurately.
4.2. 3D process data visualization Nowadays, 3D visualizations are rarely used, mainly because the creation of 3D views and models is quite time-consuming and cost-intensive and there is little experience how such a visualization performs. In addition little research has been conducted on the question how a 3D visualization should be created. In either case, the views and 3D models have to be created anew for each problem that should be visualized. This process is significantly more complex than building a 2D representation. To research different possibilities on how to display data in 3D, Pantförder et al. (2005) developed and evaluated a prototypical 3D object library of multiple so called 3D patterns. The analyzed patterns include the 3D histogram, the surface plot, the helix, the cone tree and the 3D line plot. These 3D
4. STATE OF THE ART IN PROCESS VISUALIZATION AND VISUALIZATION TECHNIQUES As stated before, visualization of data in 3D in general is nothing new. Many different applications, for example in the field of CAD or CAM, use 3D visualizations to present 3D data to a user. In contrast to this work those existing applications do not use online process data, but more or less invariable 3D object data. This data is often rendered for the purpose of presenting and is not intended as process data visualization, but more as a method to better visualize a 3D object and WR VXSSRUW WKH XVHU¶V LPDJLQDWLRQ The user can interact with 318
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In order to be able to have just one global visualization for all kinds of personnel, another approach is needed. By now HTML is a standard that can be displayed by many different devices and that can easily be programmed. HTTP and HTML are old standards that were almost exclusively used to display static content to a user so far. Because HTTP is a stateless web protocol, there is no possibility to exchange data after the initial query. This last point was addressed by using background services and by the new HTML5 standard. During the last years, there have been major efforts to update the old HTML standard, which originates from 1998, to better reflect the requirements of the modern internet. The new W3C HTML5 standard (2013), which is currently available online as a draft, should ensure a more dynamic and more flexible internet, with easier integration of complex data. One of the major innovations is the support of accelerated 3D content via WebGL, which is a slimmed-down OpenGL implementation that also runs on embedded devices, since more and more chip manufacturers integrate a graphic accelerator into their chips. Therefor WebGL content can be displayed on any modern device, independently of its size and origin, as long as a recent web browser is installed, to fulfill the need for a so called canvas element, required by the current HTML5 draft. Since the HTML5 standard is very new, not final and still evolving, there are currently no 3D process data visualizations that use HTML5 and WebGL. Some Siemens PLCs permit the creation and display of HTML webpages, e.g. the CP343-1 IT series. This series has a build-in webserver that can communicate directly with the CPU module of the PLC and can thus display process data. The webpage itself consists of a HTML page and a java applet, both programmed in a Siemens specific way. In contrast the new concept in this work enables the operator to view the visualization on any device he wishes, as long as it is reasonably old.
patterns use standard interfaces like OPC or ActiveX and can be integrated in most state-of-the art SCADA systems as easy as 2D visualization objects. This approach simplified the use of a 3D visualization for the application engineer significantly. A 3D visualization should help the operator building a realistic mental representation of the process, to ensure an easy, fast and secure operation. The correlation between mental model, visualization and reality should be bidirectional, so that identified problems can easily be located, either from the visualization to the process or the other way around. There are numerous ways of rendering 3D data and many ways of interacting with the 3D visualization, but the amount of research done concerning 3D process data visualizations is comparatively low. The benefit of using integrated 3D visualizations and the portability of Wickens PCP for process data was shown in some projects. Beuthel (1997) and Hoppe et al. (2004) showed, based on the application example of a coal-fired power plant and a highvoltage energy storage system that the visualization of relevant process data in 3D on top of a 2D schematic leads to faster intervention by the operator. Also, the benefit of using just one 3D diagram over multiple 2D diagrams could be shown. Zeipelt & Vogel-Heuser (2003) developed a 3D visualization of an ethylene reactor. The old visualization sensor data was displayed in tables on four separate displays. 7KH RSHUDWRU¶V WDVN LV WR SUHYHQW KRW VSRWV within the reactor as this can lead to an explosion of the reactor. He had to check every single display in order to identify possible critical situations. $FFRUGLQJ WR :LFNHQV¶ 3&3 with a new 3D approach, the demands on the operator got significantly lower. The sensor values are now combined in a single 3D diagram and visualized together with further information. Pantförder et al. (2009) evaluated the benefit of a 3D visualization in a process monitoring task of a continuous hydraulic press. 70 subjects were randomly divided into five groups. Each group got a different training prior to the evaluation and had to use either a 2D or a 3D representation of a given process. During the monitoring task, the subjects had to find different kinds of problem situations in four diagrams divided into two groups: simple and complex problem situations. The analysis of error rates showed significantly better results in the 3D group, but only if the subjects had the possibility to interact with the 3D Scene.
5. APPLICATION EXAMPLE The data that is displayed in the 3D process data visualization is gathered from a continuous thermo-hydraulic press used in the timber industry. The hydraulic press is used to produce different kinds of fiber boards. The data was recorded during the initial operation of the plant and was saved in a MySQL database. A glued material mat runs into the press and is pressed between two moving steel belts. The heat that is necessary for the technological process, is transferred by roller rods from the heating plate to the steel belt. Hydraulic cylinders generate the pressure to press the material mat to set values of distance. The cylinders are located equally spaced along the whole length and width of the press. The thickness of the mat is measured by means of distance transducers and is controlled by increasing or reducing the pressure. Thickness, temperature and pressure are continuously measured and displayed for the operator in the control room (Vogel-Heuser 2008). Without a process model, the operator has to manually adjust various parameters of the plant from time to time, in order to compensate for specific issues, e.g. the moisture of the wood,
In the described experiment, the PCP was implemented by the spatial proximity of the visualization. The process values pressure and distance were further represented in two diagrams, but with a close spatial relationship to the process. However, both process values have a direct relationship to each other. The following concepts describe the integrated visualization of both process values, pressure and distance, in just one diagram. The achieved spatial and mental proximity supports the operator to gather the actual system state and reduce the demand and the RSHUDWRUV¶ ZRUNORDG 4.3. HTTP, HTML & WebGL Almost all visualizations are manufacturer specific solutions and thus cannot be used widely by other projects or devices. 319
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its temperature, or thickness variations of the material at the beginning of the press. For high-quality fiber boards it is important that the operator recognizes potentially problematic situations very early. It is also important WKDW WKH RSHUDWRU¶V DFWLRQV DUH IDVW DQG HIIL cient.
Saturation or by additional charts which are integrated into the surface plot. The following charts were considered in this work: Bar charts Pie charts Line charts Bubble chart Radar chart Ternary plot
6. REQUIREMENTS FOR THE VISUALIZATION As stated before, the data that was recorded during the initial operation of continuous thermo-hydraulic press and saved in a MySQL database. There are two data sets, each for a time of about a few minutes. The outline of the press, i.e. the frames, should be visible in the visualization to ensure the bidirectional correlation between visualization and real plant. Furthermore, the material within the press should be visualized in 3D (surface plot), together with all other relevant process data, i.e. distance, pressure and temperature at every cylinder of every frame. The final visualization should be clearly and easily readable (clarity), so that the operator does not confuse different process values (unambiguousness). To further support the operator the difference between set pressure and current pressure, as well as the difference between set distance and current distance should be displayed if needed. To further relieve the operator, the criticality of the current situation at a given frame should also be calculated and displayed. All this data should be integrated in one 3D diagram to maximize mental and spatial proximity (integration), without cluttering the display and without the need to constantly manipulate the view (operating effort). This should ensure a better and faster reaction by the operator. In order to be able to evaluate different approaches and styles, it should also be possible to easily switch between those variants in the final demonstrator. In order to be able to evaluate the final visualization on multiple devices without customization, the visualizations should be done in HTML5 and WebGL. In summary the following aspects have to be bore in mind:
To allow for a maximum of spatial and mental proximity, the charts should be integrated into the 3D surface plot. Apart from bar, pie and bubble charts, every other chart cannot be read properly when placed on top of the plot, especially when viewed from the distance. Therefor only those three diagram types were investigated. To simplify the initial development, in a first step only the values for distance and pressure were used. Together with the 3D parameters (color, transparency, texture, luminance, saturation), the following 13 different variants in table 1 were chosen and considered: Position 1 2 3 4 5 6 7 8 9 10 11 12 13
XY
Distance Z + RGB-Color Z + RGB-Color Z + RGB-Color Z + RGB-Color Z + RGB-Color Z + RGB-Color Z + RGB-Color Z + RGB Frames Z + RGB-Color Z + Red-Channel Z + Planes Z + Contour Colored Bars
Pressure Transparency Saturation Luminance Texture / Pattern Colored Bars Colored Bubbles Colored Frames RGB-Color Texture  Transp. Blue-Channel RGB-Color RGB-Color Colored Bars
Table 1: Overview of the 13 possible visualization variants
Usage of MySQL as database Display of the frames 3D surface plot for the fiber board Integrated visualization of pressure, temperature and distance Integrated visualization of the criticality Multiple variants for evaluation
The position is always displayed on the x- and y-axis, whereas the distance is in all but the last variant displayed on the (vertical) z-axis. The first four variants are relatively close to each other. The distance of the material is displayed in vertical direction. Additionally, the plot has a color gradient from green to red, dependent on the distance variance comparison. Pressure is displayed as transparency, saturation, luminance or texture of the colored surface plot. The next two variants (five and six) differ in the way the pressure is displayed. It is not displayed as a surface plot component, but as a chart. For each variant a different chart is used that is placed on top of the surface plot. In variant seven and eight, the frames around the surface plot are colored according to the distance or pressure variance comparison. The ninth variant displays the distance in vertical direction and the plot gets redder the greater the difference between set distance and current distance gets. The pressure is displayed as an overlay texture that becomes more and more visible and
7. CONCEPT OF AN INTEGRATED 3D VISUALIZATION As stated before, the distance of the material, measured at three positions across each frame as well as the pressure and the temperature at those cylinders should be displayed. To achieve this, there are numerous possible diagram types and graphic renditions. The distance of the material and its expansion are represented by the 3D surface plot. The current pressure and temperature at a given position can be visualized either by additional parameters of the surface plot, i.e. Color Transparency Texture Luminance 320
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dense the greater the difference between set pressure and current pressure gets. The next variant (ten) splits the color components. The red channel is used to further visualize the distance ± the surface plot gets redder, whereas the blue channel is used to visualize the pressure ± the surface gets bluer. In conclusion, very critical areas (areas where distance and pressure are critical) are purple through the combination of red and blue. Variants eleven and twelve have additional elements to emphasize the distance of the material. For variant eleven there are two planes that represent the minimum and the maximum allowed thickness. That means the distance of the material should always stay between the two planes. In the 13th variant there are contour lines to better show the width. The pressure is displayed as a color gradient. The last variant does not show the distance of the material in vertical direction. The third dimension is only used for bar charts that display the distance and pressure.
10 11 12 13
Detection of critical situations
Operating effort
++ ++ ++ + 0 + 0 0 +
++ 0 0 0
+4 ±0 ±0 -1
1 4
Position XY XY
Distance Z + RGB Z + RGB
5 7 9
XY XY XY
10
XY
Z + RGB Z + RGB Z + RGBColor Z + RedChannel
Pressure Transparency Texture / Pattern Colored Bars Colored Frames Texture / Transparency Blue-Channel
Sum +3 +3 +3 +3 +3 +4
Table 3: Selection of variants to implement 8. IMPLEMENTATION To enable a richer and easier internet, the old HTML standard was recently revised. Many new features were introduced and the new standard for the next generation HTML is almost finished. The majority of modern browsers already support the new HTML5 standard, at least a sufficient subset. HTML5 is especially interesting for mobile devices, because it enables them to bring a whole new experience to users without the need for completely rewritten applications or websites. Nowadays, mobile devices are also powerful enough to handle lots of data and also to display this data in 3D. HTML5 makes it possible to render this 3D data in a web browser at real time. Those new standards and possibilities enable the creation of just one visualization that runs on many different devices. The creation of this visualization is presumably easy, because no manufacture specific tools or languages are used. There is also a vast amount of libraries and templates available.
Sum
Unambiguousness
0 0 0 0 0 0 0 0
Integration
Clarity
+ ++ 0 + 0
0 0 0
In summary, table 3 shows the implemented variants.
The 13 different variants were evaluated by experts to narrow down the number of variants to a number suitable for implementation. Tthe requirements from the last section were used as criteria.
+ + + + + + + + +
+ -+
As seen in Table 2 there are quite a lot notable variants that should be further evaluated. However, variants one, two and three are very similar and thus it was sufficient to only implement one of them. Likewise it is possible to implement bar charts (variant five) and colored frames (variant seven) independently from other variants, because they do not interfere with the other variants. In conclusion, the variants one, four, nine and ten were implemented separately. In addition, and to further assist the operator the variants five and seven were implemented as add-ons.
The most important thing for the operator is to reliably identify critical situations, i.e. areas with very high pressure and high distance deviation. It should also be possible to read the diagram easily and quickly. Critical situations should be visible by the operator without turning and zooming the diagram. Because the values are integrated into one diagram there is a higher risk of confusing them. As this is to be avoided, values should be easily assigned to the correct real world values. It is not desired that the operator has to manipulate the displayed diagram constantly in order to view every important detail. The relevant information should be easily recognizable. Finally, to achieve a high spatial and mental proximity, as required by Wickens¶ proximity compatibility principle, the displayed values should be displayed in a highly integrated way.
+ + + 0 0 0 + +
0 ++ + -
Table 2: Rating of the 13 variants
7.1. Rating and Selection
1 2 3 4 5 6 7 8 9
+ 0 0 -
+3 +3 +3 +3 +3 +2 +3 -2 +3
The final application consists of the following parts which are illustrated in Figure 1: A MySQL database for all the data that is required by the frontend A PHP application on a web server that gets the data from the database by request and preprocesses it, so that the client can handle it A HTML5 website that displays the data 321
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REFERENCES
as can be seen in Figure 5. At the same time non-critical areas are still fully visible. The critical situation at the entry of the plant is very easy to spot with this variant. It is also visible, that the area around the end of the press has very high pressure (very blue) but little distance (no red content).
Beuthel, C.M. (1997). Dreidimensionale Prozessvisualisierung zur Führung technischer Anlagen am Beispiel eines Kohlekraftwerks. PhD Thesis University of Clausthal-Zellerfeld. Papierflieger, Clausthal-Zellerfeld. Friedrich, D., Soeren, M., Pantförder, D., Vogel-Heuser, B. (2005). 3D-Patterns for the data analysis and operation of process plants. International Association of Science and Technology for Development - Human Computer Interaction (IASTED-HCI). Phoenix. Hoppe, S.M., Essenberg, G.R., Wiegmann, D., Overbye, T.J. (2004). Three-Dimensional displays as an effective visualization technique for power systems. Technical Report. University of Illinois.
Fig. 5: Example for variant ten
Pantförder, D., Vogel-Heuser, B., Schweizer, K. (2009). Benefit and evaluation of interactive 3D process data visualization for the presentation of complex problems. 13th International Conference on Human-Computer Interaction (HCI). San Diego.
9.2. Bar charts In the next figure the optional bar charts for the difference between set pressure and current pressure are show. Together with the colored frames, the bar charts help the operator to better evaluate the current situation and estimate the remaining scope of action.
St. John, M., Cowen, M.B., Smallman H.S., Oonk, H.M. (2001). The use of 2D and 3D displays for shapeunderstanding versus relative-position task. Human Factors, 43 (1), 79-98. Vogel-Heuser, B., Schweizer, K., v. Burgeler, A., Fuchs, J., Pantförder D. (2007). Auswirkungen einer dreidimensionalen Prozessdatenvisualisierung auf die Fehlererkennung. Zeitschrift für Arbeitswissenschaften, 61 (1), 2334.
Fig. 6: Example for optional bar charts
Vogel-Heuser, B. (2008): Automation in wood and paper industry. In Nof, S.Y. (ed.), Handbook of Automation, 1015-1026. Springer, New York.
The bar charts can be customized to show the absolute pressure or the difference between set pressure and current pressure. Additionally they can be turned off if they interfere with the surface plot. 10.
Vogel-Heuser, B., Zeipelt, R. (2003). Nutzen der 3DProzessvisualisierung in der industriellen Prozessführung. Automatisierungstechnische Praxis (atp). 45 (3), 45-50.
SUMMERY AND OUTLOOK
Operating a plant is a complex task that requires highly attentive operators. Managing multiple independent diagrams with process data makes this task even more difficult. This paper presents a concept for an integrated view of process data in a single diagram. Multiple variants to achieve this task were developed and inspected afterwards. Four of the variants promised good results and were therefor implemented for evaluation. In order to be able to give a final rating for the four variants an empiric evaluation has to be done in the future. To achieve this, more variants have to be implemented and some minor problems with the already implemented variants have to be solved, e.g. mainly aliasing effects in some variants and the invisibility of certain areas of the surface plot in one variant. After solving those issues, an empiric evaluation with already existing state-of-the-art semi-3D and 2.5D visualizations in comparison to the newly developed integrated visualization can be conducted. This task will be part of future work. In case of the final evaluation being positive, the integration of a third process data ± the temperature ± can be done, in order to demonstrate and evaluate further possibilities.
W3C HTML5.1 Draft (2013). HTML5.1 1LJKWO\ (GLWRU¶V Draft 26 March 2013. Online: http://www.w3.org/html/ wg/drafts/html/master/single-page.html Wickens, C.D., Merwin, D.H., Lin, E.L. (1994). Implications of graphics enhancements for the visualization of scientific data: Dimensional integrity, stereopsis, motion, and mesh. Human Factors, 36 (1), 44-61. Wickens, C.D., Carswell, C.M. (1995). The Proximity Compatibility Principle: Its psychological foundation and relevance to display design. Human Factors, 37 (3), 473-494.
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