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3D design reference framework
3D design reference framework E Sittas
The paper presents a new user-interface method for creating and manipulating 3D geometries with geometric modellers. The method is particularly useful during the initial stages of conceptual design, where designers have rapidly to generate concepts of components and assemblies with no immediate considerations of scale and accuracy. Application examples are included demonstrating how the method is used in conjunction with a solid-modelling system CAD user interlace, 3D geometric modelling, conceptualdesign
One important activity during the initial stages of conceptual design is sketching. Sketching in two dimensions is perhaps the most useful tool for conceptualizing because it enables designers to explore, analyse and communicate their personal thoughts and ideas rapidly. The sketched concepts that appear to be more promising are developed further into more definite forms, while others are temporarily or permanently rejected ~ In mechanical-assembly design, sketching is a demanding task. It involves creating complex interface features between a cluster of components in 3D space ~. This often requires the definition of many auxiliary construction planes and considerable effort until an acceptable configuration is perceived. Typical examples of interface features include ducts, complex sweep sections, support clips, wires and pipes. One of the difficulties with any surface or solid modeller is that of rapidly generating 3D geometries on a 2D screen in the form of a sketch s ~". For example, the usual methods used for defining 3D coordinates on commercially available systems include: • input of the x, y, z values via the keyboard, • input of the x, y values directly on the screen set up in a chosen plane by mouse or stylus, and then insertion of the z value of depth via the keyboard or by digitization of a number from a calculator menu displayed on the screen, • definition of an oblique construction plane (infinite), and then the use of a projection normal to the plane to define 3D points, lines or curves; the problem CAD ResearchGroup, Department of Manufacturing and Engineering Systems,Brunel University,Uxbridge, MiddlesexUB8 3PH, UK Paper received: 30 October 1989. Revised:26 April 1990
380
here is that the definition of an oblique plane requires 3D information in the first place. What is clear from the above observations is that 3D information can easily be obtained by reference to points of existing objects (models or construction planes). However, if the user wants to define 3D points on a blank screen, it becomes very cumbersome to start thinking in 3D, especially when there is a need to describe entities such as points and lines. A blank screen does not provide any feel for the 3D space. In this paper, a 3D design reference framework is presented. Its main purpose is to provide a space in which a designer can easily develop his/her ideas. The second section discusses briefly the activities involved in sketching, and the remaining sections explain the tools that have been created to support these activities.
ACTIVITIES INVOLVED IN SKETCHING 3D OBJECTS When sketching a new component or assembly, the designer often needs to: • determine what are the key design features to be emphasized, and select the best viewpoint, • select values as proportions of other entities in convenient units, • determine points within an overall space defined by the length, width and depth of the object, • define a framework of multiple subspaces within which sketches of different parts of an assembly can be developed, • locate a sketched feature by using as a reference the coordinate system of the working subspace, • associate an individual sketch with one or more other sketches related to the design task in hand, • associate text with each sketch. The set of sketching activities described so far is common practice in all design offices, but is often ignored when modelling techniques are being developed. Within a CAD/CAM environment, this kind of information should be captured and organized in a way that can enhance the creativity and productivity of the designer. The problem addressed in this paper is that of creating a framework of multiple subspaces within which sketches of different parts of an assembly can be generated quickly.
0010 4485/91/050380-05 ((~ 1991 Butterworth-Heinemann Ltd
computer-aided design
CREATION OF 3D DESIGN REFERENCE FRAMEWORK To tackle the 3D point-definition and sketching difficulties, a design reference framework has been created. This framework is a set of 3D grids, which, in their simplest form, can be used to insert rapidly 3D coordinates directly on the screen. In more complex forms, the grid framework can be used for sketching design concepts, defining lines, free form curves and surfaces, defining features on finite construction planes, arranging the relative position of components in an assembly layout etc. The objective is to produce a pictorial view of a space enclosed by three or more orthogonal planes lying along the edges of the working space.
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C u r s o r is o u t s i d e the d e s i g n space
Method of defining the grid The user must define an origin and dimensions of the space. The origin is a point set in this space, i.e. P0(X0, Y0, Z0), and the values of three orthogonal axes, i.e. (XL, YL, ZL), are entered using the keyboard or a mouse or stylus. The default value of the origin in the development program is (0, 0, 0). The default values of X~, Y~, Z1 are those used to define the world window. These values are then used in the construction of four reference points that describe the orthogonal vectors of the axes. Po(X0, Y0, Zo)
where
Xa = X0 + XL
PI(Xa, Yo, Zo)
Ya = Y0 + YL
P2(Xo, Ya,
Za:
Zo)
Z 0 -l- Z L
P3(Xo, Yo, Za) This set of coordinates is then transformed into the chosen view representation being displayed. Using the dimensions of the projected axes, three polygons are created, representing the XY, YZ and ZX planes, respectively. Having defined these polygons, the user is then asked to insert the size of the grid, as in any 2D case. Examples of such grid pictures are given in Colour Plate 1. The step value of the grid can be used if accuracy of the inserted points is required.
Use of the grid With a defined set of reference grids, the user can then define 3D coordinates by pointing two locations within the space defined by the total grid. The position of the cursor with respect to the grid polygons is detected with a point in the polygon algorithm. Figure 1 illustrates the principle. Geometric-modelling operations usually involve the definition of points, lines, control points of a space curve or a surface, translation or rotation vectors for moving objects in space etc. All these operations can be performed without ambiguity within the 3D grid (see examples in Colour Plate 2).
volume 23 number 5 [une 1991
Figure 1. Defining 3D coordinates within the grid space
SKETCHING ON CONSTRUCTION PLANES OF FINITE SIZE The same equations that define the grid can be used to generate construction planes with finite boundaries in the space of the grid, and embed different-size grids on such planes. A construction plane is given by three points (altogether requiring six points on the grid): the origin (P1) and the two vectors (P2, P3), which uniquely define a local coordinate system on the plane. The coordinate system of the plane can be orthogonalized and provided with scale to form a rectangular reference grid (see Colour Plate 3). On such a grid, features such as circles, ellipses or other curves can be speedily generated. As an example, consider the drawing of an elemental part model, as shown in Colour Plate 4. Each point on the sketch is projected in the viewing direction to the point where it hits the plane, and its local coordinates are computed.
Benefit of using finite construction planes To clarify further the way in which these finite construction planes are intended to be used, an example is given in Figure 2. As can be seen, the process starts by defining a reference grid, as described earlier. Within that grid, a set of finite construction planes can be defined. So that overlapping construction planes may be used without confusion, the hidden-lineelimination algorithm can be used. Via change of the viewpoint, some regions of each construction plane will be visible, while others not. With partially visible, and thus concave, reference polygons, the cursor point is projected onto the plane, within whose visible area it is located. One salient aspect of this process is that segments of different part features can be defined using one view on different planes simultaneously and quickly.
381
The paper presents a new user-interface method for creating and manipulating 3D geometries with geometric modellers. The method is particularly useful during the initial stages of conceptual design, where designers have rapidly to generate concepts of components and assemblies with no immediate considerations of scale and accuracy. Application examples are included demonstrating how the method is used in conjunction with a solid-modelling system CAD user interlace, 3D geometric modelling, conceptualdesign
One important activity during the initial stages of conceptual design is sketching. Sketching in two dimensions is perhaps the most useful tool for conceptualizing because it enables designers to explore, analyse and communicate their personal thoughts and ideas rapidly. The sketched concepts that appear to be more promising are developed further into more definite forms, while others are temporarily or permanently rejected ~ In mechanical-assembly design, sketching is a demanding task. It involves creating complex interface features between a cluster of components in 3D space ~. This often requires the definition of many auxiliary construction planes and considerable effort until an acceptable configuration is perceived. Typical examples of interface features include ducts, complex sweep sections, support clips, wires and pipes. One of the difficulties with any surface or solid modeller is that of rapidly generating 3D geometries on a 2D screen in the form of a sketch s ~". For example, the usual methods used for defining 3D coordinates on commercially available systems include: • input of the x, y, z values via the keyboard, • input of the x, y values directly on the screen set up in a chosen plane by mouse or stylus, and then insertion of the z value of depth via the keyboard or by digitization of a number from a calculator menu displayed on the screen, • definition of an oblique construction plane (infinite), and then the use of a projection normal to the plane to define 3D points, lines or curves; the problem CAD ResearchGroup, Department of Manufacturing and Engineering Systems,Brunel University,Uxbridge, MiddlesexUB8 3PH, UK Paper received: 30 October 1989. Revised:26 April 1990
380
here is that the definition of an oblique plane requires 3D information in the first place. What is clear from the above observations is that 3D information can easily be obtained by reference to points of existing objects (models or construction planes). However, if the user wants to define 3D points on a blank screen, it becomes very cumbersome to start thinking in 3D, especially when there is a need to describe entities such as points and lines. A blank screen does not provide any feel for the 3D space. In this paper, a 3D design reference framework is presented. Its main purpose is to provide a space in which a designer can easily develop his/her ideas. The second section discusses briefly the activities involved in sketching, and the remaining sections explain the tools that have been created to support these activities.
ACTIVITIES INVOLVED IN SKETCHING 3D OBJECTS When sketching a new component or assembly, the designer often needs to: • determine what are the key design features to be emphasized, and select the best viewpoint, • select values as proportions of other entities in convenient units, • determine points within an overall space defined by the length, width and depth of the object, • define a framework of multiple subspaces within which sketches of different parts of an assembly can be developed, • locate a sketched feature by using as a reference the coordinate system of the working subspace, • associate an individual sketch with one or more other sketches related to the design task in hand, • associate text with each sketch. The set of sketching activities described so far is common practice in all design offices, but is often ignored when modelling techniques are being developed. Within a CAD/CAM environment, this kind of information should be captured and organized in a way that can enhance the creativity and productivity of the designer. The problem addressed in this paper is that of creating a framework of multiple subspaces within which sketches of different parts of an assembly can be generated quickly.
0010 4485/91/050380-05 ((~ 1991 Butterworth-Heinemann Ltd
computer-aided design
CREATION OF 3D DESIGN REFERENCE FRAMEWORK To tackle the 3D point-definition and sketching difficulties, a design reference framework has been created. This framework is a set of 3D grids, which, in their simplest form, can be used to insert rapidly 3D coordinates directly on the screen. In more complex forms, the grid framework can be used for sketching design concepts, defining lines, free form curves and surfaces, defining features on finite construction planes, arranging the relative position of components in an assembly layout etc. The objective is to produce a pictorial view of a space enclosed by three or more orthogonal planes lying along the edges of the working space.
*P2
.I " / ~
.
(
t J
. ~ _
,
l
t tin.valid
117oI
i
I
design space _ ~
P3
C u r s o r is o u t s i d e the d e s i g n space
Method of defining the grid The user must define an origin and dimensions of the space. The origin is a point set in this space, i.e. P0(X0, Y0, Z0), and the values of three orthogonal axes, i.e. (XL, YL, ZL), are entered using the keyboard or a mouse or stylus. The default value of the origin in the development program is (0, 0, 0). The default values of X~, Y~, Z1 are those used to define the world window. These values are then used in the construction of four reference points that describe the orthogonal vectors of the axes. Po(X0, Y0, Zo)
where
Xa = X0 + XL
PI(Xa, Yo, Zo)
Ya = Y0 + YL
P2(Xo, Ya,
Za:
Zo)
Z 0 -l- Z L
P3(Xo, Yo, Za) This set of coordinates is then transformed into the chosen view representation being displayed. Using the dimensions of the projected axes, three polygons are created, representing the XY, YZ and ZX planes, respectively. Having defined these polygons, the user is then asked to insert the size of the grid, as in any 2D case. Examples of such grid pictures are given in Colour Plate 1. The step value of the grid can be used if accuracy of the inserted points is required.
Use of the grid With a defined set of reference grids, the user can then define 3D coordinates by pointing two locations within the space defined by the total grid. The position of the cursor with respect to the grid polygons is detected with a point in the polygon algorithm. Figure 1 illustrates the principle. Geometric-modelling operations usually involve the definition of points, lines, control points of a space curve or a surface, translation or rotation vectors for moving objects in space etc. All these operations can be performed without ambiguity within the 3D grid (see examples in Colour Plate 2).
volume 23 number 5 [une 1991
Figure 1. Defining 3D coordinates within the grid space
SKETCHING ON CONSTRUCTION PLANES OF FINITE SIZE The same equations that define the grid can be used to generate construction planes with finite boundaries in the space of the grid, and embed different-size grids on such planes. A construction plane is given by three points (altogether requiring six points on the grid): the origin (P1) and the two vectors (P2, P3), which uniquely define a local coordinate system on the plane. The coordinate system of the plane can be orthogonalized and provided with scale to form a rectangular reference grid (see Colour Plate 3). On such a grid, features such as circles, ellipses or other curves can be speedily generated. As an example, consider the drawing of an elemental part model, as shown in Colour Plate 4. Each point on the sketch is projected in the viewing direction to the point where it hits the plane, and its local coordinates are computed.
Benefit of using finite construction planes To clarify further the way in which these finite construction planes are intended to be used, an example is given in Figure 2. As can be seen, the process starts by defining a reference grid, as described earlier. Within that grid, a set of finite construction planes can be defined. So that overlapping construction planes may be used without confusion, the hidden-lineelimination algorithm can be used. Via change of the viewpoint, some regions of each construction plane will be visible, while others not. With partially visible, and thus concave, reference polygons, the cursor point is projected onto the plane, within whose visible area it is located. One salient aspect of this process is that segments of different part features can be defined using one view on different planes simultaneously and quickly.
381