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Structure visibility change with contrast reversal
RESEARCH
NOTE
STRUCTURE VISIBILITY CHANGE CONTRAST REVERSAL
WITH
JOSEPH J. MEZREH RCA Laboratories. Princeton, NJ 0854% U.S.A. in recised
(Receired 12 Mar 1977;
The visibility of structure in a pattern depends on the visibility of the physical elements of the pattern. However, as is shown here, one can modify the visibility of portrayed structure by stimulus manipulations that do not produce corresponding changes in pattern element visibility or clarity. This is demonstrated with Fig. 1. The two random-dot patterns in this figure are identical; they are contrast reversals of each other. Dots have been deleted from the random-dot patterns in a systematic fashion’ so that if the dot density were sufficiently high that individual dots coutd not be resolved a line drawing of a triangle would actually be presented in each half of the figure. A white triangle on black background would be shown in the top and a black triangle on white background wouid be shown in the bottom as in Fig. 2. If Fig. 1 is viewed from a distance of a few feet the structure portrayed in the bottom dot pattern should, after some inspection, pop into view. The triangle can be perceived as having subjective solid lines.’ The figure in the top dot pattern can also be perceived, with sufficient viewing distance, but it is less salient than the black triangle of the bottom pattern. The difference in structure visibility in the top and bottom patterns is emphasized by decreasing the viewing distance so that the triangle in the lower pattern is just apparent. in this case the structure in the top pattern is not seen. The structure visibility for either dot pattern can be increased by decreasing the i~um~ation level, or increasing the viewing distance, or blurring the pat’ The dot deletion was accomplished by placing a transparency of the original of the bottom triangle in Fig. 2 over a random pattern of white dots on a black background A number of dots were, therefore, obscured by the triangle. A high contrast film photograph of the resulting pattern is reproduced in the black background portion of Fig. 1. and its contrast reversal is reproduced in the white background The technique of systematic deletion of dots from a random-dot pattern was introduced by Rose (1974) as a device for characterizing signal detection properties of visual processing. ’ Upon inspection of the detail of these solid lines they are realized to be not physically present The perceptual construct of solid lines in these dot patterns is reminiscent of the phenomenon of an apparent edge at a texture boundary (Julesz, 1965) and aiso reminiscent of the phenomenon of subjective contours (Coren, 1972; Gregory, 1972; Kaniza, 1976). 3 A similar blur&g benefit is observed with block quantitized pictures (Harmon and Juiesz. 1973).
form
4 August
1977)
terns. These procedures for making the triangle stand that effectively filter out high spatial frequency content. It is, therefore, Iikely that a requirement for the visibility of structure in either of these dot patterns is the low-pass filtering out of some high spatial frequency masking noise.’ However, this filtering hypothesis does not account for the effect of contrast reversal. It is the case that the bottom pattern, with white dots, is dimmer than the top pattern with black dots. But. SO long as the darker pattern with white dots has a mean luminance of at least 1Omi (which it can have with good room lighting) there should be negligible difference in the amount of visible high frequency detail for the opposite contrast patterns. The change in spatial resolution above 10ml is minimal (Shlaer, 1939: van Ness and Bouman, 1967; Patel, 1966). Indeed. the white dots in the bottom pattern seem to be no less sharply defined than the black dots in the top pattern. Figure 3 is a variation on Fig. 1 in which a constant, gray background is used. Contrast reversal of the dot patterns is accomplished with minimal change in mean luminance. The e&et of contrast reversal is slightly weaker in this case than in Fig. 1, but it is still quite evident. The dot-deletion triangle is more visible, or seems to have greater apparent contrast, in the pattern of white dots. While the mean luminance values for the opposite contrast patterns of Fig. 3 are close, it is noteworthy that we now have a situation in which the brighter of the two patterns yields the greater structure visibility. Figures 1 and 3 taken together should make it clear that the influence of contrast reversal on structure visibility is not simply a secondary effect of the influence of pattern mean huninance. For example, more than lowpass spatial filtering is involved The triangle structure, when visible, is seen against a background random-dot pattern. A brightness eontrast between the structure and its supporting randomdot pattern does not exist; nevertheless there is a sense of contrast. This sense of contrast can be better understood if one assumes that when the triangle structure is apparent, the randomly distributed dots are visually connected to form a unitary background texture. A perceived unitary texture should have a perceived globaf brightness, which could be equivalent to the mean pattern brightness. Consequently, the effective texture brightness would be different from the brightness of the dot-vacant regions which define the portrayed structure. Hence, there is a specifiable contrast for the triangle, a texture contrast.
321
out are operations
328
Research Note
This approach allows an explanation for the observed difference in structure visibility in the opposite contrast dot patterns. The contrast of a pattern is a measure of the signal as a fraction of its background. Therefore, a reasonable definition of texture contrast for the dot patterns is the magnitude of the difference between the dot-vacant structure location luminance and the mean luminance of the randomdot pattern, divided by the mean luminance of the randomdot pattern.* To apply this definition some notation has to be introduced. Let C, be the texture contrast of the triangle in the pattern of white dots and C, be the corresponding quantity for the pattern of black dots. Let Db be the luminance of a black dot and D, be the luminance of a white dot. The luminance of the blank, inter-dot area will be denoted B, for the pat?m of white dots and Bb for the pattern of black dots. L, is the mean luminance of the pattern of white dots and& is the mean luminance of the black-dot pattern. Using the definition of texture contrast, the equation of interest for explaining the contrast reversal effects in Figs 1 and 3 is: (C,/C,l = (L,IG$.) (D, - B,W,
- &A.
(1)
white-dot pattern texture contrast is about six times that of the biack-dot pattern. Contrast reversal. therefore. produces a larger texture-contrast difference in Fig. 1 than in Fig. 3. Implicit in equation (1) is a statement about the influence of dot den_sity. The dot density determines the values of 2, and I+. hence their ratio. The ma@tude of the contrast reversal effect should depend on dot density since (C,jC,) decreases as dot density increases The sets of patterns in Figs 4 and 5 show the effect of changing dot density by as much as a factor of 5. This change in dot density is fairly modest as far as changes in Lb and L, are concerned, yet a tendency of increasing dot density to diminish the magnitude of the contrast reversal effect is noticeabIe. In the absence of the stimulus manipulations that modify the visibility of the structure in the presented dot patterns, an explanation for the appearance of the triangle based on the perceptual analysis of dot pattern regularities would have seemed appropriate. One could have argued that a dot pattern is analyzed for arrangements of dots that do not appear to be completely random, and the perception of structure is a consequence of being satisfied that certain dot arrangements are not entirely haphazard. However, the fact that structure visibility can be varied without changing the pattern arrangement indicates that perception of the portrayed structure involves more than an analysis of the arrangement dots. It seems also to involve more than Iinear spatial frequency analysir5
For the typ of contrast reversal used in Fig. 1, the term (D, - B,)/(B, - Db) equals one and the term (&,/zJ is larger than one. Hence C, > C,; the triangle has greater texture contrast in the pattern of white dots in Fig. 1. For Fig. 3. the quantity (D, - B,)/(B, - D,) is larger than one because the gray background is closer in luminance to the black level than it is to the white level. Mean luminance Acknowledgements-The concept of texture brightness and change due to contrast reversal is su~~entiy small its potential explanatory vaiue. as pointed out here, were that, compared--to the value of (DW- B,)/(B, - D,,), developed during discussions with A. Rose. I thank him the value of (LdL,) can be taken as one. Hence, for for this, and for helpful comments_ I also thank R. W. Cohen, J. R. Moeller and D. Staebler for critical reading Fig, 3 we also have f, > C,: the triangle still has of the manuscript. This work was supported in part by greater texture contrast in the pattern of white dots. ONR. This type of analysis can be taken a step further to explain why the contrast reversal effect in Fig. 3 is weaker than in Fig. 1. For Fig. 3 the term Rf F’fRENCfS (DW- &J/(4 - D,) has a value of about three, so the white-dot pattern texture contrast is about three Coren S. (1972) Subjective contours and apparent depth. tunes that of the black-dot pattern. In Fig. 1 i, is Psychol. Rec. 79, 359-367. on the order of six times the vafue of i,, so the Harmon L. D. and Juleu B. (L973) Masking in visual *For dot densities sufficiently high that the pattern is essentially a line drawing as in Fig. 2. the texture contrast is equivaIent to the line contrast, and agrees (within a proportionahty constant) with previous definition of line contrast (Tolhurst and Dealy, 1975). J It would seem reasonable that a role of low-pass spatial filtering in enhancing structure visibility in Figs I and 3 is one of enabling the coIIection of dots to be visualty connected to form the unitary background texture. For example, high spatial frequency content of sharp dot boundaries might suppress the cooperative effect of visual connection, so that some low-pass filtering would be useful for perception of a dotted background supporting a triangle. In’ any event, the contrast reversal effect appears to demonstrate issues other than consequences of linear spatial filtering.
recognition: effects of two-dimensional filtered noise. Science 180, 1194-I 197. Julesz B. (1971) Foundodons of Cyclopean Perceprion. University of Chicago Press. chi&go.Nes F. L. van and Bouman M. A. (1967) Spatial modulation transfer in the human eye. J. opr.‘Soc. Am. 57. 401406. Pate! A. S. (1966) Spatial resolution by the human visual system: the effect of mean retinal ifluminance. 1. opt. Sot. Am. 56, 689-694. Rose A. (1974) Vision. Human and Electronic. Plenum Press, New York. Shlaer S. (1937) The relation between visual acuity and iilumination. J. gen. Physiof. 21, 165f88. Tolhurst D. J. and Dealy R. S. (1975) The detection and identification of lines and edges Vision Res. 15. 1367-1372.
Fig. 1. If these dot patterns are inspected from a distance of a few feet_ a dot-vacant triangle should appear in each of the patterns (it is helpful to use Fig. 2 to develop a visual template of the tri anglesk The shape is typically more pronounced in the bottom pattern. Its visibility is enhanced. irI either dot pattern. by decreasing the iIlu~nadon level. increasing viewing distance, or introducir ig blur.
319
Fig. 2. Line drawings which would result from applying the systematic dot deletion procedure of Fig. I to dot patterns that are too dense to resolve individual dots.
330
Fig. 3. Variant of Fig. I in which a constant. gray background is used. (The dot patterns were transfated so as to be side-by-side in this figure as a practicai convenience.)
331
c
Fig. 4. Dot patterns portritying a triangle in the manner of Fig. 1. There are 11.5 times as many dots in B as in A. and twice as many dots in C as in B. The dot densitl; tn C is less :han W, The effect of contrast reversal on structure visibility. or on apparent contrast of the triangle structure when it is clearly visible. tends to drcrcase with increasing dot densit!. The biack background triangle still seems to have a contrast advantage m C.
Fig. 5. Variant
of Fig. 4 in which a constant.
333
gray background
is used.
NOTE
STRUCTURE VISIBILITY CHANGE CONTRAST REVERSAL
WITH
JOSEPH J. MEZREH RCA Laboratories. Princeton, NJ 0854% U.S.A. in recised
(Receired 12 Mar 1977;
The visibility of structure in a pattern depends on the visibility of the physical elements of the pattern. However, as is shown here, one can modify the visibility of portrayed structure by stimulus manipulations that do not produce corresponding changes in pattern element visibility or clarity. This is demonstrated with Fig. 1. The two random-dot patterns in this figure are identical; they are contrast reversals of each other. Dots have been deleted from the random-dot patterns in a systematic fashion’ so that if the dot density were sufficiently high that individual dots coutd not be resolved a line drawing of a triangle would actually be presented in each half of the figure. A white triangle on black background would be shown in the top and a black triangle on white background wouid be shown in the bottom as in Fig. 2. If Fig. 1 is viewed from a distance of a few feet the structure portrayed in the bottom dot pattern should, after some inspection, pop into view. The triangle can be perceived as having subjective solid lines.’ The figure in the top dot pattern can also be perceived, with sufficient viewing distance, but it is less salient than the black triangle of the bottom pattern. The difference in structure visibility in the top and bottom patterns is emphasized by decreasing the viewing distance so that the triangle in the lower pattern is just apparent. in this case the structure in the top pattern is not seen. The structure visibility for either dot pattern can be increased by decreasing the i~um~ation level, or increasing the viewing distance, or blurring the pat’ The dot deletion was accomplished by placing a transparency of the original of the bottom triangle in Fig. 2 over a random pattern of white dots on a black background A number of dots were, therefore, obscured by the triangle. A high contrast film photograph of the resulting pattern is reproduced in the black background portion of Fig. 1. and its contrast reversal is reproduced in the white background The technique of systematic deletion of dots from a random-dot pattern was introduced by Rose (1974) as a device for characterizing signal detection properties of visual processing. ’ Upon inspection of the detail of these solid lines they are realized to be not physically present The perceptual construct of solid lines in these dot patterns is reminiscent of the phenomenon of an apparent edge at a texture boundary (Julesz, 1965) and aiso reminiscent of the phenomenon of subjective contours (Coren, 1972; Gregory, 1972; Kaniza, 1976). 3 A similar blur&g benefit is observed with block quantitized pictures (Harmon and Juiesz. 1973).
form
4 August
1977)
terns. These procedures for making the triangle stand that effectively filter out high spatial frequency content. It is, therefore, Iikely that a requirement for the visibility of structure in either of these dot patterns is the low-pass filtering out of some high spatial frequency masking noise.’ However, this filtering hypothesis does not account for the effect of contrast reversal. It is the case that the bottom pattern, with white dots, is dimmer than the top pattern with black dots. But. SO long as the darker pattern with white dots has a mean luminance of at least 1Omi (which it can have with good room lighting) there should be negligible difference in the amount of visible high frequency detail for the opposite contrast patterns. The change in spatial resolution above 10ml is minimal (Shlaer, 1939: van Ness and Bouman, 1967; Patel, 1966). Indeed. the white dots in the bottom pattern seem to be no less sharply defined than the black dots in the top pattern. Figure 3 is a variation on Fig. 1 in which a constant, gray background is used. Contrast reversal of the dot patterns is accomplished with minimal change in mean luminance. The e&et of contrast reversal is slightly weaker in this case than in Fig. 1, but it is still quite evident. The dot-deletion triangle is more visible, or seems to have greater apparent contrast, in the pattern of white dots. While the mean luminance values for the opposite contrast patterns of Fig. 3 are close, it is noteworthy that we now have a situation in which the brighter of the two patterns yields the greater structure visibility. Figures 1 and 3 taken together should make it clear that the influence of contrast reversal on structure visibility is not simply a secondary effect of the influence of pattern mean huninance. For example, more than lowpass spatial filtering is involved The triangle structure, when visible, is seen against a background random-dot pattern. A brightness eontrast between the structure and its supporting randomdot pattern does not exist; nevertheless there is a sense of contrast. This sense of contrast can be better understood if one assumes that when the triangle structure is apparent, the randomly distributed dots are visually connected to form a unitary background texture. A perceived unitary texture should have a perceived globaf brightness, which could be equivalent to the mean pattern brightness. Consequently, the effective texture brightness would be different from the brightness of the dot-vacant regions which define the portrayed structure. Hence, there is a specifiable contrast for the triangle, a texture contrast.
321
out are operations
328
Research Note
This approach allows an explanation for the observed difference in structure visibility in the opposite contrast dot patterns. The contrast of a pattern is a measure of the signal as a fraction of its background. Therefore, a reasonable definition of texture contrast for the dot patterns is the magnitude of the difference between the dot-vacant structure location luminance and the mean luminance of the randomdot pattern, divided by the mean luminance of the randomdot pattern.* To apply this definition some notation has to be introduced. Let C, be the texture contrast of the triangle in the pattern of white dots and C, be the corresponding quantity for the pattern of black dots. Let Db be the luminance of a black dot and D, be the luminance of a white dot. The luminance of the blank, inter-dot area will be denoted B, for the pat?m of white dots and Bb for the pattern of black dots. L, is the mean luminance of the pattern of white dots and& is the mean luminance of the black-dot pattern. Using the definition of texture contrast, the equation of interest for explaining the contrast reversal effects in Figs 1 and 3 is: (C,/C,l = (L,IG$.) (D, - B,W,
- &A.
(1)
white-dot pattern texture contrast is about six times that of the biack-dot pattern. Contrast reversal. therefore. produces a larger texture-contrast difference in Fig. 1 than in Fig. 3. Implicit in equation (1) is a statement about the influence of dot den_sity. The dot density determines the values of 2, and I+. hence their ratio. The ma@tude of the contrast reversal effect should depend on dot density since (C,jC,) decreases as dot density increases The sets of patterns in Figs 4 and 5 show the effect of changing dot density by as much as a factor of 5. This change in dot density is fairly modest as far as changes in Lb and L, are concerned, yet a tendency of increasing dot density to diminish the magnitude of the contrast reversal effect is noticeabIe. In the absence of the stimulus manipulations that modify the visibility of the structure in the presented dot patterns, an explanation for the appearance of the triangle based on the perceptual analysis of dot pattern regularities would have seemed appropriate. One could have argued that a dot pattern is analyzed for arrangements of dots that do not appear to be completely random, and the perception of structure is a consequence of being satisfied that certain dot arrangements are not entirely haphazard. However, the fact that structure visibility can be varied without changing the pattern arrangement indicates that perception of the portrayed structure involves more than an analysis of the arrangement dots. It seems also to involve more than Iinear spatial frequency analysir5
For the typ of contrast reversal used in Fig. 1, the term (D, - B,)/(B, - Db) equals one and the term (&,/zJ is larger than one. Hence C, > C,; the triangle has greater texture contrast in the pattern of white dots in Fig. 1. For Fig. 3. the quantity (D, - B,)/(B, - D,) is larger than one because the gray background is closer in luminance to the black level than it is to the white level. Mean luminance Acknowledgements-The concept of texture brightness and change due to contrast reversal is su~~entiy small its potential explanatory vaiue. as pointed out here, were that, compared--to the value of (DW- B,)/(B, - D,,), developed during discussions with A. Rose. I thank him the value of (LdL,) can be taken as one. Hence, for for this, and for helpful comments_ I also thank R. W. Cohen, J. R. Moeller and D. Staebler for critical reading Fig, 3 we also have f, > C,: the triangle still has of the manuscript. This work was supported in part by greater texture contrast in the pattern of white dots. ONR. This type of analysis can be taken a step further to explain why the contrast reversal effect in Fig. 3 is weaker than in Fig. 1. For Fig. 3 the term Rf F’fRENCfS (DW- &J/(4 - D,) has a value of about three, so the white-dot pattern texture contrast is about three Coren S. (1972) Subjective contours and apparent depth. tunes that of the black-dot pattern. In Fig. 1 i, is Psychol. Rec. 79, 359-367. on the order of six times the vafue of i,, so the Harmon L. D. and Juleu B. (L973) Masking in visual *For dot densities sufficiently high that the pattern is essentially a line drawing as in Fig. 2. the texture contrast is equivaIent to the line contrast, and agrees (within a proportionahty constant) with previous definition of line contrast (Tolhurst and Dealy, 1975). J It would seem reasonable that a role of low-pass spatial filtering in enhancing structure visibility in Figs I and 3 is one of enabling the coIIection of dots to be visualty connected to form the unitary background texture. For example, high spatial frequency content of sharp dot boundaries might suppress the cooperative effect of visual connection, so that some low-pass filtering would be useful for perception of a dotted background supporting a triangle. In’ any event, the contrast reversal effect appears to demonstrate issues other than consequences of linear spatial filtering.
recognition: effects of two-dimensional filtered noise. Science 180, 1194-I 197. Julesz B. (1971) Foundodons of Cyclopean Perceprion. University of Chicago Press. chi&go.Nes F. L. van and Bouman M. A. (1967) Spatial modulation transfer in the human eye. J. opr.‘Soc. Am. 57. 401406. Pate! A. S. (1966) Spatial resolution by the human visual system: the effect of mean retinal ifluminance. 1. opt. Sot. Am. 56, 689-694. Rose A. (1974) Vision. Human and Electronic. Plenum Press, New York. Shlaer S. (1937) The relation between visual acuity and iilumination. J. gen. Physiof. 21, 165f88. Tolhurst D. J. and Dealy R. S. (1975) The detection and identification of lines and edges Vision Res. 15. 1367-1372.
Fig. 1. If these dot patterns are inspected from a distance of a few feet_ a dot-vacant triangle should appear in each of the patterns (it is helpful to use Fig. 2 to develop a visual template of the tri anglesk The shape is typically more pronounced in the bottom pattern. Its visibility is enhanced. irI either dot pattern. by decreasing the iIlu~nadon level. increasing viewing distance, or introducir ig blur.
319
Fig. 2. Line drawings which would result from applying the systematic dot deletion procedure of Fig. I to dot patterns that are too dense to resolve individual dots.
330
Fig. 3. Variant of Fig. I in which a constant. gray background is used. (The dot patterns were transfated so as to be side-by-side in this figure as a practicai convenience.)
331
c
Fig. 4. Dot patterns portritying a triangle in the manner of Fig. 1. There are 11.5 times as many dots in B as in A. and twice as many dots in C as in B. The dot densitl; tn C is less :han W, The effect of contrast reversal on structure visibility. or on apparent contrast of the triangle structure when it is clearly visible. tends to drcrcase with increasing dot densit!. The biack background triangle still seems to have a contrast advantage m C.
Fig. 5. Variant
of Fig. 4 in which a constant.
333
gray background
is used.