Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Color Spreading, Neon Color Spreading, and Watercolor Illusion

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_267

Synonyms

Definition

The color spreading is a long-range assimilative spread of color emanating from a thin colored contour running in the same direction/continuation or being contiguous/adjacent to a darker chromatic contour and imparting a figure-ground effect across a large area. The two main examples of color spreading are the well-known neon color spreading and the watercolor illusion.

Neon Color Spreading

In 1971, Varin [1] studied a novel “chromatic spreading” phenomenon induced when four sets of concentric black circumferences, arranged in a cross-like shape, are partially composed of blue arcs, thus producing a virtual large central blue circle (Fig. 1a). The virtual circle appears as a ghostly transparent circular veil of chromatic translucent diffusion of bluish tint spreading among the boundaries of the blue arcs and filling in the entire illusory circle, induced by the terminations of the black arcs (Fig. 1).
Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 1

Neon color spreading: The central virtual circle (a) and the inset virtual diamond shape (b) appear as a ghostly overlapping transparent veil of bluish tint spreading among the boundaries of the blue components

This phenomenon was independently reported a few years later from Varin’s discovery by van Tuijl [2], who named it “neon-like color spreading.” He used a lattice of horizontal and vertical black lines, where segments of different colors (e.g., blue) create an inset diamond shape. The main outcome reveals again a tinted transparent diamond-like veil above the lattice (Fig. 1b).

Geometrically, the main geometrical property of all the known cases of neon color spreading is the continuation of one contour, usually black, in a second contour with a different color or, differently stated, a single continuous contour changing from one color to another. Phenomenally, color spreading manifests a coloration and a figural effect described in detail in the following sections.

Coloration Effects in Neon Color Spreading

The phenomenology of the coloration effect within neon color spreading depends on the luminance contrast between the two inducing contours and is summed in the next points [2, 3]. (i) The color is perceived as a diffusion of a certain amount of pigment of the inset chromatic segments. (ii) The appearance of the coloration is diaphanous like a smoggy neon upon the background or (under achromatic conditions) like a shadowy, dirty, or filmy transparent veil. (iii) Under conditions where the inset figure is achromatic and the surrounding inducing elements chromatic, the illusory veil of the inset figure appears tinted not in the achromatic color of the embedded elements, but in the complementary color of the surrounding elements, e.g., the achromatic components appear to spread greenish or bluish illusory colors, respectively, with red or yellow inducers.

Figural Effects in Neon Color Spreading

The apparent coloration of neon color spreading is related to its figural effects. Phenomenally, (i) the illusory neon coloration manifests a depth stratification appearing in front of the inducing elements; (ii) it is also perceived as a transparent film; (iii) by reversing the relative contrast of inset versus surrounding components, the depth stratification reverses accordingly (Fig. 2); (iv) the illusory colored region, under different chromatic conditions, may assume different figural roles by becoming, for example, a superimposed “light,” a “veil,” a “shadow,” or a “fog.”
Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 2

By reversing the relative contrast of inset versus surrounding components (cf. Fig. 1), the depth stratification of neon color spreading reverses accordingly

Watercolor Illusion

The “watercolor illusion” is a long-range spread of color diffusing from a thin colored contour adjacent to a darker chromatic contour and imparting a clear figural effect within large regions [4, 5, 6, 7, 8, 9]. In Fig. 3, purple wiggly contours flanked by orange edges are perceived as undefined curved solid shapes, similar to peninsulas emerging from the bottom, evenly colored by a light veil of orange tint spreading from the orange edges.
Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 3

Two undulated peninsulas emerging from the bottom are perceived illusorily colored by a light veil of orange tint spreading from the orange edges

By reversing purple and orange contours of Fig. 2, the coloration and figure-ground organization are reversed, and, thus, two orangish peninsulas, going from the top to the bottom, connected to the mainland at the top, are now perceived (Fig. 4). Briefly, what appears as illusory colored and segregated in one figure is perceived as empty space without a clear coloration in the other figure and vice versa. Therefore, the peninsulas of Figs. 3 and 4 pop up as totally different objects not referable to the same juxtaposed contours.
Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 4

By reversing the purple and orange contours of Fig. 3, the coloration and the figure-ground segregation are reversed: the two peninsulas going from the top to the bottom and connected to the mainland at the top

Coloration Effects in the Watercolor Illusion

The phenomenology of the coloration effect in the watercolor illusion reveals the following main attributes:
  1. (i)

    As in neon color spreading, the illusory color is approximately uniform. As shown in Fig. 5, the coloration, within the regions where the orange contours are closer, is the same as within the regions where they are distant.

     
  2. (ii)

    The coloration extends up to about 45°.

     
  3. (iii)

    It is complete by 100 ms.

     
  4. (iv)

    Similarly to neon color spreading, all the colors can generate the illusory coloration, as shown in Fig. 6, where an undefined irregular peninsula appears filled with a light blue tint. It should be noted that this peninsula is the Mediterranean Sea when the two adjacent chromatic contours are reversed.

     
  5. (v)

    The coloration occurs on colored and black backgrounds. In Fig. 7, an undefined irregular peninsula (the Mediterranean Sea when the contours are reversed) appears filled with a purple tint.

     
  6. (vi)

    The optimal contour thickness is approx. 6 arcmin.

     
  7. (vii)

    The effect is stronger with wiggly contours, but it also occurs with straight contours and with chains of dots as shown in Fig. 8 [5, 6].

     
  8. (viii)

    High luminance contrast between inducing contours shows the strongest coloration effect; however, the color spreading is clearly visible at near equiluminance (see Fig. 9) [5, 6].

     
  9. (ix)

    The contour with a lower luminance contrast relative to the background spreads proportionally more than the contour with a higher luminance contrast.

     
  10. (x)

    The color spreads in directions other than the contour orientation.

     
  11. (xi)

    By reversing the colors of the two adjacent contours, the coloration reverses accordingly.

     
  12. (xii)

    Phenomenally, the coloration appears solid, impenetrable, and epiphanous as a surface color.

     
  13. (xiii)

    Similarly to neon color spreading [1, 2, 3], the watercolor illusion induces a complementary color when one of the two juxtaposed contours is achromatic and the other chromatic (see Fig. 10) [5]. The inside of the zigzagged annulus appears yellowish.

     
Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 5

The apparent coloration is approximately uniform, i.e., the coloration, within the regions where the orange contours are closer, is the same as the regions where they are distant

Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 6

All the colors can generate the coloration effect of the watercolor illusion: the undefined irregular peninsula appears filled with a light blue tint

Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 7

The coloration occurs on colored and black backgrounds: an undefined irregular peninsula appears filled with a purple tint

Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 8

The watercolor illusion is stronger with wiggly contours, but it also occurs with chains of dots

Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 9

High luminance contrast between inducing contours shows the strongest coloration effect; however, the color spreading is clearly visible at near equiluminance

Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 10

The inside of the zigzagged annulus appears yellowish

Figural Effects in the Watercolor Illusion

The main properties of the figural effect are the following:
  1. (i)

    The watercolor illusion strongly enhances the “unilateral belongingness of the boundaries” [6, 7, 10].

     
  2. (ii)

    As in neon color spreading, the figural effect of the watercolor illusion is clearly perceived although it occurs in a different mode of appearance. The figure shows a strong depth segregation and a volumetric rounded and three-dimensional attribute, while the perceived variation of color, going from the boundaries to the center of the object, is seen as a gradient of shading, as if light were reflected onto a volumetric and rounded object. Figure 11 shows undefined, rounded, and volumetric shapes differing from one row to another on a shapeless empty space due to the unilateral belongingness of the boundaries. The stars are totally invisible.

     
  3. (iii)

    By reversing the colors of the two adjacent contours, the figure-ground segregation reverses accordingly. In Fig. 12, the same elements of Fig. 11, illustrated with reversed purple-orange contours, appear like juxtaposed stars. The undefined shapes differing from one row to another are now invisible.

     
  4. (iv)

    Under the previous conditions, the figure-ground segregation is not reversible and unequivocal.

     
  5. (v)

    The watercolor illusion determines grouping and figure-ground segregation more strongly than the Gestalt principles of proximity, good continuation, Prägnanz, relative orientation, closure, symmetry, convexity, past experience, similarity, surroundedness, and parallelism [6, 7, 10]. In Fig. 13, some examples showing the watercolor illusion respectively against and in favor of surroundedness, relative orientation, good continuation, past experience, and parallelism are illustrated.

     
  6. (vi)

    By reversing the luminance contrast of the background, e.g., from white to black, while the luminance contrast of the contours is kept constant, the figure-ground segregation reverses (Fig. 14) [10]. Going from the bottom to the top of the figure, the crosses become stars. These results are in contrast to Gestalt claim that the currently figural region is maintained even on black/white reversal.

     

This suggests that the watercolor illusion includes a new principle of figure-ground segregation, the asymmetric luminance contrast principle, stating that, all else being equal, given an asymmetric luminance contrast on both sides of a boundary, the region whose luminance gradient is less abrupt is perceived as a figure relative to the complementary more abrupt region, which is perceived as a background [10].

Similarities and Differences In Between the Two Illusions

By summing up the phenomenology of coloration and figural effects in both neon color spreading and watercolor illusion, the former differs from the latter in both the appearance of the coloration (respectively, transparent vs. solid and impenetrable and diaphanous vs. epiphanous) and in the figural effects (respectively, transparent vs. opaque and dense and appearance as a “light,” a “veil,” a “shadow,” or a “fog” vs. rounded thick and opaque surface bulging from the background).

In spite of these differences, the two illusions are phenomenally similar in their clear color spreading and depth segregation. It is suggested [5] that, while the similarities may depend on the local nearby transition of colors, equivalent in both illusions, the differences may be attributed to the global geometrical boundary conditions, dissimilar in the two illusions. As a matter of fact, while the neon color spreading is elicited by the continuation in the same direction of two contours of different colors, the watercolor illusion occurs through their juxtaposition.

If this is true, the phenomenal differences between the two illusions can be reduced or eliminated through geometrical variations that bring both phenomena to a common limiting case placed in between and based on the local nearby transitions of colors.
Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 11

Watercolored undefined shapes differing from one row to another

Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 12

The same elements of Fig. 11, illustrated with reversed purple-orange contours, appear like juxtaposed stars

Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 13

Some examples showing the watercolor illusion respectively against and in favor of surroundedness, relative orientation, good continuation, past experience, and parallelism

Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 14

By reversing the luminance contrast of the background, from white to black, while the luminance contrast of the contours is kept constant, the figure-ground segregation reverses: going from the bottom to the top of the figure, the crosses become stars

A Limiting Case

Figure 15 shows four conditions that gradually introduce the limiting case. Figure 15a illustrates a neon color spreading that represents the starting condition where concentric purple arcs continue by becoming orange. Now, the resulting inset square annulus appears as a transparent veil of orange color not like a ghostly circular veil of translucent color as in Fig. 1. This difference in the color and figural appearance is likely related to the high contrast between the two inducing colors.
Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 15

(a) The neon color spreading defined by the continuation of lines of different colors; (b) a condition in between neon color spreading and watercolor illusion, where the orange inset arcs are reduced to short dashes; (c) a condition in between the two illusions, where the purple surrounding arcs of (a) are reduced to short dashes; (d) the two-dot limiting case

Gradual steps toward the final combination of the two illusions in a limiting case are illustrated in Figs. 15b and c. Geometrically, in Fig. 15b, the orange inset arcs are reduced to short dashes, creating a condition in between neon color spreading and the watercolor illusion: from the neon color spreading perspective, the inducing elements are contours continuing in short dashes (or elongated dots), but from the watercolor perspective, the terminations of the inducing arcs contain juxtaposed short dashes. Under these conditions, a coloration effect, not weaker than that of Fig. 15a, is perceived. However, it manifests a poor diaphanous and surface appearance. The illusory figure appears as a fuzzy square annulus, yellowish and brighter than the background. It is worthwhile to note that the further reduction of the dashes to dots does not change significantly the strength of these effects.

The geometrical reduction in between neon color spreading and the watercolor illusion and opposite to the one of Fig. 15b is illustrated in Fig. 15c. Under these further conditions, all else being equal, short dashes become the purple arcs of Fig. 15a. Now the coloration effect is weaker than that of Fig. 15a.

Given these geometrical prerequisites, the final step toward the limiting case becomes immediate and consists in putting together the previous opposite reductions as shown in Fig. 15d. The results show that by reducing both the purple and orange arcs of Fig. 15a to short dashes, the coloration and figural effects do not change significantly [5]. This is corroborated by previous outcomes according to which the watercolor illusion occurs not only by using juxtaposed lines but also by using juxtaposed chains of dots [6, 7, 10]. Under these conditions both coloration and figural effects become weaker and weaker as the density of the dots becomes sparser and sparser.

The two-dot juxtaposition of Fig. 15d can be considered as a true limiting case for neon color spreading and the watercolor illusion. As a matter of fact, (i) the two-dot limiting case can be considered as the geometrical common condition, beneath. (ii) The strength of both coloration and figural effects does not change significantly; therefore, the specific mode of appearance of coloration and figural effects in the two illusions is elicited by different local and global distributions of nearby transitions of colors that, in their turn, induce different boundary organizations. (iii) Phenomenally, the differences between the two illusions, where the inner changes are based on continuation and juxtaposition of contours, can now be reconsidered and unified in terms of transition. This is not only a linguistic alternative but also it can bring advantages by providing support for a simple common neural model. (iv) The limiting case can suggest variations of the two illusions that manifest coloration and figural attributes in between the neon color spreading and the watercolor illusion, as shown in the next section.

Near the Limiting Case

By increasing the width of one of the two juxtaposed contours of the watercolor illusion to such an extent that the contour becomes a surface, the watercolor illusion manifests geometrical properties similar to the neon color spreading and, as a consequence, also shows different coloration and figural effects. The resulting coloration does not assume surface color properties, but properties more similar to the neon color spreading. It appears diaphanous like a foggy coloration diffusing everywhere in the background or as a colored light (Fig. 16).
Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 16

A light blue coloration filling in the inset square appears surrounded by a red spreading. The coloration effect is not accompanied by a figural effect with a plain volumetric property, but it appears diaphanous like a foggy veil of color

Similarly to the previous condition, the coloration effect of Fig. 17 gives to the illusory star a fuzzy luminous quality. While in Fig. 16 the coloration belongs to the background, in Fig. 17 it belongs to the figure; however, the star does not manifest a strong surface appearance, but its inner surface appears brighter and yellowish and foggy and smooth.
Color Spreading, Neon Color Spreading, and Watercolor Illusion, Fig. 17

The illusory coloration of the star appears fuzzy and luminous and manifests a poor surface appearance

Taken together, these figures suggest that (i) the modes of appearance of coloration are strongly related to boundary conditions that induce specific figural effects; (ii) by changing the boundary conditions, coloration and figural attributes are perceived more similar to one, to the other illusion, or in between; and (iii) given this variety of appearances on the basis of different conditions, a simpler set of boundary cases, like in the limiting case, can unify both effects using local transitions of colors and can help to explain similarities and dissimilarities of the two illusions.

Neural Mechanisms Underlying the Two Illusions

On the basis of the previous results, coloration and figural effects may derive from parallel processes. At a feature processing stage, the short-range interaction area around and in between the two dots produces the color spreading common to both illusions, and at a parallel boundary processing stage, the different geometrical structures in both illusions organize the color spreading to elicit different figural effects. Moreover, the reduction of the neon color spreading and watercolor illusion to a common limiting case can suggest a common and an easier explanation that can be based on the FACADE neural model of biological vision [5]. The model posits that two processes, boundary grouping and surface filling-in [11, 12] substantiated by the cortical interblob and blob streams, respectively, within cortical areas V1 through V4, are responsible of how local properties of color transitions activate spatial competition among nearby perceptual boundaries, with boundaries of lower-contrast edges weakened by competition more than boundaries of higher-contrast edges. This asymmetry induces spreading of more color across these boundaries than conversely. These boundary and surface processes show complementary properties that can also predict how depth and figure-ground effects are generated in these illusions.

Other related findings to both illusions [13, 14, 15] showed that neurons in V2 respond with different strength to the same contrast border, depending on the side of the figure to which the border belongs, implying a neural correlate process related to the unilateral belongingness of the boundaries. Figure-ground segregation may be processed in areas V1 and V2, in inferotemporal cortex and the human lateral occipital complex. Also the color spreading of the two illusions might have its explanation in the cortical representation of borders [9].

Summary

The color spreading is a long-range assimilative spread of color emanating from a thin colored contour running in the same direction/continuation or being contiguous/adjacent to a darker chromatic contour and imparting a figure-ground effect across a large area. Two main examples of color spreading are the well-known neon color spreading and the watercolor illusion. The coloration and the figural properties of the two illusions, studied using phenomenal and psychophysical observations, can be reduced to a common limiting condition, i.e., a nearby color transition called the “two-dot limiting case,” which explains their perceptual similarities and dissimilarities and suggests a common explanation.

Cross-References

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© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Department of Humanities and Social SciencesUniversità degli Studi di SassariSassariItaly