Color Constancy or is it the Power of Gray?

by Howard Lyon

There as an article that I recently came across (you can read it here) that was really fascinating.  Dan Dos Santos sent it to me as well and I thought I would write a quick post about it.  It shows a plate of strawberries (keep reading, that isn't the fascinating part :)).



The contrast has been reduced and the shadow color is shifted to green.  The strawberries look, as you would expect, red.  But when you examine the image closer with the eye dropper tool, you soon see that there is no red.  It seems impossible.  The entire image is in fact made of greens of various saturations.

The article concludes that the reason we see the strawberries as red is due to "color constancy".  The article quotes Bevel Conway, and expert on visual perception from the National Eye Institute:

Conway said this illusion is also helped out by the fact that we recognize the objects as strawberries, which we very strongly associate with the color red, so our brain is already wired to be looking for those pigments

I think that color constancy is legitimate, and it might add to our perception in this image, but I don't think it fully explains what is happening.  I believe it is because of a different phenomenon. That is that when a gray is placed next to a color of higher saturation and similar hue and value, that gray will take on the appearance of the complementary color.

The reason the strawberries look red is because they are actually less saturated greens next to higher saturated greens and so they start to look like the complement of green.  Let's take a closer look at the image and the palette.

I reduced the image down to 256 colors to simplify the colors and get cleaner color samples.  It looks the same at this point:



Here is what the palette (below) looks like for the above image.  It is arranged according to hue.  The top rows being a little cooler and warming as it gets to the bottom rows.  Look at how the grays fluctuate in color temperature according to how much saturation there is.

*if you take the palette in to photoshop, there are a couple pixels in the palette that are outliers


Here is a crop of the image with colors swatches picked out.  Note how in the close up the "red" is still perceived even though you can't really tell that strawberries are the subject.


The swatches across the top of the image correspond to the pixel at the center of the circle.  Look at how the colors that are more blue or green are higher in saturation.  As the color starts to appear more red, the hue shifts a tiny amount (still blue/green), but the saturation drops off significantly and the gray looks more and more red.  The most "red" color, the one 4th from the left is actually the least saturated color in the crop with a saturation of just 13%.



If we shift the colors more yellow/green, the strawberries start to look purple/magenta:


Here is the 256 color palette for the image above.


This corresponds to the traditional color wheel showing complements across from each other



Here is the image shifted more towards blue.  The strawberries now look distinctly orange.  If color constancy were the only principle in effect, we should still see red strawberries, because we know they are red, but in fact they look orange, reflecting the complement of blue.




I have written about this in a previous post if you want to see this in action with some paintings:

Feeling Grey Today

I look forward to hearing your thoughts.  Thanks!

**update

I created a new image to remove some of the recognizable elements and just focus on the effect.  Here the image has been run through the mosaic filter, rotated and cropped.  I did this to remove any perception of strawberries.  If you take the image into PS and check the value range of the "strawberries" you will see that there is very little change in value, just saturation.  This is important to get the effect.


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