Gradient "white mark" optical illusion

Question

I have a solid colour background. Diagram 1.

Over the top of this is a transparent gradient going left to right. Diagram 2. It is RGB 0,0,0, the left edge is 85% opacity, and the right edge fades off to 0% opacity. In the screenshot it is 30px wide (diagram 3 shows where the 0% right edge ends).

Ok, my issue is I can see a white smudge area toward the right edge of the gradient. So it appears to go to a lighter green colour before the solid green background. But zooming in on Photoshop and checking the colours proves to me that this isn't actually happening.

To see for yourself, look at diagram 2, and see if you think it fades from dark green to the slid background green smoothly, or whether you can see a lighter area too. Diagram 4 shows the position that the light area appears in diagram 2.

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1. Why does this optical illusion occur?

2. How can I solve this optical illusion so the blend looks smooth?

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Answer 1

Nice question.

It is a compensation the brain makes of tonal contrast.

When you have a color you see it lighter if it is next to a dark color, and you see it darker if it is next to a light one.

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Let us concentrate on the central rectangles of this next image.

Each of them are next to a darker one to the left. So the brain see each as a litle lighter. But they are also next to a lighter color, so the brain see them as darker...

But all rectangles are just plain color. The result is a gradient. As a wavy ilusion.

The same is happening on youe rectangle. The green is next to a darker color on the left... The brain compensates it with a lighter green next to it.

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There is not much you can on a tiny sample, but on a larger sample:

1) You can try to make a "logaritmic or curved progrssion" of the dark side, so the gradient crashes with the green smoother.

In other words, adjust a bit the curves of the shadow, and extend it a bit more so it has more time to fade.

2) Make an intermediate patch and drag it to the dark side.

3) Do not use a "flat green" make a gradient all the way. The example above keeps the gradient all the way and you do not see a white patch.

4) Another possible option is to add a "chromathic shift", to fool the brain with is stares at this direct tonal contrast. Take a look at this: How can I make the color white look as white as possible?

Answer 2

Make sure that the gradient layer is absolutely transparent on the transparent side. I could imagine a single row of non-transparent pixels creating this error. Also ensure that the blending mode for the black gradient layer is set to multiply.

I'd recommend using layer effects for creating gradients like these, instead of using separate layers superimposed over the background. Double-click the background layer's bar in the Layers palette, or choose Layer > Layer Style > Gradient Overlay from the top menu.

Answer 3

I run into this issue often. my hard rule for it is, don't use black, use a very dark green (or whatever color it is). i'd start around #002200, which would be nearly black but has some green in it.

your brain i trying to compensate for the lack of green in the black and that's where the "white" comes from, too much contrast. i'm not sure what exactly you're trying to achieve, but in reality, a shadow is almost never black, it takes the colors of what it's being cast on.