THREE.MultiplyBlending behavior

[emkajeej]

Description
Adding a black and white material with MultiplyBlending on results in a darker image than it should be.

It's my understanding that in multiply blend mode, each pixel value should be multiplied by the pixel value below it. When adding a colored layer to a black and white layer, this should is useful for perserving details or coloring shadows without distorting the original colors.
Using software as photoshop gives the correct result. Doing this in THREE results in a darker shade of the original color.

Consider the pen below.
The plane has one StandardMaterial, full white. Adding another white material on top of this should not have any visual effect on the render. However it appears slightly darker than the original material was. It's noticeable in default tonemapping; Using ACESFilmicToneMapping amplifies this problem.

Click the button to toggle the top layer.
https://codepen.io/emkajeej/pen/XWqWMVa

Same with actual images:
https://codepen.io/emkajeej/pen/abGoByg
The mutiplied image is mostly white, with some light gray streaks. Notice how the entire image is darkened, however.

r144

[Mugen87]

With basic renderer settings, the result is as expected: https://jsfiddle.net/35nfmkxb/

The problem is that blending is not performed correctly when tone mapping and color space conversion is done inline via GLSL (so prior to blending). This issue is already discussed here: #23019

[mrdoob]

The problem is that blending is not performed correctly when tone mapping and color space conversion is done inline via GLSL (so prior to blending). This issue is already discussed here: #23019

Oh, I missed that issue. That's annoying... we can't use OpenGL blend modes anymore...

[emkajeej]

Does that mean that the only way to get proper blending and tonemapping, each frame has to be color corrected in post? That seems like a massive overhead, and quite unfortunate if that's the case.
That would also only account for part of the issue, as physicallycorrectlights has an impact on this as well.. and I assume that's not something that can be changed in post.

My workaround for now, I think, is to blend the images in a separate canvas first and push that as a texture to the material.

[Mugen87]

Does that mean that the only way to get proper blending and tonemapping, each frame has to be color corrected in post?

Yes. As mentioned above, a post processing step is required for the most color accurate rendering.