Issue with shaders and alpha

[mateuszJS]

Hey!

I'm trying to make calculations in a shader, and use all 4 channels, but unfortunately the alpha channel does not work as expected. I was thinking that if I will set
gl.blendFunc(gl.ONE, gl.ZERO)
then the output should be only just the source, without any blending, but it's not. So I have tried to combine it also with premultipling alpha (and keep gl.blendFunc(gl.ONE, gl.ZERO)), I get sort of closer results to the expected, but still behaviour is strange.

In the demo below I'm just filling mesh with one color, and console.log values of the first pixel, was expecting [25, 178, 229, 25] but I got [255, 255, 255, 25] (without premultipled alpha) and [31, 184, 235, 25] (with premultipled alpha)
https://www.pixiplayground.com/#/edit/785nGjqG5wSZoa9r2Xn7I
 

Thank you in advance for a response!

[ivan.popelyshev]

extract assumes that data in buffer is premultiplied, so it postdivides it

premultiplied

ok, so, your R*A = 0.1 * 0.1 , that's 0.01, that's only 2.55 if interval is 0..255, it gets rounded to just 2. Then it divides it by alpha, which is 0.1, and that's how I get red=20 in my console.

Welcome to 8-bit per channel premultiplied hell

Edited November 9, 2020 by ivan.popelyshev

[ivan.popelyshev]

What can you do: modify extract that way it doesnt postdivide stuff. If you want to just use that renderTexture somewhere else, you can use non-premultiplied alpha and everything will be fine.

[mateuszJS]

Awesome! Thank you! Just in case if someone else will run into that issue, here is the fixed code sample

https://www.pixiplayground.com/#/edit/JBJ_NTNOIwcxv64JdNMZE

1. Output from the fragment shader are not premultipled by alpha
2. Created function extractPixelsWithoutPostdivide, it's just copy of https://github.com/pixijs/pixi.js/blob/068e0af7f2851c51618b463b7fccc555e6ffb4b0/packages/extract/src/Extract.ts#L192 but with removed Extract.arrayPostDivide(webglPixels, webglPixels);

Edited November 10, 2020 by mateuszJS
Typo