Hi,
I'm working on an algorithm where I need to detect if the current `gl_FragCoord` is an even or odd row/column. A typical implementation would look something like `if (mod(gl_FragCoord.x, 2.0) < 1.0) { ... } else { ... }`. After running into issues with this approach a quick google search pointed me to quite some good information:
Especially this forum post is pretty much identical to the issue I'm looking into.
I've created this repository to experiment with this issue and to solve it. I'm using a MiBox MDZ-16-AB. This repository is created for Android Studio. It will setup a basic custom `GLSurfaceView` that uses the `TestGlView` that instantiates the `TestGlRenderer` class. In `TestGlRenderer` I create a simple filter shader that applies the `if (mod(..))` logic to draw different colors for odd/even columns. I create a FBO with a size of 3840 x 2160 to detect floating point precision issues. When rendering into a FBO with a color attachment of a texture with a size of 1920 x 1080 the issues is a lot less.
In the image below (3840 x 2160) you can clearly see the issue. It should show vertical red and black lines from left to right.
When rendering using a 1920 x 1080 things get a little bit better, but still not 100% correct.
In `TestGlRenderer` I create an instance of `GlRenderToTexture` which is just a thin wrapper that create a FBO with one texture attachment. In `onSurfaceCreated()` of the `TestGlRenderer` class I create an instance of `GlRenderToTexture`. In the code I've added (a commented) version which creates either a 1920x1080 or 3840x2160 FBO.
Now I'm curious what would be a workaround or solution to be able to distinguish between odd and even rows/columns?
Thanks
Thanks for your reply and great suggestion. This was one of the two possible solutions I was thinking about; the other solution I was thinking about was creating some sort of tiled based rendering where I render / execute the fragment shader only on portions of the framebuffer; but I did not research if this is a sound solution yet. Using a texture lookup with 1 pixel height seems to be the best alternative.
If you go for tiled framebuffers, for FP16 you get 10 bits of mantissa so you should only be able to accurately address 2^10 = 1024 pixel wide framebuffers using gl_FragCoord.