Bug 119473 – [CSS Shaders] Validated fragment shaders do too much math

• Status: RESOLVED
• Resolution: INVALID
• Component: CSS

[Ralph T]

A trivial CSS fragment shader (empty css_main) performs a mat4 * vec4, at least one vec4 * vec4 and a bunch of things that look like mix. I'd like to:

 1. not multiply by css_ColorMatrix unless css_main writes it.
 2. add a compositing/blending fast path for some combination of modes.

My goal is to do vertex-based lighting with a fragment shader that looks more like:

  gl_FragColor = texture2D(css_u_texture, css_v_texCoord) * css_MixColor;

(1) is easy enough to detect. Is there a combination of compositing and blending that equates to the above shader (or something close to it)?

(My motivation is that the current generated shaders run very slowly on IMGTEC SGX GPUs, which generally don't want to do much fragment math).