[Webkit-unassigned] [Bug 119473] New: [CSS Shaders] Validated fragment shaders do too much math

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=119473

           Summary: [CSS Shaders] Validated fragment shaders do too much
                    math
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: UNCONFIRMED
          Severity: Normal
          Priority: P2
         Component: CSS
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: ralpht+bugs at gmail.com
                CC: achicu at adobe.com, mvujovic at adobe.com


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).

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