[Webkit-unassigned] [Bug 57499] New: Speed up alpha-blending in ImageDecoder.h by avoiding floating-point

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=57499

           Summary: Speed up alpha-blending in ImageDecoder.h by avoiding
                    floating-point
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: Enhancement
          Priority: P2
         Component: Images
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: pkasting at google.com
                CC: abarth at webkit.org


ImageFrame::setRGBA() uses floating-point ops to do alpha premultiplication.  There are two methods we can try to make this faster:

* For Skia, use SkPreMultiplyARGB()
* For others, use "x = (x * a * 32897) >> 23", which produces the same output for all x and a from 0 - 255.

Pascal Massimino did a quick test of method 2 on a Chromium build and got a noticeable speedup (about 5 ms savings out of an original 75 ms of total paint time for his testcase).

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