[Webkit-unassigned] [Bug 93430] New: incorrect transparent WebP image rendering

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=93430

           Summary: incorrect transparent WebP image rendering
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
               URL: https://developers.google.com/speed/webp/
        OS/Version: Unspecified
            Status: UNCONFIRMED
          Severity: Normal
          Priority: P2
         Component: New Bugs
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: pascal.massimino at gmail.com
                CC: noel.gordon at gmail.com, jzern at google.com


Hi,

this is mostly a heads up for now. The best way to fix is still open and debated.

Summary:

* libwebp tree was updated in chromium to 0.2.0 with alpha support:
     http://codereview.chromium.org/10832153/

* WEBPImageDecoder.cpp now needs an update to correct handle compressed image with alpha
  we're setting "buffer.setHasAlpha(false);" always, at line 122
  Rendering of webp-with-alpha is 'weird', as if values rgb values were not pre-multiplied.

* But even when setting "setHasAlpha(true)" (when there's alpha channel only), we need to
address the pre-multiplication of rgb values (cf m_premultiplyAlpha and line 79).

* libwebp can now emit premultiplied values, using MODE_bgrA instead of MODE_BGRA as outputMode()

i'll poke around the code further and try to get a patch, but any insight from Noel or others is welcome!

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