[Webkit-unassigned] [Bug 41682] New: Certain page elements cause XGetImage and software fallback rendering

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=41682

           Summary: Certain page elements cause XGetImage and software
                    fallback rendering
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: PC
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: Normal
          Priority: P2
         Component: WebKit Qt
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: tsaviran at gmail.com


Created an attachment (id=60610)
 --> (https://bugs.webkit.org/attachment.cgi?id=60610)
Simplified version of aol.co.uk landing page

When going to certain heavyweight websites such as aol.co.uk, we noticed extremely poor performance on particular hardware which has a very high penalty for software fallbacks in the graphics pipeline.

While investigating this, we locally added several features to Qt's X11 backend, allowing us to run transformed pixmaps, patterns, and linear gradients on the GPU.  However, we discovered that Webkit does not take advantage of several of these features (including specifically linear gradients), and persists in using client-side software rendering for some page elements.

The specific page element causing the most trouble on aol.co.uk is an "atlas map" implemented using CSS.  The source images for this are extremely tall, 10k or 20k pixels in height, with large amounts of blank space between icons.  Every icon drawn using this atlas map is apparently rendered in software within WebKit, despite no special features technically being required to accelerate them.

We also noticed many instances when XPutImage is called to populate a pixmap, and (sometimes immediately) XGetImage is used to retrieve the exact same image data, which has not been changed in the interim.  This data shuffling across the X11 socket is inherently inefficient.

We have prepared a number of simplified test cases which expose the problem even on desktop hardware, through examination of X11 protocol traces.  The main symptom is the excessive use of XGetImage.


WebKit was built using sources from Subversion trunk on 5th of July 2010 and tested on desktop using QtTestBrowser with tiled backing store enabled.

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