Hi everyone! A few weeks ago some folks from company100.net have started a thread of multithreaded (parallel) image decoding in WebKit. We have worked together since then to get a better idea how to complete this feature. I would like to report on the progress and our plan.
(The goal for Chromium is slightly different but it will reuse most of the architecture discussed here.) Master bug: https://bugs.webkit.org/show_bug.cgi?id=90375 In WebKit today image rendering is progressive, meaning that image is rendered as bytes are received from the network. This is done through ImageObserver<http://trac.webkit.org/browser/trunk/Source/WebCore/platform/graphics/ImageObserver.h> and CachedImageClient<http://trac.webkit.org/browser/trunk/Source/WebCore/loader/cache/CachedImageClient.h> interfaces. Multithreaded image decoding uses the same notification architecture, clients of BitmapImage are notified when a new region is decoded and available for painting. Today image decoding happens synchronously when a BitmapImage is drawn into a GraphicsContext. We plan to use the draw operation as a trigger to start image decoding asynchronously. Which means the first draw of BitmapImage will get a transparent image, subsequent draws will have the most recently decoded bitmap. This architecture will take several steps: https://bugs.webkit.org/show_bug.cgi?id=93467 This modifies ImageSource to be asynchronous. ImageSource is used as the public interface for decoding images. By making this interface asynchronous individual port can implement parallel image decoding or a similar architecture. This change is ready for review. I would like to get more feedback on the interface since this touches all ports. https://bugs.webkit.org/show_bug.cgi?id=93590 Progressive painting of an image may not be possible everywhere. For example <canvas> and accelerated-composited <img> requires synchronous decoding. We plan to keep synchronous decoding the default case. This change identifies code location where asynchronous decoding is possible and tell BitmapImage asynchronous image decoding is requested. After these two changes we will be able to implement multithreaded image decoder inside BitmapImage and ImageSource. I will report on the progress once we get to this point. Alpha
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