[Webkit-unassigned] [Bug 255869] New: [WebCodecs] Poor video frame rate exporting from Construct Animate

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=255869

            Bug ID: 255869
           Summary: [WebCodecs] Poor video frame rate exporting from
                    Construct Animate
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Media
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ashley at scirra.com

Our animation software Construct Animate (https://animate.construct.net) uses WebCodecs to export animations to videos. It does this faster than realtime by rendering each frame to a canvas as fast as possible, and encoding each frame on the canvas with WebCodecs. Then it uses a library like mp4box.js to mux the resulting video to a playable media file which the user can preview or download.

This works fine in Chrome but in Safari 16.4 the resulting video has a poor framerate. It's inconsistent but seems to be around 1 frame per second, which is not enough to be useful. I also observed a tab crash in Safari following these steps once or twice but was not able to reliably reproduce that.

Steps to reproduce:
1. Open https://animate.construct.net/#open=make-animations-with-construct
2. Choose Menu > Project > Export...
3. Double-click "Video"
4. Click OK with default settings (should specify MP4 format with H.264 codec)
5. Approve any popup prompt (it is easier to allow popups for the site in advance)
6. The video export process should complete after a moment and show the result in a dialog. There the video can be previewed or downloaded.

Observed result:

In Safari the resulting video has a poor frame rate.

Expected result:

A smooth 30 FPS frame rate, as Chrome is able to produce.

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