Bug 241402 – Decoded image data is not reused by canvas causing long task blocking on the main thread

• Component: Canvas
• Status: NEW

[Xidorn Quan]

Created attachment 460082 [details]
testcase

STR:
1. download the attachment and serve it and open it
2. click the button in the page

Expected result:
The decoding takes sometime, but drawing is instant, and there is no long task detected on the main thread.

Actual result:
Both the decoding and the drawing takes roughly the same amount of time, and there is a long task indicates the main thread is blocked.


Seemingly that the drawing the image on canvas then `getImageData` forces a decoding of the image even if the image is already decoded through `img.decode()`. It makes it hard to avoid a long task on main thread when dealing with large images.

Blink has similar issue: https://bugs.chromium.org/p/chromium/issues/detail?id=1334448

On Gecko, I get the expected result.

[Radar WebKit Bug Importer]

<rdar://problem/95171712>

[Kimmo Kinnunen]

Thanks for the test-case!
For WebKit internals:
The long duration after initial decode() is due to the way WP transfers the data to GPUP the first time. The new NativeImage gets sent over to GPUP during this duration.

There are at least 3 different undesirable behaviors in WebKit:
1. initial draw after decode() is slow
2. Multiple .src assignments and decode()s are slow for same url. 
3. Big getImageData() is slow (10ms on WebKit, ~0ms on Firefox)

[Thomas Steiner]

See https://bugs.chromium.org/p/chromium/issues/detail?id=1334448#c9 for a hosted demo and https://bugs.chromium.org/p/chromium/issues/detail?id=1334448#c10 for a theory of why the seemingly double decoding might happen.

[Xidorn Quan]

Thomas suggested to us that for this testcase, we can set `image.decoding` to `async`, and that seems to help on Chrome (where `draw` call is now immediate most of time), but not on Safari.

[Xidorn Quan]

No, I was wrong. It didn't help on Chrome either.

Chrome was to some extent cheating by somehow reusing a decoded image from a previous image load. If I randomize the URL by adding `?` with `Math.random()` to the end of the URL, double decoding happens every time again.