Proposal: CanvasRenderingContext2D#getImageData should accept optional target ImageData parameter

[oguzeroglu]

At this point CanvasRenderingContext2D#getImageData returns a copy of the existing buffer. This may be a performance problem for applications where one needs to process the ImageData ideally 60 times per second.

One nice feature to have for getImageData would be an additional targetImageData parameter, in that case getImageData would write the bytes over the target buffer rather than creating a new ImageData.

Example usage:

var targetImageData = ctx.createImageData(width, height);

ctx.getImageData(x, y, width, height, targetImageData);

// The buffer of targetImageData may now be transferred to workers via transferables for processing.

Any feedback on that?

[Kaiido]

Was already proposed in #4748
But it's been closed in favor of #4785, which I think is more aimed at short-circuiting video -> ImageData.
I personally agree that having this option on the canvas's getImageData would still be useful too, so I'm not sure it should be closed because of #4785, but maybe the discussion should be done there anyway.

[annevk]

cc @whatwg/canvas

[kdashg]

Yes, this is low-hanging fruit and we should do it.

[annevk]

Small quibble on the signature, presumably you don't need width and height parameters as the target already has those. And since you don't need a return value perhaps we should have a new method, e.g.,

getImageDataInto(x, y, destination)

It seems that should fail though if destination points to a detached buffer so you cannot transfer to workers, but rather have to copy, right?

[Kaiido]

It seems that should fail though if destination points to a detached buffer so you cannot transfer to workers, but rather have to copy, right?

The ImageData can be constructed from a TypedArray, so you can still transfer the buffer back and forth and only create a new ImageData wrapper.

[trusktr]

But it's been closed in favor of #4785,

Wouldn't that mean an unnecessary extra allocation of memory to hold the pixels, thus mem and CPU waste?

I personally agree that having this option on the canvas's getImageData would still be useful too

Not just useful, but necessary for avoiding memory allocation.

[annevk]

Let's forward duplicate this into #10855 as that has some momentum.