On Wed, Apr 15, 2015 at 1:15 PM, Kenneth Russell <k...@google.com> wrote:

> If repeatedly transferring ImageBitmaps of the same size into the same
> <img> won't cause repeated re-layouts, that alleviates my concern
> about efficiency of using images as the output path. I expect that a
> single OffscreenCanvas will be used to produce output to multiple
> areas on the page (whether into images or canvases) but that it will
> be sized large enough so that all of the ImageBitmaps it produces will
> cover the largest of those areas, avoiding repeatedly resizing the
> OffscreenCanvas.

So you want to produce multiple ImageBitmaps of the same (largest) size and
then render them to multiple elements in the page, each element cropping
its ImageBitmap to a (potentially) different size? Isn't that going to be
wasteful of memory?

There is one other problem which has come up repeatedly in canvas
> applications: needing to be able to accurately measure the number of
> device pixels covered by a canvas, so pixel-accurate rendering can be
> done. https://wiki.whatwg.org/wiki/CanvasRenderedPixelSize addresses
> this and it's currently being implemented in Chromium. There is no
> such mechanism for images. It would be necessary to understand exactly
> how many device pixels the output image is covering in the document so
> that the OffscreenCanvas can be sized appropriately. Using a canvas
> element for the display of these ImageBitmaps avoids this problem. It
> also makes the API more symmetric; a canvas-like object produces the
> ImageBitmap, and a canvas element is used to view it. Do you have any
> suggestion for how this issue would be solved with <img>?

We could support renderedPixelWidth/Height on <img> as well as <canvas>.
But this is a valid argument in favor of rendering those ImageBitmaps
through <canvas>.

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