[whatwg] Adding features needed for WebGL to ImageBitmap

Justin Novosad junov at google.com
Thu Jul 11 08:29:51 PDT 2013

On Wed, Jul 10, 2013 at 9:37 PM, Rik Cabanier <cabanier at gmail.com> wrote:

> On Wed, Jul 10, 2013 at 5:07 PM, Ian Hickson <ian at hixie.ch> wrote:
> > On Wed, 10 Jul 2013, Kenneth Russell wrote:
> > >
> > > ImageBitmap can cleanly address all of the desired use cases simply by
> > > adding an optional dictionary of options.
> >
> > I don't think that's true. The options only make sense for WebGL --
> > flipping which pixel is the first pixel, for example, doesn't do anything
> > to 2D canvas, which works at a higher level.
> >
> > (The other two options don't make much sense to me even for GL. If you
> > don't want a color space, don't set one. If you don't want an alpha
> > channel, don't set one. You control the image, after all.)
> >
> >
> > > I suspect that in the future some options will be desired even for the
> > > 2D canvas use case, and having the dictionary already specified will
> > > make that easier. There is no need to invent a new primitive and means
> > > of loading it.
> >
> > If options make sense for 2D canvas, then having ImageBitmap options
> would
> > make sense, sure.
> >
> >
> yeah, these options seem a bit puzzling.
> From the spec:
> An ImageBitmap object represents a bitmap image that can be painted to a
> canvas without undue latency.
> note: The exact judgement of what is undue latency of this is left up to
> the implementer, but in general if making use of the bitmap requires
> network I/O, or even local disk I/O, then the latency is probably undue;
> whereas if it only requires a blocking read from a GPU or system RAM, the
> latency is probably acceptable.
> It seems that people see the imageBitmap as something that doesn't just
> represent in-memory pixels but that those pixels are also preprocessed so
> they can be drawn quickly. The latter is not in the spec.
> I think authors will be very confused by these options. What would it mean
> to pass a non-premultiplied ImageBitmap to a canvas object? Would the
> browser have to add code to support it or is it illegal?
> Maybe it's easier to add an optional parameter to createImageBitmap to
> signal if the ImageBitmap is for WebGL or for Canvas and disallow a Canvas
> ImageBitmap in WebGL and vice versa.

You are implying a pretty heavy imposition as to what constitutes undue
I think the spec should stay away from forcing implementations to pin
decoded image buffers in RAM (or on the GPU), so that the browser may have
some latitude in preventing out of memory exceptions. In its current form,
the spec implies that it would be acceptable for an implementation to
discard the decoded buffer and only retain the resource in encoded form in
RAM.  Do we really need to make further optimizations explicit? For
example, an implementation could prepare the image data for use with WebGL
the first time it is drawn to WebGL, and keep it cached in that state. If
the same ImageBitmap is subsequently drawn to a 2D canvas, then it would
use the non-WebGLified copy, which may be cached, or may require
re-decoding the image. No big deal.

Fundamental question: Do we really need the caller to be able to specify
what treatments need to be applied to prepare an image for WebGL, or is it
always possible to figure that out automatically? As long as there is a way
to do it automatically, shouldn't we avoid adding complexity to the API?

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