[whatwg] Offscreen canvas (or canvas for web workers).

David Levin levin at google.com
Mon Feb 22 15:36:54 PST 2010

On Mon, Feb 22, 2010 at 3:10 PM, Jonas Sicking <jonas at sicking.cc> wrote:

> On Mon, Feb 22, 2010 at 11:13 AM, David Levin <levin at google.com> wrote:
> > I've talked with some other folks on WebKit (Maciej and Oliver) about
> having
> > a canvas that is available to workers. They suggested some nice
> > modifications to make it an offscreen canvas, which may be used in the
> > Document or in a Worker.
> What is the use case for this? It seems like in most cases you'll want
> to display something on screen to the user, and so the difference
> comes down to shipping drawing commands across the pipe, vs. shipping
> the pixel data.

The other use case I can think of is doing image manipulation and then
> sending the result directly to the server, without ever displaying it
> to the user. However this is first of all not supported by the
> suggested API, and second I can't think of any image manipulation that
> you wouldn't want to display to the user except for scaling down a
> high resolution image. But that seems like a much simpler API than all
> of canvas. And again, not even this simple use case is supported by
> the current API.
A simple use case is image resizing which was what started the last thread
and that is a similar use case to what I heard internally.

I don't understand what you mean about things not being supported.

1. Given the current structure clone support, it is certainly possible to
transfer image data to and from a worker, so it seems possible to display
the result to the user. It is orthogonal to this feature but adding
something like toFile (your proposal) and a corresponding fromFile/load
would also aid in this (as well as aid in sending things to the server).

2. Resize may be done using the scale(x, y) method.

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