[webkit-dev] Feature announcement: canvas pixel access API additions for high-resolution backing stores

[Maciej Stachowiak]

On Apr 16, 2012, at 10:52 AM, Darin Fisher wrote:

> Could this be an opportunity to design an asynchronous API for fetching the pixel buffer?  It seems like many implementations are GPU backed now, and fetching the pixel buffer is an expensive (blocking) operation.  Had you considered making such a change?

Adding async support was suggested on the whatwg thread about this. I think async support is useful, but should not be tied to high DPI support. In particular, we shouldn't, in my opinion, require authors to rewrite their existing sync code to an async model just to properly support higher resolutions.

In addition, the whatwg thread revealed that there are many hidden complexities in the design of get/putImageData, in particular designing how they work in the face of sychronous drawing operations to the same canvas. The HiDPI problem is much more straightforward and does not need to be gated on resolving the async design issues.

 - Maciej

> 
> Regards,
> -Darin
> 
> 
> 
> On Thu, Apr 12, 2012 at 6:17 PM, Dan Bernstein <mitz at apple.com> wrote:
> [This is actually an enhancement announcement to an existing feature]
> 
> Over at <http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2012-March/035112.html>, Edward O’Connor has proposed enhancements to CanvasRenderingContext2D to allow authors to take full advantage of high-resolution backing stores, when available (whereas the existing API hides the fact that the backing store resolution is not CSS pixel resolution, for compatibility with existing content). The enhancements include a backingStorePixelRatio attribute and {get,put}ImageDataHD functions on CanvasRenderingContext2D.
> 
> Through <https://bugs.webkit.org/show_bug.cgi?id=83619> and <https://bugs.webkit.org/show_bug.cgi?id=83836>, I am making these enhancements, with the names prefixed with “webkit”.
> 
> There is no build-time option to disable these enhancements. Ports that don’t opt into ENABLE_HIGH_DPI_CANVAS get a working, albeit boring, version of the additional API. Ports that opt into high-DPI canvas need to enhance their ImageBuffer implementation to fully support the resolutionScale and CoordinateSystem parameters.
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