[whatwg] Proposal: ImageData constructor or factory method with preexisting data

[Rik Cabanier]

On Tue, Mar 12, 2013 at 4:16 PM, Glenn Maynard <glenn at zewt.org> wrote:

> On Tue, Mar 12, 2013 at 12:14 AM, Boris Zbarsky <bzbarsky at mit.edu> wrote:
>
> > CSE can get rid of the redundant .data gets.  Similarly, .data gets can
> be
> > loop-hoisted in many cases.
> >
>
> Doing COW based on page-faults is nicer anyway, but I don't know about the
> data structures of JS engines to know whether this is feasible.  (For
> example, if an object in JS is preceded by a header that gets written by
> the engine now and then, it'll probably lie on the same page as the data,
> which would trigger an expensive fault each time.)
>
> I suppose padding the backing store so it doesn't share pages with anything
> else might be reasonable here: up to about 8k of waste on a system with 4kb
> pages.  The cost of marking the pages read-only would only have to be paid
> when the copy-on-write action (eg. a call to putImageData) is actually
> made.  Very small buffers could simply disable copy-on-write and always
> perform a copy, where the waste for padding is more significant and the
> benefits of avoiding a copy are smaller.
>
> (For what it's worth, marking a 128 MB buffer read-only in Linux with
> mprotect takes on the order of 3 microseconds on my typical desktop-class
> system.  I don't know if Windows's VirtualProtect is slower.)
>
> On Tue, Mar 12, 2013 at 4:04 PM, Rik Cabanier <cabanier at gmail.com> wrote:
>
> > > I don't think it is necessary to provide a createImageDataHD in this
> >
>  > interface. The caller will know the devicePixelRatio and determine
> > > whether to generate high-DPI data.
> >
> > That probably won't work since it results in code that executes
> differently
> > on devices that are HD.
> >
>
> The difference between getImageData(HD) and putImageData(HD) is in the
> canvas operation, not the ImageData: it determines how pixels are scaled
> when being read out of and written into the canvas backing store.  It
> doesn't apply to this API; ImageData objects don't know anything beyond
> their pixel data and dimensions.
>
> (Code executing differently on high-DPI devices is a bridge we've already
> crossed.  getImageData scales pixels down from the device's pixel ratio;
> getImageDataHD doesn't, copying backing store pixels one to one.)
>
> There is no real reason why you could use it in both HD and non-HD APIs.
> >
>
> Rather, there's no reason you couldn't.  You can definitely create an
> ImageData with getImageData and then pass it to putImageDataHD (which would
> cause the image to be scaled on devices with a pixel ratio other than 1, of
> course).


It feels like something is missing. How does putImageDataHD know that the
bitmap should be scaled? Width and height refer to the pixel dimensions and
not the 'px' unit