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Re: [whatwg] WebGL and ImageBitmaps

From: Rik Cabanier <cabanier@gmail.com>
Date: Wed, 14 May 2014 21:27:59 -0700
Message-ID: <CAGN7qDBCxqXHvSWr22aHdggVmBsV9P5zo=wKt79oE78whsWVDw@mail.gmail.com>
To: Glenn Maynard <glenn@zewt.org>
Cc: "whatwg@whatwg.org" <whatwg@whatwg.org>, Ian Hickson <ian@hixie.ch>, Katelyn Gadd <kg@luminance.org>
On Wed, May 14, 2014 at 7:45 PM, Glenn Maynard <glenn@zewt.org> wrote:

> On Wed, May 14, 2014 at 6:27 PM, Glenn Maynard <glenn@zewt.org> wrote:
> > That's only an issue when sampling without premultiplication, right?
> >
> > I had to refresh my memory on this:
> >
> > https://zewt.org/~glenn/test-premultiplied-scaling/
> >
> > The first image is using WebGL to blit unpremultiplied.  The second is
> > WebGL blitting premultiplied.  The last is 2d canvas.  (We're talking
> about
> > canvas here, of course, but WebGL makes it easier to test the different
> > behavior.)  This blits a red rectangle surrounded by transparent space on
> > top of a red canvas.  The black square is there so I can tell that it's
> > actually drawing something.
> >
> > The first one gives a seam around the transparent area, as the white
> > pixels (which are completely transparent in the image) are sampled into
> the
> > visible part.  I think this is the problem we're talking about.  The
> second
> > gives no seam, and the Canvas one gives no seam, indicating that it's a
> > premultiplied blit.  I don't know if that's specified, but the behavior
> is
> > the same in Chrome and FF.
> >
> It looks right on red, but if the background is green you can still see the
> post-premultiplied black being pulled in.  It's really just GL_REPEAT that
> you want, repeating the outer edge.
> On Wed, May 14, 2014 at 9:21 PM, K. Gadd <kg@luminance.org> wrote:
> > The reason one pixel isn't sufficient is that if the minification
> > ratio is below 50% (say, 33%), sampling algorithms other than
> > non-mipmapped-bilinear will begin sampling more than 4 pixels (or one
> > quad, in gpu shading terminology), so you now need enough transparent
> > pixels around all your textures to ensure that sampling never crosses
> > the boundaries into another image.
> >
> I'm well aware of the issues of sampling sprite sheets; I've dealt with the
> issue at length in the past.  That's unrelated to my last mail, however,
> which was about premultiplication (which is something I've not used as
> much).
> > I agree with this, but I'm not going to assume it's actually possible
> > for a canvas implementation to work this way. I assume that color
> > profile conversions are non-trivial (in fact, I'm nearly certain they
> > are non-trivial), so doing the conversion every time you render a
> > canvas to the compositor is probably expensive, especially if your GPU
> > isn't powerful enough to do it in a shader (mobile devices, perhaps) -
> > so I expect that most implementations do the conversion once at load
> > time, to prepare an image for rendering. Until it became possible to
> > retrieve image pixels with getImageData, this was a good, safe
> > optimization.
> >
> What I meant is that I think color correction simply shouldn't apply to
> canvas at all.  That may not be ideal, but I'm not sure of anything else
> that won't cause severe interop issues.

Maybe the color correction described here is happening:

If so, the image that's drawn on the canvas should match what the browser
is showing on screen.
Without an example, it's just speculation of course.

> To be clear, colorspace conversion--converting from sRGB to RGB--isn't a
> problem, other than probably needing to be specified more clearly and being
> put behind an option somewhere, so you can avoid a lossy colorspace
> conversion.  The problem is color correction that takes the user's monitor
> configuration into account, since the user's monitor settings shouldn't be
> visible to script.  I don't know enough about color correction to know if
> this can be done efficiently in an interoperable way, so the data scripts
> see isn't affected by the user's configuration.

Yes, color correction from sRGB to your monitor should not affect drawing
on canvas. (What if you had multiple monitors :-))
Received on Thursday, 15 May 2014 04:28:27 UTC

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