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Re: Filter Effects and High DPI

From: Gregg Tavares <gman@google.com>
Date: Fri, 15 Mar 2013 15:26:12 -0700
Message-ID: <CAKZ+BNpb-G2F=7eDbQ8T9BAffWmCmYLVipgG7dgcxHkuX36eqQ@mail.gmail.com>
To: Dirk Schulze <dschulze@adobe.com>
Cc: "Tab Atkins Jr." <jackalmage@gmail.com>, Stephen White <senorblanco@chromium.org>, "public-fx@w3.org" <public-fx@w3.org>
On Fri, Mar 15, 2013 at 3:09 PM, Dirk Schulze <dschulze@adobe.com> wrote:

>
> On Mar 15, 2013, at 2:40 PM, Gregg Tavares <gman@google.com> wrote:
>
> >
> >
> >
> > On Fri, Mar 15, 2013 at 2:33 PM, Dirk Schulze <dschulze@adobe.com>
> wrote:
> >
> > On Mar 15, 2013, at 1:50 PM, Gregg Tavares <gman@google.com> wrote:
> >
> > > This brings up an issue that I don't *think* is addressed by the
> current custom filters proposal?
> > >
> > > In custom filters, assuming they use GLSL, there are global values
> available. For example
> > >
> > >      gl_FragCoord
> > >
> > > that are in device coordinates. If we want CSS custom filters to be
> device independent the spec will probably need to mention that shaders
> using gl_FragCoord are disallowed or else that implementations must
> re-write the shader so that gl_FragCoord is in CSS pixels and not device
> pixels.
> > >
> >
> > gl_FragCoord must be in device coordinates to provide access to the
> texture data. For shaders it doesn't makes sense that the pixel information
> ar win CSS coordinates. Ditto for the textureSize uniform which represents
> the actual texture size. I added a description to the spec to clarify this
> fact.
> >
> > The problem you'll have is the texture size will be different on an
> HD-DPI display vs a LO-DPI display and effects like generating a
> checkerboard from gl_FragCoord will generate the wrong size checkerboard.
>
> Since the author has the texture size it should be no problem at all to
> normalize all relevant data.
>

It's not about being possible. It's about being portable.

I make a texture 256x256. I make a shader that goes

   bool check = mod(floor(gl_FragCoord.x / 128.0) + floor(gl_FragCoord.y /
128.0), 2.0) < 1.0;
   gl_FragColor = check ? vec4(1,0,0,1) : vec4(0,1,0,1);

On an HD-DPI machine that texture is magically made some other size. For
example 512x512. Because if that I get a 4x4 checkerboard instead of the
2x2 checkerboard I wanted.

If gl_FragCoord was in CSS pixels, the same units I specified the size of
the texture, then the code is magically portable.


> Greetings,
> Dirk
>
> >
> >
> > Greetings,
> > Dirk
> >
> > >
> > >
> > >
> > > On Fri, Mar 15, 2013 at 12:54 PM, Tab Atkins Jr. <jackalmage@gmail.com>
> wrote:
> > > On Fri, Mar 15, 2013 at 11:59 AM, Stephen White
> > > <senorblanco@chromium.org> wrote:
> > > > In particular, in Chrome's accelerated implementation, on a high-DPI
> > > > display, we get high-DPI input images from the compositor.  Right
> now,  we
> > > > filter the high-DPI image by the original (unscaled) parameter
> values,
> > > > which, for the filters whose pixel's result depends on more than a
> single
> > > > input pixel value (e.g., blur(), drop-shadow()), results in less
> blurring
> > > > than would be visible on a non-HighDPI display.  This seems wrong.
>  (Last
> > > > time I checked, the non-composited path was downsampling the input
> > > > primitive, giving a non-high-DPI result but correct amounts of blur,
> > > > although that may have been fixed).
> > >
> > > This is a bug in our implementation, then.  The values in the
> > > functions are CSS values, so a length of "5px" means 5 CSS pixels, not
> > > 5 hardware pixels.  The browser has to scale that to whatever internal
> > > notion of "pixel" it's using.
> > >
> > > > For blur() and drop-shadow(), It would be straightforward to scale
> the
> > > > parameter values by the devicePixelRatio automatically, and achieve
> the
> > > > correct amount of blurring without affecting the resolution of the
> result.
> > > > Of course, we could downsample the input primitive for all filters,
> but that
> > > > would lose the high DPI even for those filters which are unaffected
> by this
> > > > problem, e.g., brightness() etc.
> > > >
> > > > However, for the reference filters, in particular feConvolveMatrix,
> it's not
> > > > clear what the optimal behaviour is.  It's tempting to simply
> multiply the
> > > > kernelUnitLength by the devicePixelRatio, and apply the convolution
> as
> > > > normal.  However, that also loses high DPI, and incurs the cost of a
> > > > downsample where it otherwise wouldn't be required (also note that
> > > > kernelUnitLength seems to be unimplemented in WebKit, but that's our
> > > > problem).  Would it be a possibility to simply upsample the kernel by
> > > > devicePixelRatio instead, and apply that kernel to the original
> unscaled
> > > > image?   (Or perhaps size' = (size - 1) * devicePixelRatio + 1 for
> odd
> > > > kernel sizes?)   This would result in a similar effect range, while
> > > > preserving the resolution of the source image.
> > > >
> > > > I have no idea if the convolution math is really correct this way,
> though.
> > > > I'm guessing not, since if it was, presumably the spec would have
> allowed
> > > > its use for kernelUnitLength application in general.
> > >
> > > I'm not sufficiently familiar with feConvolveMatrix to know how to
> > > handle it well.  However, if you get a substantially different result
> > > (beyond rendering/scaling artifacts), the implementation is definitely
> > > wrong in some way.  None of SVG or CSS should require knowledge of the
> > > device's DPI.
> > >
> > > ~TJ
> > >
> > >
> >
> >
>
>
Received on Friday, 15 March 2013 22:26:40 GMT

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