Re: Filter Effects and High DPI

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.


>
> 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
> >
> >
>
>