Re: Filter Effects and High DPI

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.




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
>
>