Filter Effects and High DPI

Hi folks,

Sorry if this has been addressed before, but I'm wondering whether High DPI
support has been discussed for W3C Filter Effects.

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).

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.

Stephen