- From: Tab Atkins Jr. <jackalmage@gmail.com>
- Date: Fri, 15 Mar 2013 12:54:00 -0700
- To: Stephen White <senorblanco@chromium.org>
- Cc: "public-fx@w3.org" <public-fx@w3.org>
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 19:54:48 UTC