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Re: Filter Effects and High DPI

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Fri, 15 Mar 2013 12:54:00 -0700
Message-ID: <CAAWBYDA9rsTZGrouK8eEt+-sUCHiq+=DnuskJE7XE8QyiJ6o_Q@mail.gmail.com>
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 GMT

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