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Re: Filter Effects and High DPI

From: Dirk Schulze <dschulze@adobe.com>
Date: Fri, 15 Mar 2013 13:36:53 -0700
To: "Tab Atkins Jr." <jackalmage@gmail.com>
CC: Stephen White <senorblanco@chromium.org>, "public-fx@w3.org" <public-fx@w3.org>
Message-ID: <044752E5-D546-42C9-A6CF-933613A71A8A@adobe.com>

On 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.

That is correct. WebKit's SVG implementation should do the right thing, but I guess we do not apply the devicePixelRatio yet.

> 
>> 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.
> 

I agree here as well. This is an implementation issue and doesn't need to be specified separately.

Greetings,
Dirk

> ~TJ
> 
Received on Friday, 15 March 2013 20:37:22 GMT

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