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Re: Filter Effects and High DPI

From: Stephen White <senorblanco@chromium.org>
Date: Mon, 18 Mar 2013 14:31:30 -0400
Message-ID: <CAPeKFThSkZa2deFH3CMj4u2_YjstpkWW6po9mQ_De8WhNTdcNQ@mail.gmail.com>
To: Dirk Schulze <dschulze@adobe.com>
Cc: Rik Cabanier <cabanier@gmail.com>, "Tab Atkins Jr." <jackalmage@gmail.com>, "public-fx@w3.org" <public-fx@w3.org>
On Mon, Mar 18, 2013 at 11:11 AM, Dirk Schulze <dschulze@adobe.com> wrote:

>
> On Mar 18, 2013, at 7:02 AM, Stephen White <senorblanco@chromium.org>
> wrote:
>
> > On Sun, Mar 17, 2013 at 10:01 PM, Rik Cabanier <cabanier@gmail.com>
> wrote:
> >
> >
> > 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.
> >
> > From the Filter Effects spec [1]:
> > Because they operate on pixels, matrix convolutions are inherently
> resolution-dependent. To make ‘feConvolveMatrix’ produce
> resolution-independent results, an explicit value should be provided for
> either the ‘filterRes’ attribute on the ‘filter’element and/or attribute
> ‘kernelUnitLength’.
> >
> > So, this is a case where the device's DPI is allowed to make a
> difference. 'FilterRes' [2] should be used if you want device independent
> output.
> > It seems that for feConvolveMatrix, 'FilterRes' should have been
> required. (It's weird that 'FilterRes' is not a resolution but a number of
> device pixels)
> >
> > Thanks!  I didn't catch that.
> >
> > So it seems the author's options are:
> >
> > 1)  Set filterRes, and have the image downsampled before all filtering
> (lo-DPI results)
> > 2)  Set kernelUnitLength, and have the image downsampled before
> convolution only (lo-DPI results).
> > 3)  Check window.devicePixelRatio, and provide a kernel sized
> appropriately to the DPI (hi-DPI results).
> >
> > Does that sound right?
>
> You forgot transformations on the element or it's parent elements that
> influence the element size. Rotations make it even harder :)
>

I'm not sure what you mean by this.  Do you mean the spec doesn't take
these into account, or that the current implementations don't?  For
example, I do know that (last time I checked) kernelUnitLength is
completely unimplemented in WebKit, so #2 won't work, but I was more
wondering from the spec side what are the options for the user, once the
implementations are spec-compliant.  Do your concerns affect all three
options?  It seems like providing a DPI-friendly kernel size (#3) should
work regardless of rotations, scaling, etc, applied to the primitive, but
maybe I'm not understanding the problem.

I need to look at this. This is not the way it is implemented in WebKit or
> Firefox IIRC.
>

Thanks for your feedback,

Stephen


>
> Greetings,
> Dirk
>
> >
> > Stephen
> >
> > 1:
> https://dvcs.w3.org/hg/FXTF/raw-file/tip/filters/index.html#feConvolveMatrixElement
> > 2:
> https://dvcs.w3.org/hg/FXTF/raw-file/tip/filters/index.html#FilterElementFilterResAttribute
> >
>
>
Received on Monday, 18 March 2013 18:31:58 GMT

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