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Re: [filter-effects] resolution dependent filter primitives

From: Michael Mullany <michael@sencha.com>
Date: Mon, 4 Nov 2013 16:01:32 -0800
Message-ID: <CABTYPJm_10bYfPig+KRjb6R6wcH7-BxsVVvbK4ZMEnb47XarwQ@mail.gmail.com>
To: Dirk Schulze <dschulze@adobe.com>
Cc: "public-fx@w3.org" <public-fx@w3.org>, www-svg list <www-svg@w3.org>
On Mon, Aug 26, 2013 at 9:28 PM, Dirk Schulze <dschulze@adobe.com> wrote:

> Hi,
> This is a follow up on Stephen White's mail "Filter Effects and High
> DPI"[2]
> The Filter Effects spec[1] has a couple of filter primitives that are
> resolution dependent. Namely: feConvolutionMatrix, feDiffuseLighting,
> feSpecularLighting and feCustom.
> With removing 'filterRes', there is just 'kernelUnitLength' that can be
> used to "hard code" a resolution for a primitive. 'filterRes' defined the
> resolution for the whole filter chain which reduced or increased pixel
> density for each filter primitive instead of one. This either caused
> unnecessary oversampling of filter operations but more often: unnecessary
> reduction of quality. With 'kernelUnitLength' the affect of over- or
> undersampling can be localized to exactly one primitive where the primitive
> values can not be scaled for resolution independent results.
> If 'kernelUnitLength' is not set, all the primitives in the introduction
> may depend on the device pixel ratio and are therefore platform dependent.
> This is actually hardly a "retina" vs "normal" screen problem. SVG is
> scalable by nature and therefore you can see the problem on any screen
> today. Open the following example in a browsers and resize the window:
> http://www.w3.org/Graphics/SVG/Test/20110816/svg/filters-conv-01-f.svg
> You'll see that you don't have consistent results. If you make the window
> smaller, the top left image gets more blurry and the top right image more
> sharpen. Same happens if you zoom into a page where one of the above
> primitives was applied.
> We could create stable results by default. UAs could set
> 'kernelUnitLength' such that the kernel size is always relative to a CSS
> pixel in the current local coordinate system of the filtered object. This
> is something that is nearly impossible to determine for the author if he
> wants to use the same filter on different objects. With the auto calculated
> 'kernelUnitLength' the proportion of the filter chain result looks always
> the same across platforms but might be more pixelated on one platform and
> less pixelated on another one.
> It is unclear what is better for the users:
> * best results that are highly platform dependent and indeed differ
> significantly, or
> * platform independent results that might be pixelated.
> During my research I couldn't find one SVG example where the above
> primitives are used in combination of either 'filterRes' or
> 'kernelUnitLength'. Not even in the SVG test suite. That means these
> examples in the web out there are all suffering from device and zoom
> dependent filter results.
> I would even suggest removing 'kernelUnitLength' as well and choose one of
> the above two ways (high DPI but resolution dependent results, or pixelated
> proportion stable results). Or let the author choose between one of the two
> options and have one as fallback.
> Does anyone have a comment?

I got around to doing a thorough testing of feDiffuseLighting +
fePointLight cross browser and generated a test page for people to play

The page is here: http://codepen.io/mullany/pen/wKJeu (note this takes a
while to render in IE10 due to lack of support for objectBoundingBox units
for kernelUnitLength)

It's not very surprising that there is not much content using these
settings because they don't work consistently cross browser. Here's what I

   - kernelUnitLength: supported by IE10, Firefox (no support in Safari or
   - filterRes: supported by Chrome, Safari, Firefox (no support in IE10)
   - primitiveUnits = userSpaceOnUse -> supported everywhere
   - primitiveUnits = objectBoundingBox -> supported by Firefox,  (no
   support in Safari, Chrome, IE10: units are interpreted as userSpace units)
      - This makes kernelUnitLengths specified in OBB units cause very very
      long render times in browsers (aka IE10) that support
kernelUnitLength, but
      don't support objectBoundingBox units - because it thinks I'm
asking for a
      very small kernelUnitLength

This test page raised one or two notes for the current spec:

   - The spec is not clear what Z position units are calculated relative
   to, when primitiveUnits = objectBoundingBox -> X? Y? Some average thereof?
   - The spec is not clear that surfaceScale units are in pixels, not in
   primitiveUnits (or if they're supposed to be primitiveUnits, it should say
   that - no browser currently supports this)

In any case, the use case I'm running into is not a high dpi vs. low dpi
problem (because OBB units should be able to solve this, I think?) Instead,
it's the fact that Firefox (specifically) chooses a fast, low quality
default for lighting effects which are too coarse to use in image filters.
The mozilla implementor has told me to use a higher filterRes, but since
all positions are scaled by the ratio of filterRes to default filterRes
(aka when you double the filterRes, the X, Y and Z positions of your image
source are also scaled up by 2x), you end up with what is effectively a new
unit system just for lightsource positions that has nothing to do with any
other unit system (unless you also reduce the kernelUnitLength to
compensate).  [As an aside, the practical effect of this in Firefox was no
increase in quality (and I've filed a bug against that)].

It would all be a lot simpler if there was an attribute similar to
shape-rendering (filter-rendering?) that would allow optimizeSpeed vs.
optimizeQuality for these primitives.

> Greetings,
> Dirk
> [1] http://dev.w3.org/fxtf/filters/
> [2] http://lists.w3.org/Archives/Public/public-fx/2013JanMar/0149.html

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Received on Tuesday, 5 November 2013 00:02:34 UTC

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