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RE: ISSUE-2335: Clarify feConvolveMatrix bias property [SVG 1.1 F2 Last Call]

From: Anthony Grasso <Anthony.Grasso@cisra.canon.com.au>
Date: Fri, 10 Dec 2010 04:57:06 +0000
To: Jasper van de Gronde <th.v.d.gronde@hccnet.nl>
CC: SVG Working Group WG <public-svg-wg@w3.org>
Message-ID: <923A4276C1903646BE5299CA8462B0A0173E5353@EXM01-WVP.cisra.canon.com.au>
Hi Jasper,

Thank you for help regarding the feConvolveMatrix 'bias' property.

I was given the task of resolving this issue. The SVG Working Group decided that the bias should be defined as you suggested Option 2. To address this problem, the following changes have been made to the feConvolveMatrix section [1] of the specification:
 - Made a correction to the convolution matrix formula [2]
 - Added wording to the 'bias' attribute [3]
 - Clarified how the 'preserveAlpha' attribute effects the convolution matrix formula [4]

Please advise the SVG Working Group if these changes have satisfactorily address the issue you have raised.

Kind Regards,

Anthony Grasso


[1] http://dev.w3.org/SVG/profiles/1.1F2/master/filters.html#feConvolveMatrixElement

[2] http://dev.w3.org/SVG/profiles/1.1F2/master/filters.html#feConvolveMatrixElementFormula

[3] http://dev.w3.org/SVG/profiles/1.1F2/master/filters.html#feConvolveMatrixElementBiasAttribute

[4] http://dev.w3.org/SVG/profiles/1.1F2/master/filters.html#feConvolveMatrixElementPreserveAlphaAttribute



> -----Original Message-----
> From: public-svg-wg-request@w3.org [mailto:public-svg-wg-
> request@w3.org] On Behalf Of SVG Working Group Issue Tracker
> Sent: Wednesday, 23 June 2010 6:14 PM
> To: public-svg-wg@w3.org
> Subject: ISSUE-2335: Clarify feConvolveMatrix bias property [SVG 1.1 F2
> Last Call]
>
>
> ISSUE-2335: Clarify feConvolveMatrix bias property [SVG 1.1 F2  Last
> Call]
>
> http://www.w3.org/Graphics/SVG/WG/track/issues/2335

>
> Raised by: Erik Dahlström
> On product: SVG 1.1 F2  Last Call
>
> I would really like the bias property clarified in SVG 1.1 SE. The
> problem with the current explanation is that it seems to be
> contradictory (and if you take the most explicit explanation, frankly
> not that useful).
>
> In the explanation of the property it is said to be intended to have
> (as one possibility) a gray zero response. However, in the basic
> convolution formula the bias is added directly to the premultiplied
> color values. If you follow this definition the bias property does not
> give you any effective control over the zero response! Depending on the
> alpha component a bias of 0.5 could be anything from out-of-range white
> to 0.5 gray.
>
> We've discussed this problem before and the last post in that thread
> that I could find was by me (on Nabble, as my laptop, with a lot of my
> e-mails is being serviced) and summarized the progress as follows:
>
> <quote>
> For reference, the pseudo-code in the SVG  specification suggests the
> following (preserveAlpha=false):
> -------------------------------------
> Option 1:
>    aF = filtered alpha without bias
>    cF = filtered (premultiplied) color without bias
>    aR = aF + bias
>    cR = cF + bias
> -------------------------------------
>
> I previously suggested:
> -------------------------------------
> Option 2:
>    cR = cF + bias*aR
> -------------------------------------
>
> You seem to suggest:
> -------------------------------------
> Option 3:
>    cR = cF + bias*(aR-bias)
> -------------------------------------
> I'm not sure what the advantage of this would be though. For example,
> if you have a uniformly colored shape (opacity also uniform) and apply
> the filter -1 1 (with bias=0.5) I would expect a response of
> (0.25,0.25,0.25,0.5), and not (0,0,0,0.5).
>
> Another option would be to do an explicit conversion from and to
> premultiplied colors:
> -------------------------------------
> Option 4:
>    cR = (cF/aF + bias)*aR
> -------------------------------------
> Conceptually this might be cleaner, but it does have the disadvantage
> of needing a division. Specifically, what color should result if aF=0?
> </quote>
>
> Original mail:
> http://www.w3.org/mid/4C21BDBC.3090800@hccnet.nl

>
>

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Received on Friday, 10 December 2010 04:57:16 GMT

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