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[ISSUE-2335] feConvolveMatrix bias property in SVG 1.1 SE

From: Anthony Grasso <Anthony.Grasso@cisra.canon.com.au>
Date: Wed, 10 Nov 2010 01:06:27 +0000
To: Jasper van de Gronde <th.v.d.gronde@hccnet.nl>, "www-svg@w3.org" <www-svg@w3.org>
CC: W3C SVG Public List <public-svg-wg@w3.org>
Message-ID: <923A4276C1903646BE5299CA8462B0A013CBAF69@EXM01-WVP.cisra.canon.com.au>
Hi Jasper,

I was given ACTION-2900 [1] to investigate the bias attribute.

As you're aware the specification currently says when preserveAlpha="false" to apply the convolution to all channels. Currently in the specification, the formula to do the convolution for that case is:

cR = (cF * kernel)/divisor + bias
aR = (aF * kernal)/divisor + bias

I agree that the definition of 'bias' is not clear at all and should be changed.

Regarding how it is used in a filter mathematically, your options listed below do not take into account the filter 'divisor' that the current formula does. I would like to know where you think the 'divisor' value fits in with your options that you have specified.


[1] http://www.w3.org/Graphics/SVG/WG/track/actions/2900

> -----Original Message-----
> From: www-svg-request@w3.org [mailto:www-svg-request@w3.org] On Behalf
> Of Jasper van de Gronde
> Sent: Wednesday, 23 June 2010 5:55 PM
> To: www-svg@w3.org
> Subject: feConvolveMatrix bias property in SVG 1.1 SE
> 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>

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Received on Wednesday, 10 November 2010 01:07:06 UTC

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