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Re: feConvolve matrix bias property premultiplied or not?

From: Jasper van de Gronde <th.v.d.gronde@hccnet.nl>
Date: Mon, 15 Feb 2010 12:41:32 +0100
Message-ID: <4B7932EC.6020606@hccnet.nl>
To: Robert Longson <longsonr@gmail.com>
CC: www-svg@w3.org
Robert Longson wrote:
> Jasper,
> What version of Firefox are you testing with. Firefox prior to version 
> 3.6 does not process bias values correctly. Firefox 3.6 displays your 
> testcase the same as Opera.

I was still using 3.5, I've updated and now it does indeed look better 
(at least it gets rendered). But it does surprise me. There are roughly 
two kinds of output I would expect:

1. If you add the bias unmodified to all channels (except the alpha 
channel in this case, as preserveAlpha=true) then you should get 
(0.5,0.5,0.5,0.5), which is transparent white.

2. If you premultiply the bias by the resulting alpha channel you should 
get (0.25,0.25,0.25,0.5) (which would result in the same transparent 
gray as the other square, on a white background).

There is one other option I've heard mentioned (premultiplying by 
ResultAlpha-bias, but this makes little sense when preserveAlpha=true).

So I guess I know now have even more questions :)
  - What on earth does Firefox/Opera do with bias.
  - Is there any reason NOT to use premultiplied bias?
(And if so, what is the reason? If not, can we clarify the specification?)
Received on Monday, 15 February 2010 11:42:14 UTC

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