koffice/krita/image/filter

[Ed .]

The problem with COMPOSITE_OVER is that it is additive. So instead of copying the filter result to the paint device, it overlays it over the original. That yields incorrect results when then the alpha channel is not totally opaque.

> From: cberger at cberger.net
> To: dimula73 at gmail.com
> Subject: Re: koffice/krita/image/filter
> Date: Thu, 6 May 2010 10:26:42 +0200
> CC: kimageshop at kde.org
> 
> On Thursday 06 May 2010, you wrote:
> > On Thu, May 6, 2010 at 1:26 AM, Cyrille Berger wrote:
> > > Hello,
> > > 
> > > I am confused by this change, why remove the bitBlt with selection ?
> > 
> > Well, yes. It's not clean form the point that we are going to remove
> > selections from filters. You are right.
> hum indeed. So at least I can backport.
> 
> > But in this very case the code does nothing, because COMPOSITE_COPY still
> > does not take selections into account. I think we need a special composite
> > for this. Do we have one?
> Remind me why COMPOSITE_OVER does not work ? The commit message that changed 
> it to COPY does not explain why...
> Because I have no clue what a COMPOSITE_COPY would do with selection. I can 
> understand what happen when SELECTION = 0 or SELECTION = 255, but the result 
> is totally undefined between those two values.
> 
> And anyway, COMPOSITE_OVER would do what I would expect for that use.
> 
> -- 
> Cyrille Berger
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