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Re: Algorithm for soft-light blend mode (ISSUE-2095)

From: Erik Dahlström <ed@opera.com>
Date: Thu, 09 Oct 2008 09:21:24 +0200
To: "Doug Schepers" <schepers@w3.org>, "Benjamin Otte" <otte@gnome.org>
Cc: www-svg@w3.org
Message-ID: <op.uiqxdyhdgqiacl@gnorps.linkoping.osa>

On Wed, 08 Oct 2008 23:59:06 +0200, Doug Schepers <schepers@w3.org> wrote:

>
> Hi, Benjamin-
>
> Thanks for bringing this to our attention.
>
> Benjamin Otte wrote (on 10/8/08 2:02 PM):
>>
>> I'm in the process of implementing bend modes in the Cairo imaging
>> library[1]. As Cairo is used as the renderer for a wide variety of
>> formats, I wanted to have an as comprehensive list of blend modes as
>> possible.
>
> Without looking too deeply into the issue, that seems like a reasonable
> goal.

I agree.

>> And it turns out that the blend mode algorithms used are
>> almost always equal. However, there is one exception: soft-light. The
>> latest SVG draft specifies a different algorithm than PDF.
>>
>> So I was wondering if it was possible to change the algorithm in SVG
>> to match the one of PDF, as that would solve a lot of problems.
>
> Which SVG spec are you referencing (presumably the Filters module)?  Is
> there a PDF spec you can point to, and how does it differ from SVG?

I believe this would be in the compositing module[1], since SVG 1.1 (and the filters module[2]) don't have a 'soft-light' blendmode. In any case, I'd like the same blendmodes to be available both in the compositing module and in the filters module going forward.

> It's possible we could change it, though we'd have to look into the
> reasons it's specified the way it is, and what the costs of changing it
> would be.  Since SVG is a royalty-free language, that would also need to
> be a consideration.

I would think the cost of changing it is low, since the compositing module hasn't yet been published as a working draft (and 1.2 full which included this was also in working draft status).

Cheers
/Erik

[1] http://dev.w3.org/SVG/modules/compositing/master/SVGCompositing.html#comp-op-property
[2] http://www.w3.org/TR/SVGFilter12/#feBlendElement

-- 
Erik Dahlstrom, Core Technology Developer, Opera Software
Co-Chair, W3C SVG Working Group
Personal blog: http://my.opera.com/macdev_ed
Received on Thursday, 9 October 2008 07:24:02 GMT

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