W3C home > Mailing lists > Public > www-svg@w3.org > July 2012

Re: Compositing math in SVG

From: Rik Cabanier <cabanier@gmail.com>
Date: Sat, 28 Jul 2012 16:06:36 -0700
Message-ID: <CAGN7qDALwm8Pew3B9n558VjxmcghKzR9BK2TjPKaCHiMx4=PZA@mail.gmail.com>
To: Calculemus <calculemus1988@gmail.com>
Cc: Dirk Schulze <dschulze@adobe.com>, "steve@fenestra.com" <steve@fenestra.com>, "www-svg@w3.org" <www-svg@w3.org>
On Sat, Jul 28, 2012 at 2:44 PM, Calculemus <calculemus1988@gmail.com>wrote:

> Wooow, thanks Rik! :)
> I have some questions:
> 1. Why don't people mention this when they write about blend
> modes? :) I read even the Adobe PDF reference and it was also
> talking about the compositing formula the way I did it.
> Where can I read more about this?

PDF explicitly doesn't have color spaces with alpha so it didn't have to
cover this case. I think it was excluded because it causes exactly the
confusion you are facing.

In the case of a soft image (which what you have) you can find the
description in the PDF reference manual (
in chapter

> 2. I am confused about the second case. Does not alpha
> always come from the images?

No, for most cases alpha will come from the 'color' or 'opacity' value.

> It is kinda strange to think
> of a blend mode with some alpha, while both images have
> alpha 1.
> It seems what I want always is the first case, since alpha
> will always come from the images. Not sure when would
> I need the second case though?

If you look at the layer palette in Photoshop, you will see an entry for
If you want to implement that or you want to apply alpha to a group of
images, you need the second case.

> I was so excited when I saw this, I wrote this quickly, I am
> going to the shop to get some coffee and chocolate and
> I am getting back to try if this change will fix my problems :D

I am getting myself a beer :-)
Received on Saturday, 28 July 2012 23:07:05 UTC

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