RE: [css-compositing] some proposals

Hi Rik, Alex,

Just to make sure I understand the proposal. What would happen if you apply the 'dst-atop' compositing operator with partially transparent objects together with say a 'plus' blending operator. I understand that the 'plus' would only apply on the intersection but I'm not sure about the maths. Would you compose the equations, in which order? It doesn't look like you can consider that the result of the (alpha-)compositing becomes the source (or destination) of the (color-compositing) blending ,does it?


From: Rik Cabanier []
Sent: Friday, 18 November 2011 4:11 PM
To: www-svg;
Subject: [css-compositing] some proposals

Last week I sent out some ideas to update the SVG composting spec:
Some people suggested some improvements which I integrated.

Here is the updated list of proposals:
a. split up the spec in 2 sections: Porter-Duff & Blending.
Porter-Duff are compositing primitives that describes how 2 images (src+dst) can be merged.
Porter-Duff will continue to use the existing comp-op property.

Blending describes how the top image will 'blend' with its underlying colors. The result of this is a new blended image. In the Adobe imaging model and the existing SVG compositing spec it is assumed that this blended image is composited with src-over.
However, there are use cases where you might not want 'src-over'. To facilitate other composting operators, we will introduce a new keyword 'blend-mode'.

b. simplify Porter-Duff
-remove the clip-to-self property. The default behavior becomes 'clip-to-self: object'.

c. complete blend modes
- add the following missing blend modes: hue, saturation, color & luminosity
- define 'normal' as the default blend mode

d. update knockout
- change the value to be more descriptive. The new value ('replace'/'preserve') is confusing.
- provide a better description + correct the compositing formula
- knockout will only affect groups that use the new 'blend-mode' operator

e. update enable-background
- remove the ability to set up x, y, width & height.
- clarify the description and refer to the filters spec which already defines this parameter
- replace the keyword with 'isolate' which is Adobe's equivalent. 'isolate: isolated' = 'enable-background: new'. Keep the existing parameter for backward compatibility.
- Any comp-op apart from 'src-over' will establish a new stacking context.

f. update all the formulas on the page
- fix incorrect math
- always do the math with pre-multiplied alpha.

g. replace 'src'/'dst' in the spec with 'source'/'dest'

Let me know if I missed something.


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Received on Monday, 21 November 2011 00:29:51 UTC