[whatwg] several messages

Ian Hickson ian at hixie.ch
Wed May 16 16:25:31 PDT 2007

On Fri, 21 Jul 2006, Henri Sivonen wrote:
> I gather that a normative reference to the Porter–Duff paper is needed: 
> http://keithp.com/~keithp/porterduff/p253-porter.pdf

On Mon, 24 Jul 2006, L. David Baron wrote:
> I've written tests for the 11 operators defined in the paper, plus a 
> test for 'darker' that assumes Quartz's PlusDarker operator [1]:
>   http://dbaron.org/tests/canvas/composite-operations4
> It seems like 'darker' is currently not interoperable, and might be 
> unlikely to become interoperable, due to different availability of 
> compositing operators across platforms.  (Quartz has PlusDarker. Mozilla 
> uses the saturate operator that is provided by Cairo/libpixman (and also 
> by XRender), which is described in [2] and [3], but seems to me to be 
> quite different.)
> One solution to that problem would be to remove 'darker' from the spec 
> rather than defining it one way or the other.  Thoughts?

I've referenced the paper and dropped 'darker'.

On Wed, 28 Mar 2007, Philip Taylor wrote:
> [snip a very long and detailed discussion of the operators, with much 
> research]

Wow. Thanks! Based on David's input above and on your own advice, and on 
Vlad's later comments, I've simply removed 'darker' and referenced 
Porter-Duff for the others.

On Wed, 4 Apr 2007, Vladimir Vukicevic wrote:
> Well, if we have lighter, we should keep darker; I think that for 
> mozilla at least, we can implement this using some slow-boat fallback 
> mechanism -- basically, render the path/image to a separate surface, 
> then manually do the requested math if things don't map directly to one 
> of our operators; this is what our SVG impl does now for many of the SVG 
> filters.

According to David, 'lighter' is just Porter-Duff's 'plus'. Is that not 
right? For now I've left it, with that definition.

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'

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