- From: L. David Baron <dbaron@dbaron.org>
- Date: Sun, 20 May 2007 15:30:15 -0700
On Sunday 2007-05-20 17:53 +0100, Philip Taylor wrote:
> If you care about alpha, and take cX as premultiplied colour
> components (and CX as non-premul, and aX as alpha), then you get:
> Mozilla:
> cO = cA * min(1, (1-aB)/aA) + cB
> aO = aA * min(1, (1-aB)/aA) + aB
Correct. This is the cairo "saturate" operator, which I think is
the same as Xrender's saturate operator.
> Safari:
> CO = 1 - (1-CA)*aA - (1-CB)*aB except not quite
> aO = aA + aB
I thought what Safari does is basically "lighter", but with the
color components inverted (where inverting is CX = 1-CX, or cX = aX
- cX), so that it comes out to:
aO = min(1, aA + aB)
cO = aO - min(1, aA - cA + aB - cB)
> you change alpha from 0.0 to 0.01. In all the implementations,
> 'lighter' is confusingly unlike 'darker' - names like 'plus' and
> 'multiply' make much more sense.)
Hrm. I thought lighter was confusingly *like* darker, in that they
act exactly the same when the alphas add to less than or equal to 1.
(I think that's true for plus (a.k.a. lighter), Cocoa's darker, and
for saturate.)
-David
--
L. David Baron <URL: http://dbaron.org/ >
Technical Lead, Layout & CSS, Mozilla Corporation
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Received on Sunday, 20 May 2007 15:30:15 UTC