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

Compositing Blend Modes

From: Bob Holmes <rangsynth@gmail.com>
Date: Sun, 5 Aug 2012 11:36:09 +0200
Message-ID: <CAMvo67bmi3b0UNgx3F7bbidBn8S62XaPcoEtF2AqWCNh3ighAg@mail.gmail.com>
To: www-svg@w3.org
Description of pre-multiplication is not adequate and is confusing...
Both for input source colors and for the destination result color and
for the destination color.

SOURCE OVER SPEC STATES the FORMULA AS

Sca + Dca  (1 - Sa)

So obviously the source color has been premultiplied. Fine... BUT IS
THE RESULT PREMULTIPLED? DOES IT HAVE TO BE? WHY?

======================
MULTIPLY SPEC STATES THE FORMULA AS

Sca  Dca + Sca  (1 - Da) + Dca  (1 - Sa)

Now it seems like what it is doing, is multiplying the "premultiplied
source color" with the inverse dest alpha and then multiplying the
"unpremultiplied dest color" with the inverse source alpha. So far no
problem but...

What is the result? Premul? Not premul?

For Multiply does it also then need an alpha blend or does it get stored as is?

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The main confusions are is the result f things premultiplied and also
do ALL the formulas require premultiplied source values or does it
apply to some of them.
Received on Sunday, 5 August 2012 21:22:55 GMT

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