From: Rik Cabanier <cabanier@gmail.com>

Date: Sat, 21 Jul 2012 15:02:58 -0700

Message-ID: <CAGN7qDB4b2Ub8RFuH+U8QW6sTfgtmh6cEOtSqBVRfp4q7usqug@mail.gmail.com>

To: Calculemus <calculemus1988@gmail.com>

Cc: steve@fenestra.com, www-svg@w3.org

Date: Sat, 21 Jul 2012 15:02:58 -0700

Message-ID: <CAGN7qDB4b2Ub8RFuH+U8QW6sTfgtmh6cEOtSqBVRfp4q7usqug@mail.gmail.com>

To: Calculemus <calculemus1988@gmail.com>

Cc: steve@fenestra.com, www-svg@w3.org

On Sat, Jul 21, 2012 at 1:26 PM, Calculemus <calculemus1988@gmail.com>wrote: > Having problems with the formulas from SVG spec... > > I tested the blend modes and my results match > perfectly Photoshop and GIMP results, but only > for regions in the image where alpha values > are either 0 or 1 for the input images. > > If the top image for example has Opacity, or > alpha 0.5 then my results are darker for some reason > I can't figure out whole day. > > I am using the formulas for blend modes and Over compositing > from SVG spec: > > http://www.w3.org/TR/SVGCompositing/ > > What could possibly be the reason for this behaviour? > > The general formula is: > > A*aa*(1-ba) + B*ba*(1-aa) + aa*ba*Blend(A, B) > This formula will return you the color in premultiplied alpha. I think you still need to calculate alpha of the end result and matte with white to get the final color. See example 4 in https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#backdropexamples > > where A and B are non-premultiplied color values and aa and ba > are alpha values. > > When aa and ba are both 1 my results are fine, the formula reduces > to Blend(A,B). This confirms that I have all formulas > for blend modes correct. It is the other part before > Blend(A,B) that must somehow causing this problem. > > So, this is the case for all blend modes, not just for one or two. > I wonder if the formula for compositing is correct? > The blend formulas are correct though. >Received on Saturday, 21 July 2012 22:03:27 UTC

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