MultiplyCompositeOp behaviour

[anthony]

Delete the alpha from both images, before multiplying them.
You can just turn it off with the 'matte' controls.

That hopefully will solve the problem.

[anthony]

While reviewing...

As a first draft.

test image...

Code: Select all

   convert -size  100x100 gradient: \( +clone -rotate 90% \) \
                  -compose CopyOpacity -composite   test.png

this reads the image, 'saves' a backup for later reuse.
then separates the alpha channel, and multiplys ALL the channels.
The save image is then used to restore the original alpha.

Code: Select all

   convert test.png -write MPR:save \
          \( +clone -alpha extract \) -compose multiply -composite \
          MPR:save -compose DstIn -composite \
          pre-multiplied.png

NOTE: most of the Duff-Porter Compose Methods are defined in terms of pre-multiplied pixels, and Im does this internally. You would only need to do pre-multiplication if you were creating a new composition or something like it.

What Are you planning to use it for? It may already be in IM, or can be added to IM relativally easilly.