Caveat: I am not at all familiar with the tools you are using.

However, from your brief description, it sounds like you are trying to produce a vignetting effect around a non-rectangular region,using Alpha compositing.

A long time ago in a different life I had a similar problem to solve. I achieved an amazingly convincing result by the following technique. (This is from vague memories going back almost 20 years, so If I miss a step; or re-order them; or use non-current terminology; sorry!):

  1. Draw a black background.
  2. Open a path.
  3. Draw your shape(s) with an alpha of 0%.
  4. Scale your shape(s) reducing the size by -1%.
  5. Redraw the shapes with alpha +10%.
  6. Goto step 3; repeat 10 times.
  7. Close your path, and stroke it.
  8. Use this path as your destination in a Porter-Duff alpha blend XOR operation.
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
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In reply to Re: goo canvas image mask by BrowserUk
in thread goo canvas image mask by Anonymous Monk

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