not sure this can really be described as a bug as it works ok but just
differently from before.

basically, the PImage mask function in 0091 is producing quite different
results from those produced by the 0069 BImage alpha function. The 0091
mask function produces a gradient with a flat slope with all rgb values
between 0 and 255 evenly spread whereas the old alpha function under 0069
produced a more curved gradient with much less of a grey area in the
middle. I personally prefer the output from the alpha function but then I
can see that the results from the new mask function could be percieved to
be more correct.

the url included below is a page with links to download the source code for
two tests - one in 0069 using BImage.alpha() and the other to 0091 source
code using PImage.mask().

also included on the page is a gif of screen grabs from each test program
composited together to clearly show how results from the two functions
differ under 0069 and 0091.

can you use the "create a new attachment" function to add the sketch zip files and images
to this bug? we can't rely on that link still working in the future by the time we get around
to fixing the bug.

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example of BImage.alpha() output in processing 0069

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example of PImage.mask() output program for processing 0091

edit]
gif composite of screen grabs from the two attached programs