[PD] GEM problem with [alpha] on MacOS

[Matteo Sisti Sette]

Hi,

The attached patch works as expected in Linux, but in Mac OS the 
rectangle with the texture generated by pix_movement is not transparent. 
It is a black-and-white opaque image.

[pix_movement] SEEMS to be working as expected and generating the 
correct alpha channel (judging from the values I got with a [pix_blob] 
in mode "4"), so I think it is [alpha] that is not actually enabling alpha.

I have Linux and don't have a Mac, and when I observed the issue I 
couldn't take the time to do the obvious tests (such as trying with a 
solid color with alpha and no texture at all, and trying pix_image with 
a png image with alpha channel).

Can anybody confirm this?

I also tried sending [alpha] a "1" on the left inlet (not necessary in 
linux), just in case alpha was disabled by default in MacOS but that 
didn't help.

Is there some extra thing I have to do in order to have alpha work in 
Mac OS?

I am using GEM 0.92.3 in Linux and I think the same in Mac (the one 
bundled in the latest Pd Extended).

Thanks in advance
m.
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