Testcard, another smaller rotated testcard on top. The top layer is composite mode "sub".
In M8 output preview, and direct to projector, the output is as expected: full opacity, with the RGB values of the top layer subtracted from the bottom.
Send the output to syphon, and it gets frames with an alpha hole where the top layer should be, as if it is subtracting rgba rather than just rgb.
1) the output shouldn't change due to routing through syphon
2) pretty sure the expected behaviour of composite mode "sub" is to modify rgb but not a
As a workaround, I create another layer on top, testcard, with the subtractive color all the way down (black image) in composite mode "add". This seems to get the a values back to 1.0 and the hole in the syphon output disappears.
Note: this bug exists also in 2.9