[Beta 14] Syphon: composite mode treats alpha differently
  • geoffmatters
    junior Member
    Posts: 6
    Joined: Fri Sep 29, 2017 3:29 am

    [Beta 14] Syphon: composite mode treats alpha differently

    by geoffmatters » Sat Nov 18, 2017 1:50 am

    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
  • geoffmatters
    junior Member
    Posts: 6
    Joined: Fri Sep 29, 2017 3:29 am

    Re: [Beta 14] Syphon: composite mode treats alpha differently

    by geoffmatters » Sat Nov 18, 2017 1:54 am

    Attaching screenshot showing the problem
    Attachments
    Screen Shot 2017-11-17 at 6.53.13 PM.jpg
    Screen Shot 2017-11-17 at 6.53.13 PM.jpg (238.45 KiB) Viewed 537 times
  • Pierre Guilluy
    garageCube team
    Posts: 166
    Joined: Mon Oct 12, 2015 1:50 pm

    Re: [Beta 14] Syphon: composite mode treats alpha differently

    by Pierre Guilluy » Wed Nov 22, 2017 11:04 am

    This problem is due to the subtract blending operation happening to the alpha channel as well as the color ones.

    This is due to be fixed in an update.

    For now and as a workaround, in MadMapper you can use the "ignore alpha" blending mode on the surface that displays Modul8's Syphon output.

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