VPT Syphon to CamTwist…stream darkens with alpha

Home Forums v002 v002 QC Plugins Support VPT Syphon to CamTwist…stream darkens with alpha

Viewing 6 posts - 1 through 6 (of 6 total)
  • Author
  • #58955


    I’m sending a live video stream using VPT7 to CamTwist via Syphon. For some reason the stream looks darker in CamTwist, in particular with alpha adjustments, transparency, edges, etc. All other streams I’ve sent to CamTwist from other apps with syphon look fine, it’s just when using VPT.

    Please see the image linked here. When the head layer is lowered in opacity in VPT7, it is darker than it should be and has darker edges, shown in the left CamTwist window. The right most window, displaying the same syphon stream but in Simple Client, is as it should look (and is how it looks in VPT’s windows).

    Any bugs or work around with alpha in Syphon? Or a way to flatten the aplha with syphon? Is there premultiplied or unmatted aplha settings? It looks like that a bit too.

    Any help would be greatly appreciated!



    So – alpha can be pre-multiplied or un-premultiplied.

    Syphon makes NO adjustments to the incoming textures alpha channel, its up to applications to handle alpha. Sadly different applications can use different versions.

    It sounds like one app is sending premultiplied alpha, and the other is assuming unpremultiplied.

    Graphics is hard and annoying. I’d get in contact with the VPT people and see if they are assuming an alpha format. That looks like premultiplied to my eyes, and I’ve not seen that with any other apps, and recall I had similar issues in Max/MSP/Jitter without changing the blend mode in Jitter.


    Check this thread out :


    Should be a blend mode switch somewhere in VPT 🙂



    First of all you are a true hero for developing Syphon and keeping it open source. It’s an amazing technology and is changing how I think about video. Thank you. I plan to donate.

    So I found if I bring the stream from VPT into Jitter, I can change the alpha mode and it interprets without the transparency issue, which should translate into CamTwist. (Neither camtwist or vpt have options for changing how alphas are handled).

    Problem is, I’m inexperienced with Max, and I’m having trouble routing my syphon stream out of Max to syphon server. AND, I also haven’t been able to post to the Jitter forums for some reason, keep giving me a blank page when I submit the post.

    Below is the post I was trying to submit to those forums. Mind taking a look for me? Thanks again!

    (I couldn’t post the max path here, but it’s just the two Syphon help files together in one patch)

    n00b question here regarding syphon:

    I want to take incoming syphon client data, adjust the aplha @blend_enable argument, and then send it out to syphon server.

    I’ve looked around the forums and I know the answer is right in front of me in some combination using the node, texture, and gl objects, but my max skills are limited and I’m stuck.

    In the attached patch, If I could just connect the left help file’s output to the right help files syphon input, I would be very happy. Keeping in mind the @blend_enable 1 and/or erase_color 1. 0. 0. 1. arguments apply to the stream.



    Unfortunately, thats not quite how it works.

    Changing the blend mode really only affects the ‘drawing’ – it doesnt change the alpha values or anything in the texture.

    You might need to run a jit.gl.slab to change the texture from premultiplied to unpremultiplied (or, perhaps, vice versa).

    You’d do something like.

    jit.gl.slab (some slab shader here that does alpha changing)

    (in short hand)

    That could be helpful perhaps!

    • This reply was modified 5 years, 11 months ago by vade.
    • This reply was modified 1 year, 10 months ago by vade.

    Thank you! Decided to drop VPT from my setup but good to know none the less.

Viewing 6 posts - 1 through 6 (of 6 total)
  • You must be logged in to reply to this topic.