"Alpha matte" desync on Video Device Out TOP


I’m working on a show that needs SDI (aka BNC) outputs with transparency, to overlay on top of camera feeds.

So I’m generating a fill + matte in TouchDesigner, and feeding them each to a Video Device Out TOP. However, there is a sporadic “desync” between the two sources; I’ve seen it in prior builds, but it wasn’t a problem in those cases for other reasons that don’t apply here. This results in an edge of either black or white as the key and fill fall out of sync. I have searched the forums and other resources, but I haven’t found anything that solves my problem yet, which is why I’m reaching out to you now. :slight_smile:

I have tried:

  1. Adding a Timecode CHOP and using it as the reference for both of the Video Device Out TOPs;
  2. Using different video output devices (AJA 4K IO Plus over Thunderbolt, Blackmagic 8K Pro over PCIE, and the Blackmagic Ultrastudio 4K Mini over Thunderbolt in combination with the AJA 4K IO Plus);
  3. Two different computers (Windows 10 and Windows 11);
  4. Resetting the Video Device Out TOPs (toggling Active and bypass);
  5. Performing the matte/composite in different switchers (BMD ATEM 2 ME Production Studio 4K and vMix on a different computer).

If I perform a matte inside of TouchDesigner, I don’t see the “matte tear”, which is why I’m looking at the Video Device Out as the cause.

Please tell me there’s some super obvious way of adding a buffer frame and forcing sync that I’m unaware of!

Additional information:

  • Build: 2023.11510
  • I generate the matte via a Base that takes in the alpha video signal (In TOP), feeds it into a mono TOP, composites that white/alpha image over a black constant TOP, and outputs both the “key” and the “fill” via two out TOPs respectively.

Thanks for the help :slight_smile:

There is no way to sync different SDI outputs together, except for the Deltacast FLEX devices. Each will have it’s own frame queue will can result in a few frames difference.
For this usage case, can you not use the Fill+Key mode for the Output Pixel Format. This should work on Blackmagic devices at the very least.

Hi @malcolm! Thanks for the reply.

I’m set up on the Blackmagic device now, and the Video Device Out TOP is throwing an error when I set the Output Pixel Format to “8-bit + 8-bit Key (Alpha)”.
The error is “Key output not supported on selected device”. The device I’m using is the Blackmagic Decklink 8K Pro.

(edit edit: I have never been able to send Alpha or otherwise over SDI, so didn’t give it much consideration in the past when I was unable to get it to work out of TD.)

Ah, maybe that device doesn’t support it, or needs to be configured differently. In general the only way to get a synced pair of outputs from an SDI device for a case like this in by using a Fill+Key mode which uses two SDI wires, one for color and one for alpha. It’s not used very often though so it’s one of those features that tend to go stale. AJA devices should support them if you have one to try there.

Yeah, it’s remarkably difficult (for me, at least) to find somewhere where Blackmagic will state which of their cards support the Key + Fill output modes.

When I’m in that mode, and have my card (theoretically, from what I can gather) configured to send a key/fill pair from both “pairs” of connectors, the Video Device Out TOP only shows the first connector in the Device dropdown.

(It also makes TouchDesigner pretty mad, and has crashed on me twice while changing settings in the last ~20 minutes… :smile: )

I think I’ve got it going. OBS exposes the settings for the keyed output as “internal”, “external”, or “disabled”, which I’m assuming TouchDesigner is hiding “disabled” and “internal” behind the different options in the dropdown (8-bit vs. 8-bit with key), with no option for external (?).

I get the same result (with the 8K Pro in paired output, which can only be set on the first device in Desktop Video Setup, which then controls all of the other ports) using that setting in OBS and the 8-bit key+fill in TouchDesigner. Not to say that it’s a perfect solution, because I think the Decklink’s internal key generator is struggling a bit to handle some elements, but… it’s a step closer.

There’s no way to sync SDI outputs (Video Device Out TOPs), except for perhaps Deltacast FLEX devices (I cannot confirm this).

Blackmagic’s support for internal key generation is spotty and flawed, and does not give the correct outputs in my case.