Alpha channel on sdi tv broadcast output

  • Hallo

    I'm trying to use Isadora as a tv broadcast playout. I'm testing it using 2 sdi cables (one for video\fill, one for alpha\key) with embedded audio\video signals and a Black Magic broadcast video card. 
    I verified the lack of a 32 bit (rgba) output resolution: Isadora seems not to manage the alpha channel output on sdi (alpha channel is necessary on tv broadcast). 
    I just can "play" fill and audio, but not the alpha (i'm not talking about masks) rendered inside my movies\images. Is there any way to solve it?
    Thank you for your attention. 

  • There is no alpha in SDI in any standard or frequency and never has been. Alpha is always achieved by multiple streams. Maybe you are mistaking 444 colour space that is sent over dual SDI signals- this still does not contain alpha.

    10bit SDI can be achieved when your display device is capable (ie I think only one kind of kepler video card or blackmagic decklink extreme devices).
    To achieve this you need to render 2 SDI streams and then use some kind of hardware compositor to join them. Take a look at caspar CG, this is very high quality playout graphics software made by the swedish national broadcaster to solve this solution in live playout setups- again it needs a hardware compositing device to do anything with the alpha key. ProPresenter will also do this but with a €1000 extra module.
    I dont think Isadora can support 2 separate streams going to blackmagic hardware, but you may be able to use black syphon to do this. Remember for this to work the software you use needs to be able to sync the blackmagic outputs especially in the case of a moving image and alpha as if the alpha channel is not sync with the fill it will kind of break the effect.
    I have achieved this before using 2 HDMI outputs from a retina macbook pro (and software I wrote myself with openframeworks- but you can probably do some tricks with izzy to get ok sync depending on the content - especially if it is not live) and a blackmagic ATEM switcher to do the live compositing- it worked perfectly.

  • Thanks for your answer.

    Maybe i didn't explain myself very well. I'm using an Imac connected to an external Blackmagic UltraStudio4k by a thunderbolt cable, then i've two sdi (bnc) cables from UltraStudio to a common TvLogic provided with two sdi input. Using a common  Tv Playout software i can play a .mov rendered with alpha channel (for ex., an animation, apple ProRes 444 etc.) from my Imac, and on my tv logic i can switch video "fill" and "key", the former passing inside the first sdi and the latter inside the second one. Using Isadora as playout (using external module) that's not seems to be possible.
    Thank you

  • Ok, I get it- the software you are using is extracting the alpha channel and rendering it as a black and white movie over the second output. I am not sure if Isadora can extract the alpha channel from a movie with alpha and I don't have one on hand.

    I think you need to ask 2 questions the first is can Isadora extract the alpha channel from a movie with embedded alpha?
    Can Isadora access 2 blackmagic hardware outputs at the same time?
    I am pretty sure the answer to both of these questions is no, but maybe someone knows better?

  • There is a way to play out both channels.

    1. Send your alpha channel containing source to the Colorizer Actor and set all the RGB colors to -100. What is left is the alpha channel.

    2. Make your alpha channel visible by sending it to the Threshold Actor and set dark color to white. All the other parameters do not have any influence on a proper result. If you link that output to a projector you have got your gray-scale alpha channel.

    If you can use your UltraStudio's outputs as (seperate) stages I don't know, but still you have got the option to use a TH2Go and convert the HDMIs to SDI.

    I hope that helps.

  • This will work but it sucks that it is all on the CPU- especially since both HAP and prores4444 that support alpha are optimised for GPU decoding. How long until all those old CPU actors are GPU?

  • Since all the information is present it might be an option to have additional outputs on picture player and movie player that output the alpha channel directly. Like this it could be gpu-based without the use of any other actor.

  • Dear @Fred + @stoni,
    To answer one question above, you should be able to capture from more than one Blackmagic Device at a time using the new Blackmagic direct capture option available in Isadora 2.0.

    Do the new FFLG Add Alpha Channel plugins help you guys with this? See my post here:

    Best Wishes,

  • @mark, the issue is outputting to 2 devices at once not capturing. The device in question has 2 hardware outputs that are seen by the system as separate devices (I think- sometimes blackmagic need you to instantiate a stereo output device to get access to these and some broadcast playout software does this with the API and then gives the user access to each separately).@lucadiroma2004 wants to send the alpha out one output and the fill out the other.

    The plugin you linked to is great but also not what the OP is looking for, he has a movie with alpha and wants to separate it into its alpha and fill layers. Obviously the info is in the movie and making 2 separate movies creates a lot more hassle and resource use and may not stay jam sync (which is vital for keying moving images). Maybe I missed something or am not looking at an obvious solution- at any rate it would be great to get the option to play the alpha from a movie that has it (a mutable output for the movie player if it detects alpha?).

  • Dear @Fred,

    Well, what about a FFGL plugin that uses a shader to give you only the alpha? I assume it would be as simple as assigning the alpha value to r, g, and b, then setting alpha to full. (In other words, the alpha becomes a B+W mask.)
    If that's what you mean, attached you will find a shiny new FFGL plugin that does just that. (Mac OS only for now... it proves useful I'll compile it for Windows too.)
    Should be pretty efficient.
    Let me know if that helps.
    Best Wishes,

  • Tech Staff

    I could use this for sure.

  • Thank you for all your answers.

    My problem is with the External Video Output. This module doesen't output  the alpha channel on my second sdi, separated from fill layer. I always have two "fill" from the external video output. I could try to use the "add alpha channel", but that's the same...external output module doesen't separate fill and key. 

  • @lucadiroma2004  This is more complicated, as i mentioned above getting access to these outputs is tricky. I am pretty sure Isadora uses an OS level, not api level access to these outputs. In many cases these 2 outputs in the ultrastudio 4K are for full frame 3D and can usually only be accessed by asking the system to initiate a full frame 3D output (which is probably what your broadcast play out software is doing- and it probably uses the SDK from blackmagic to do that (without calling the full frame 3D settings it will duplicate the image on both outputs as you are seeing). You will have a bit of research to do to find this out but as a temporary solution you can use 2 HDMI outputs to get the same effect and run them through the ultrastudio to resync them and covert them to SDI.