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Playing out alpha channels



  • Hi guys, I was wondering if there is a way to play out key and fill separately of any file containing alpha channels? Since alpha keying is supported in Isadora as an internal key, I would like to know if there is also a way to do so externally. Any feedback on that is highly appreciated. Thank you for your help.



  • Hi Stoni
    I'm a little confused by the question can you maybe put it in context?
    thanks
    fubbi


  • Hi Fubbi,

    thank you for your answer. Here is everything a bit more detailed:

    If you use a supported image or video sequence that has got an alpha channel, e.g. a lower third and you would like to bring those graphics upon your video with proper transparencies , you would do like this: Play your video to a projector. Play your lower third to a projector, set the blend mode on transparent and set the layer at least one more layer upon your video footage. The result is proper alpha keying. So far so good. I would like to do that keying on an external device. So I have to play out my graphics (containing alpha channel) to two stages, one containing the graphics and the other one containing the alpha channel. The task is to separate the ARGB into an Alpha and an RGB and playing those two channels to different stages. I would like to know how that can be done, since all the required information is present in Isadora.

    Why would you want that and not doing the keying in Isadora? Well, delay is always a matter and like that there is no additional delay. If I did the keying in Isadora I would have to play all the video signal through Isadora even if I don't need any graphics. So to me that doesn't make sense.

    I would be very glad to get a feedback on that.

    Thank you very much, Fubbi!!!



  • In case there is anybody out there in this forum who is interested in how it works, here is the solution on how to extract the alpha channel out of an image or sequence.

    1. Send your alpha channel containing source to the Colorizer Actor and set all the RGB colors on -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 your gray-scale alpha-like image.

    Still, there is some mystery about the result. While the output of the Colorizer was still ARGB and showing a proper transparent image when holding the mouse on the connection, the output of the Threshold was RGB only and holding the mouse on the connection you see a plain white image. Furthermore, the output of the Threshold can not be used for further processing, apart from projectors. (Even though I was not trying all - I guess it is true for all Actors which are not rendering.)  Maybe Mark or one of the gurus here have got more information on that.



  • I have come back to this tread few times but honestly I do not understand what is your goal/problem still. Specially this: ”I would like to do that keying on an external device”. Like where and how and what for do you want to send this separated alpha channel?



  • The external device is a live switcher and if you would like to use a DSK you need key and fill, that's why you need your alpha channel separately.



  • Interesting! Does it work with the mixer as it is now(gray)? How do you send it to mixer(two outputs)?

    I do not have any suitable mixers at hand but I think these work in the same principle as you do it in Isadora. Like you take two video(or video+shapes, or video+picture) files and use one for alpha. Maybe you need to send two versions of your graphics - one for key and one for fill - that is pre-produced as such. Not as you trying to make alpha with manipulating on video in Isadora.


  • Basically, you have this gray-scale image which is your alpha channel (black stands for transparent - white for opaque - gray-levels for everything which is in-between) and your fill. You need both of the signals for alpha keying with a mixer. All of the recent Panasonic mixers and switchers can do. With other brands I am not really familiar, but any broadcast mixer should be able to do so.


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