Home Forums Open Forums Multi alpha layers and RGB matte

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  • #18429


    Hello everybody,

    here my issue :

    I used to work with basic one alpha layer for the color grade, and it works always great.

    I received, 2 alpha layers for 1 VFX, and I can’t find a way to use both separately in the color grade. Is it possible to do that, or RGB matte is the only possibility?

    I also tried to transform those alphas in one RGB matte, but it doesn’t work as it should be. I can grade both as if it was one selection, but not separately.

    I suppose I have to configure the external key node differently from a classic alpha channel, but I’m a little bit lost….

    Thanks in advance and all the best


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  • #18481


    This reply has been reported for inappropriate content.

    Yes, Mistika definitely does this. There are several ways to make this work. You obviously understand how to use RGBA, using EXTERNAL KEY set to SOURCE & ALPHA. Here’s how to do the others.

    1.) If the Alphas are saved as Channels within an .exr then add COLOUR GRADE in the controls you’ll see that the default is the GRADE tab. Switch it to the multi-layer tab and you’ll be able to select every channel within the .exr, including using & combining multiple within a single Vector.

    2.) If it’s not RGBA or multi-channel .exr then I’m assuming you have External Alphas delivered as separate files – up to 4 being possible in any RGBA filetype like .dpx .tiff, prores .mov, .js, etc. Some operators call them ‘Disco’ mattes’ due to the way the alphas like when they’re storage in the RGB channels as well as the alpha channel.
    In that case the best/most-efficient way to work that I know of is to place them below the image clip that work with on the timeline and extend the ‘Scope’ of the COLOUR GRADE effect down so that it covers both the clip you’re grading 1st, and then additional inputs below that. You can tell if you’ve done it correctly by opening up the NODE GRAPH and seeing how many different inputs are going into the COLOUR GRADE effect that you’re currently using.
    On the Vector that you want to use the External Alphas on activate EXTERNAL KEY (usually setting to ADD) and set the INPUT to ‘2nd’. The last control in EXTERNAL KEY is ‘Channel’ and the default is ALPHA. If you have Disco Mattes though then you can change ALPHA to RED, GREEN or BLUE depending on the channel that you want.
    If you have the EXTERNAL KEYS delivered to you as multiple Black&White images then you can combine them into a single clip using CHANNELS and then Grouping all of it so that it behaves as a single clip on your timeline (and therefore as only 1 input for COLOUR GRADE). I use that a lot for speed while grading, but also for a way to best keeping my timeline organized. And Mistika is double great if you know a few useful guidelines to make the most of it’s unique timeline, as well as tools like PROPAGATE.
    If you send me your timeline (the .env file) with the .lnk/.rnd files of your external mattes I can set up a few examples if you’d like. I don’t even need the images – just the .env and the .lnk’s. I can set up a few examples of the way that I would do it and then send it back to you. The files are so small you could email it all to me. I’m on sam@silverscreenpictures.co.uk if you’d like me to do that. No offense taken if you don’t 🙂
    By the way – one of the many ways/tricks that I used to learn Mistika was to check in the manual for each effects number of inputs. I know most operators hate manuals, and very few ever read any of them cover-to-cover. However each effect is documented in the Mistika Manual and, rather helpfully, the number of inputs per effect is on of the first topics that it provides info on. From memory (I don’t have the manual to hand right now) the ones that surprised me were:

    Number of Effect Inputs:

    CHANNELS: Unlimited
    This one is actually quite useful – every input from 2 onwards is included in Boolean operations. So lets say you’ve attempted to get a Key/Matte in 5 different passes you can stack them up, extend the CHANNELS effect to take in all 5 Keying attempts, and then set the Alpha Channel to: A1-MAX-A2. This will give you a single Alpha Channel that is the sum of all of the keying passes.

    VECTOR PAINT: At least 2
    The 2nd input is how you do things like cloning from a clip that you’ve offset by a few frames (what I use the 2nd Input for VECTOR PAINT for mostly).

    The 3rd Input is a lazy way to composite that I wouldn’t use personally – but it’s there if you need it. The 2nd Input is amazing though – it’s basically a reference for the effect that says: ‘This is an example of the colour(s) that we’re looking to key’. You can use anything like a Solid that is set to the average of the screen that you’re keying, or on a 2nd pass you can use the SCREEN BUILD tool. I consider SCENE BUILD to be one of the least  appreciated effects in Mistika, but it’s very clever if you know how to use it.


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