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  1. #1
    Join Date
    Jan 2013
    Posts
    2

    Default Nuke Amateur Help

    Hi everyone!

    I\'ve got a pretty amateur question. I have transitioned over from using After Effects as I am applying for an internship that requires Nuke.

    I\'m am doing a special effects shot and am trying to merge stock blood footage on top of a video. The stock footage consists of video footage of a blood splatter against a black background. I thought it would be easy enough to just merge the two (blood into the A pipe, and the background into the B pipe) with a screen blend mode (as I thought the Screen blend mode basically removed everything that was black). But this resulted in a very, very (very) transparent blood.

    I did do a work around where I used a Shufflecopy node to copy the Red channel into the alpha channel but this resulted in a very dark grey alpha so then needed to colour correct the alpha to make it more white. But this meant me changing the Gain to about 50 and it made the final blood slightly pixelated and also look off too.

    Is there an easier way to do this? I read somewhere this morning that if the Value in the blood footage were not between 0 and 1 then it could cause this problem but I do not know what this exactly means.

    Thanks, Steven.

  2. #2
    Join Date
    Jan 2013
    Posts
    3

    Default

    You are doing the right thing in defining the red channel a your alpha!
    Use a mult(math) to make it more solid ( highest value should be 1, clamp afterwards to ensure an alpha of 0-1).
    I always treat the alpha separately to my RGB. So see if the blood colour matches the plate, eg. the highlights and the blacks.
    Use another copy node to pipe the lovely alpha to you colour corrected blood element and the use a premult node. After this a simple merge node should work (over). Screen is best used for window reflections (in SRGB), but not for blood.
    If you have edge problems, check if your matte is not too solid. Blurring the alpha often leads to fringing, for example.
    Make sure the fine motion blur of the blood is retained, too. Otherwise it will look fake, too.

  3. #3
    Join Date
    Jan 2013
    Posts
    2

    Default

    Thank you so much, this helped a lot!

    Can I also ask, And I am sure this will be a very very common question, but why do I pre multiply it? And also, how do you clamp the alpha to stay between 0 and 1?

    Thanks again, Steven.

  4. #4
    Join Date
    Jan 2013
    Posts
    3

    Default

    Well, any image you want to merge over another and which does not come to you premultiplied (eg. a cg render) needs to be multiplied by the created alpha. Otherwise you might get stuff comped onto you BG which was not meant to be comped. If you over something which has zero alpha values it will mathematically be added (in Nuke it\'s called \"plus\").
    To clamp the alpha you can use the clamp node. Careful to only apply it to you alpha channel though, so you don\'t destroy you high values in the RGB.

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