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Thread: Colour key image - edges showing

  1. #1
    Join Date
    Aug 2008
    South Devon

    Default Colour key image - edges showing

    I'm working on a title sequence, basically there's a background still, and a bitmap logo which I want the camera to reverse through, you know, very much done before camera looking 'backwards' flies through text in space.

    So I've created the logo with a green surround, keyed it, stuck motion controls on it (a pain that zoom can only go to 600%, need more, but can sort that), it's working just as I want, except when the edge of the logo image comes into frame it can be seen, a pixel wide flashing as it zooms out. I've checked the image, fine.

    I can remove it using 'edge thin', but that trashes the image itself, I've tried various combinations of the settings but whatever I do the image is compromised.

    Is this simply a question of using a huge background so that the edge doesn't come into the frame before it's faded out?

    Or, is it wise to choose a background colour that is as far away from the logo as possible? I asked because the logo is yellow, and I went for a green screen key, right next to each other. Would, say, a purple be better so that I can adjust the tolerance more to remove these edges?

    I'm answering my own question aren't I

  2. #2
    Join Date
    Nov 2005
    Bristol uk
    Blog Entries


    If this fits your way of working, and I assuming you are rendering to hdv / dv, if so try rendering to an uncompressed file.

    Many codecs dont like contrasty vert / horixontal straight edges - might be that.

  3. #3
    Join Date
    Aug 2008
    South Devon


    The edges show up under any circumstances, in preview, and any render. But having fiddled I've found that I have in fact found the reason.

    I'd like to have my logo on a transparent background, but the original is a vector graphic. I tried importing it into Premier as a vector, but it just messes what is supposed to be an infinitely scalable image into a stepped pixel piece of junk when zoomed in.

    So, by exporting from my vector program a large bitmap, and by making the background as far away from the logo colour as possible in the spectrum, violet, I can get a reasonable file size because there's only two colours, acceptable edge definition on the logo, and no probelms with the edge of the image itself, because as suspected the tolerance can be increased far more this way.

    So I guess the lesson learned for today is:

    When chossing a background colour for colour keying, ensure it's completely isolated from the subject in the light spectrum.

    As mentioned the logo was yellow, I chose green to key out, even schoolboy physics shows that green is right next to yellow, therefore contains yellow in degrees. So any effort to adjust the tolerance was impacting on the logo itself.

    Job done, now to work on maxing out the zoom to get what I want.

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