FCG076 – The Cross Over Show (feat. Alex MacLean)


Viewer Questions & Color grading for dummies? Not exactly. On this special episode of The Grill, Alex MacLean, from digitalCINEMAcafe drops by the studio and we have a casual chat about Color grading practices, Alpha channels and the Tilde key, and we listen to YOUR voice mail messages that have been left for us on the digitalCINEMAcafe.com website.

Sponsored by premiumbeat.com

Chris Fenwick – chrisfenwick.com – @chrisfenwick
Alex MacLean – @alexm13


One thought on “FCG076 – The Cross Over Show (feat. Alex MacLean)

  1. Martin Baker

    I swear you’d already played these voicemails on previous shows…

    Anyway, premultiplied vs straight isn’t as scary as it seems. There’s only 5 things you need to know:

    1. The alpha channel is exactly the same in both premultiplied and straight files (this surprises many people because the word on the street is “straight alpha” and “premultiplied alpha”).
    2. In a premultiplied file, the pixel values in the RGB channels have been (pre)multiplied with the alpha channel and composited over a background colour (almost always black and very rarely orange). So in this case the RGB channels will look great with smooth anti-aliased edges on text etc.
    3. In a straight file, the pixels in the RGB channels remain “full level” just like the olden days of fill and key video signals and the alpha channel is used to determine the transparency of each pixel when it is composited over another image. In this case the RGB channels will usually look like crap with blocky hard edges.
    4. If you import a premultiplied file and the NLE or graphics app interprets it as straight, then you’ll see dark edges around objects. Very common in Avid which sticks to its old-school principles and ONLY supports straight. Crazy but true.
    5. If you import a straight file and the NLE or graphics app thinks it is premultiplied, then you’ll usually see a bright halo around objects.

    AFAIK most apps export premultiplied files by default is because a straight image usually looks terrible if you’re just viewing the RGB. That completely freaks people out (i.e. mostly clients) who don’t understand why the stunning graphic sequence they paid thousands for, now looks like something created in Windows Paint circa 1995.

    You’re right that straight alpha is technically “purer” because it doesn’t require unmultiplying the image before compositing it.

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