9 Replies Latest reply on Jun 23, 2013 11:05 AM by Jim_Simon

    Unexpected Alpha channel behavior in Premiere

    NickelplateSpiro Level 1

      Is there no way to interpret alphas as straight versus premultiplied in Premiere?  Why do my transparent layers and clips with alpha channels display differently with a Black Video layer below them?

       

      Background info:

      I'm exporting my film to files with codecs that support alpha channels, not because I want an alpha in the result but because I need full RGB / 444 color coding for digital cinema theaters. I'm using ProRes444, Targa and PNG Image sequences to preserve as much quality as possible in my master exports.

       

      I've discovered that there is no option to disable the alpha transparency in the Export Media step while preserving at least 10 bits per color channel (Trillions of colors in AE).

       

      To solve this problem, I've had to add a "Black Video" clip to the first video track, underneath the film's content. When this is done, the pre-rendered text elements with alphas look different. They are movie trailer style text treatments that are light text over pure black. (I'm on my ipad right now and don't see a way to upload screen shots to show examples)

       

      The result with the black video layer applied at the bottom is similar to the way QuickTime 7 displays straight alphas incorrectly. This problem occurs with both straight and premultiplied renders. The rendered text is from after effects and is stored using the animation codec.

       

      I'm aware of how to work around this, by maintaining the alpha transparency in Premiere and then removing the alpha channel of my export by running it through After Effects, but that is a ridiculous, time consuming step.

       

      I'm working with the GPU Mercury Engine on with a K5000 from NVIDIA.  Can anyone explain this behavior? Is there no way to interpret alphas as straight versus premultiplied in Premiere?  Why do my transparent layers display differently with a Black Video layer below them?

       

      Here are the screen shots

       

      UNDESIRED EFFECT using Black Video layer at bottom: Notice the fuzzy glow surrounding the text. This is happening with both straight and premultiplied renders.

      messagepart-1.png

       

       

      INTENDED LOOK when composited over nothing: The glowing edges are displayed the same way in Premiere as in After Effects.

      messagepart.png