5 Replies Latest reply on Aug 2, 2010 11:34 PM by OsakaWebbie

    Should be straight alpha, but it doesn't look like it

    OsakaWebbie Level 1

      I'm using Keylight on some footage in AE CS4, and then bringing it into PPro CS3 for the rest of the editing process.  In AE, one of my roses (a light one chosen to accentuate what is going on) looks like this:


      I have "Unpremultiply" checked in Keylight, and here are my Output Module Settings for the render:


      As far as I can tell, I should be doing everything right to end up with footage with straight alpha, but in PPro, I get the famous "premultiplied-alpha halo" anyway!  At first, the halo was only appearing a second or two after I would click in a new spot on the timeline - that really got my attention that it was something other than just bad keying on my part.  I actually tried to catch a screen capture during that moment before the halo would appear (to show the before-and-after), but by the time I decided to try that, Murphy's law had kicked in - now there is no delay and it looks like this all the time:


      I have several roses that I have processed in the same manner, and they all have the halo, but it's of course not as obvious if the rose itself and/or the background footage is darker.


      I'm pretty new to keying and alpha, but I thought I had done my homework, so to speak.  What other settings should I check?

        • 1. Re: Should be straight alpha, but it doesn't look like it
          OsakaWebbie Level 1

          I just went back to work on my PPro timeline, and discovered what makes the gray halo shy - realtime preview of unrendered footage!  So here are the before and after snapshots I was trying to get for you.  For some reason realtime preview was being stubborn on my pink rose, but this orange one is actually the one I plan to really use with this background, so here it is:

          So, the halo only shows up when the frame is actually rendered.  Weird, huh!  Is that related to the type of alpha, or something else?  If it's something else, I guess I'll need to ask over int the PPro forum, but I posted here because I though ti was something I did wrong in AE.

          • 2. Re: Should be straight alpha, but it doesn't look like it
            TimeRemapper Level 4

            NM, it looks like you can't specify the type of alpha interpretation in Premiere.

            • 3. Re: Should be straight alpha, but it doesn't look like it
              OsakaWebbie Level 1

              Any thoughts on this?  I need to send this project off to the replicator, but I don't want to do it with gray lines around my roses... Should I be trying in the PPro forum?  I don't want to get in trouble for cross-posting, but I don't really know which program is causing the problem.


              I doubt this is relevent, but my original "footage" is actually photo sequences (snapped once a minute for a couple days for each rose) - I first used VirtualDub with a deflicker plugin to bring the sequences in, remove the camera aperture flicker, and output as AVIs with Lagarith.  Then I brought them into AE for Keylight.  But none of that should matter, as there is no alpha before AE - it's how AE renders it and PPro interprets it that matter.

              • 4. Re: Should be straight alpha, but it doesn't look like it
                Mylenium Most Valuable Participant

                Try to render RGB and Alpha separately, then use Premiere's Alpha key effect to recombine them. When you do, consider dilating the RGB data by one pixel using the Minimax effect.



                • 5. Re: Should be straight alpha, but it doesn't look like it
                  OsakaWebbie Level 1

                  Thanks for the suggestion - it basically worked.  PPro CS3 doesn't appear to have an effect called "Alpha Key", so I used "Track Matte Key", specifying the track where I put the alpha clip and selecting Luma Key for the setting (setting it to Alpha Key didn't do anything, and it was clear that the "alpha" AVI was not an alpha channel per se, but just a regular video clip with a B&W representation of the transparency).  I also don't see any effect called "Minimax".  Anyway, I followed your suggestion as best I could with the tools I have - you must be using a newer version of PPro.  I can indeed see about a pixel or so that would be better if it was more  transparent - I suspect your Minimax suggestion would have taken care of  that.  But this is definitely usable.


                  Key point for those reading this later: You have to choose "Straight" for the Color setting when you render by this method, even though you get some strong warnings that dissauded me the first time I tried it (I thought they were saying I had to include an alpha channel in the stream, not the method I was about to use).


                  One more question: When rendering the separate alpha clip (essentially just a grayscale video of the transparency) to be used as the luma matte for the Track Matte Key, does it matter whether the Color setting is Straight or Premultiplied?  I can't imagine that it would matter for that clip, but if there is a subtle difference, let me know.


                  I have no idea why I can't get it to work with an embedded alpha channel, though - surely hundreds or thousands of people use these two programs to handle RGB+Alpha video files.  For this time, I'll re-render my roses (twice for each rose to get the two streams - this will give my CPU a workout!) and use this workaround, but if someone has any insight about the more direct method, please enlighten us all.