7 Replies Latest reply on Aug 18, 2009 6:04 PM by aweiis

    AE CS4 Alpha Interpretation Changes


      I'm having a hard time figuring out the changes that have been made to alpha interpretation in after effects cs4. Previously (cs3) I was able to output an flv with a perfectly clean alpha (no halo). This is no longer the case. I've tried every possible combination of alpha interpretation on import and video output color and haven't been able to get the desired result. The magical combination in cs3 was Premultiplied against black on import and straight unmatted output.  In cs4 it seems I'm only able to come close to that result with a straight unmatted import and output, and even then there's still a slight (but noticable) white halo around the image.  I've attached example images below.  Any help is greatly appreciated.thnx.

        • 1. Re: AE CS4 Alpha Interpretation Changes
          Todd_Kopriva Level 8

          See if this helps:


          Flash, FLV, and alpha channels: straight and premultiplied


          There's also some information in the first entry in the FAQ list on this forum:



          1 person found this helpful
          • 2. Re: AE CS4 Alpha Interpretation Changes
            aweiis Level 1

            Thanks for the links, Todd. I had actually checked those out before posting and wasn't able to come up with a solution. I've tried every possible combination of alpha interpretation and video output color to no avail. At this point, I'd usually assume the issue was with my source footage, but ae cs3 is able to handle the exact same footage just fine.

            • 3. Re: AE CS4 Alpha Interpretation Changes
              Mylenium Most Valuable Participant

              Are you by any chance using color management/ profiles somewhere? That would affect premultiplied colors and some of that stuff has changed in CS4. In addition, check the project bit-depth and the Render at Maximum Bit Depth option in Adobe Media Encoder. There are some combinations that cause the Alpha to be treated inconsistently.



              1 person found this helpful
              • 4. Re: AE CS4 Alpha Interpretation Changes
                Tim Kurkoski Adobe Employee



                Can you post a project that demonstrates the problem?  Source footage will probably also be helpful in this case.  You can simplify the project to just demo the problem, if that helps.


                If you're getting results in AE CS3 that are different from CS4, please also include a CS3 project so I can compare what you're expecting to what CS4 is doing.


                Oh, and of course write down the steps you're taking so I can follow along.





                -=After Effects QE

                • 5. Re: AE CS4 Alpha Interpretation Changes
                  aweiis Level 1

                  Thnx for the responses, all. I didn't have any luck with the bit depth changes and I'm not using color management/profiles. I'll PM you an .aep and a peice of footage, Tim. Most of the settings we're using are pretty standard, as far as I can tell..


                  Straight - Unmatted Alpha Interpretation (on import)

                  Video Output:

                  - Channels: RGB + Alpha

                  - Depth: Millions of Colors+

                  - Color: Straight (Unmatted)

                  Constant 240bps Bitrate


                  The CS3 settings were pretty much identical except we were Premultiplying against black on import.

                  • 6. Re: AE CS4 Alpha Interpretation Changes
                    Tim Kurkoski Adobe Employee



                    Thanks for sending the files.  I've tested this long enough now that my eyeballs hurt.  There is a problem here, but at the same time there isn't.  It's a bit hard to explain.


                    Looking closely in Flash at the same FLV output from AE CS3 and CS4, I can see colored fringing in the alpha areas from both versions of AE.  In CS3, the fringe tends to be dark.  In CS4 the fringe tends to be light.  The trick here is that the apparent visibility of the fringe is dependent depends on the color of the background.


                    Overall, the darkly fringed CS3 output looks worse to me. It's especially bad on light colored backgrounds.  But on dark colored backgrounds the lighter parts of the CS4 output stand out.


                    See the attached examples, which are screenshots from Flash at 800% view of unmatted (ie., straight) FLV output from AE CS3 and CS4.  The CS3 output is on top, CS4 output on bottom.  Sorry about the poor labeling, large size and the bright colors.  (Did I mention that my eyeballs hurt right now?)


                    aweiis, the example you're dealing with appears to be a worst case scenario.  You've got an alpha edge that appears prone to picking up these light highlights in the alpha, plus you're matting it on top of a dark background.  The best workaround I can suggest is that you try changing your AE comp's background color to the same color the animation will be composited over, then render from AE as premultiplied.  Not an ideal solution for many reasons, but it may give you better results depending on your scenario.  (You could also string together PNG files for the animation instead, but that would probably be tedious.)


                    To be clear: There should not be fringing of any color in "straight" alpha output.  This is a bug.  Flash and FLV are slightly odd critters when it comes to alpha, though.  AE has to go through some machinations just to get the alpha right.  Ideally, AE wouldn't make us think about this and just make it look correct, regardless of little niggly things like the alpha setting.


                    Summary: I think the FLV alpha output from AE CS4 looks much better than AE CS3, but it's not perfect.  aweiis, you unluckily stepped right into one of the imperfections.  Thanks much for pointing it out; I've filed the bug.



                    -=After Effects QE

                    • 7. Re: AE CS4 Alpha Interpretation Changes
                      aweiis Level 1

                      Thanks so much for the help, Tim.  The tif sequences are being compressed into flv format and then bundled into swfs which are played and layered over various backrounds using a proprietary flv player. So it sounds like I'm going to have to just deal with the fringing for the time being, since the backrounds vary in color. Thanks again for the help, though, and I'll keep my eyes open for a bug fix in the next major update.