5 Replies Latest reply on Mar 2, 2009 8:30 PM by (jaholmes)

    Image Matte Key Insanity!

      Alright, I'm hoping somebody can give me the basic theory to understand what's going on here. I think it really boils down to understanding what's expected of still images (e.g., aspect ratio) when using them on the timeline versus using them as image matte keys.


      Here's what I'm trying to do. I have a 720x540 image. In the center is an opaque, capsule-shaped object. The rest of the image is transparent. My goal is to place video over this capsule-shaped portion of the image while leaving the rest transparent. Here is what I'm doing:

      1) I place this 720x540 image on the timeline in Sequence 01.
      2) I overlay some 720x480 DV video with Blend Mode = Overlay.
      3) I use the Scale effect on the DV to downsize it so that it barely exceeds the size of the capsule.

      ...looks great so far. Now I want to "chop off" all the video that extends beyond the boundaries of the capsule.

      4) I create a new sequence (Sequence 02) and place the first sequence on this one (incidentally, all video sources and sequences are 720x480 DV)
      5) I add the Image Matte Key effect to Sequence 01 (on the Sequence 02 timeline).
      6) I select the same 720x540 image as the matte, chosing the composite mode as "Alpha".

      The resule makes no sense to me. The masked area is smaller than the capsule in the video (i.e., it's cutting off much more than I want). What's the deal? It seems that the image used for Image Matte Key is expected to have certain dimensions or aspect ratio that are different from what's expected of stills going on the timeline.

      I even tried saving a new copy of the capsule image that was corrected to DV aspect ratio. No dice.


        • 1. Re: Image Matte Key Insanity!
          Stan Jones Adobe Community Professional & MVP
          What is the aspect ratio is of your still? What are your project settings? Your best option is to set the still to the same settings as your project before bringing it in.
          • 2. Re: Image Matte Key Insanity!
            Level 1
            Hi, Stanley:

            All video (sources, sequences, etc.) is 720x480 with DV pixel aspect ratio, i.e., 4x3. All stills are 720x540 with square pixel aspect ratio, i.e., also 4x3.

            What's puzzling is that I can draw a white box on a black background, save as .BMP, and place it on a timeline. Then, I can apply the same image to itself as an image matte using "Luma" as the composite, and the white box on the timeline gets clipped. Why? That seems like the most basic test case. Using a black-and-white (not even grayscale) image as an image matte for itself with "Luma" should effectively just make the black parts transparent. The white parts shouldn't be masked at all.

            Copying my .BMP and making a matte-only version that's 720x480 (looking slightly squashed) still doesn't work :-?

            As similar experiment produced similarly bizarre results: I wanted to create a rounded-edged matte for some video. I made a 720x480 PSD file where the corners were transparent, but the bulk of the image was a solid, rounded-edged rectangle. I applied this, and the matte was squished vertically such that some of the video could be seen above and below the matte. I made the matte 720x540, and the problem disappeared. This led me to believe that the image matte key needed to be square pixels with 4x3 proportions, however that apparently is not the only catch.

            I'm mystified.
            • 3. Re: Image Matte Key Insanity!
              Stan Jones Adobe Community Professional & MVP
              Now that I look at it, I think there are some differences from CS3; all I have - in addition to the slight change in aspect ratio. In CS3, the 720x540 Square does not conform. Set your program monitor view to 50%, or whatever allows you to see outside the standard frame. Put a 720x540 image on a timeline; select it; click motion in the effects controls panel. For me, this clip is larger on all sides from the DV size frame. Try it with a DV size/par image; should fit.

              I thought you were going for the video showing through the capsule, and did not understand why you didn't just matte it. Then I realized you were going for an overlay.
              • 4. Re: Image Matte Key Insanity!
                joshtownsend Level 2
                Isn't this something more suited for After Effects? No offense just my opinion. Besides isn't the Track Matte listed under 'known problems' like QT animation and motion-Jpeg?
                • 5. Re: Image Matte Key Insanity!
                  Level 1
                  This is Image Matte Key, not Track Matte. I would hope that simply masking out a portion of the video didn't qualify as "more suited to After Effects." It's just simple compositing. If it doesn't work, it ought to be fixed. Unfortunately, "known problems" have a nasty habit of becoming de-facto "features" in Premiere.