3 Replies Latest reply on Nov 16, 2009 12:44 PM by Ed.Macke

    Track Matte Key Luma vs. Alpha

    Ed.Macke Level 3

      I've been banging my head on the Google wall for hours now with no real answer (but a very real headache): what exactly is the difference between Matte Alpha and Matte Luma in the Premiere Elements Track Matte Key?


      The best I can find is something like "The Matte Alpha uses the alpha channel for the matte", which (besides sounding like every college exam answer I ever wrote) doesn't really help because, uh, what the !$@# is an alpha channel? Even the "Photoshop for Dummies" had me lost.


      All I know is that when I create a white circle using the PRE Titler, it seems to behave the same (correctly) whether I have Alpha or Luma chosen.


      When I create a white circle on a black background in PSE, it behaves (correctly) if I choose Luma, but if I choose Alpha the matte is treated as if it was 100% black or 100% white even though it's clearly a white circle on a black background.


      So what gives?


      • Is it because the Titler has transparent backgrounds, while my PSE has a black background?


      • Or is my PSE image missing this mysterious Alpha channel, and if so how to I add it?


      • Are transparency and alpha channel related?


      • Or is the easiest rule of thumb just to create a black and white image in PSE and choose Luma?


      [Hunt: I'm going to figure out this Track Matte stuff, if it kills all of us! ]

        • 1. Re: Track Matte Key Luma vs. Alpha
          Steve Grisetti Adobe Community Professional

          It's way simpler than that, Ed.


          Luma bases the track matte on colors, i.e., the white defines the matte (transparency through your clip) and the black the opacity.


          Alpha bases the matte on transparency in the matte image. In other words, if you've got a white circle, the white circle defines the matte and the area around the white circle (which is transparent, unless you color it) defines the opacity.

          • 2. Re: Track Matte Key Luma vs. Alpha
            Ed.Macke Level 3


            You're right (as always ).  I did some experimenting, and I think it basically boils down to this:

            • If you have a transparent background, use Matte Alpha, but Matte Luma will work pretty much the same (but can give slightly different "looks")

            • If you have a black background, use Matte Luma. Matte Alpha won't work.
            • 3. Re: Track Matte Key Luma vs. Alpha
              Ed.Macke Level 3

              I figured since I went to the trouble of doing the experimentation, I'd post my results for others... just on the off chance that I'm not the only one confused by Track Matte Keys.

              Step 1. Creating a Matte in Photoshop

              First of all, I used Photoshop Elements to create an image with a transparent background that I would be using for a Track Matte in Premiere Elements.

              1. File > New > Blank File to create a new image. 640x480 pixels, 200 pixels/inch, RGB Color mode, Transparent background.
              2. New Fill Layer > Gradient. Used defaults of Normal mode, 100% opacity.
              3. Selected Radial Gradient, used defaults of 90-degree angle, 100% scale
              4. Saved as circle_gradient_transparent.png, no interlace.

              I ended up with a white, gradient circle that faded out toward the edges (it actually wasn't pure white - more on this later).

              Then, I followed the exact same steps, the only difference being that instead of selecting a transparent background for my new image, I selected a black background. I saved this file as circle_gradient_black.png.


              I now had two identical off-white, gradient circles - one with a transparent background and one with a black background.


              Creating a Matte in Premiere's Titler

              Premiere's Titler uses a transparent background, so any comments related to the transparent background image created in Photoshop would also apply to mattes created in the Titler.


              Step 2. Using the Mattes in Premiere

              I started a new project in Premiere, and

              1. Imported the two mattes above into Premiere Elements.
              2. Created a solid blue rectangle in the Titler, and put it on Video Track 1 (VT1).
              3. Created a solid green rectangle and put it on Video Track 2.
              4. Put both mattes on Video Track 3 (VT3).
              5. Put the Track Matte Key effect on VT2, and said to use VT3 for the Matte.

              You can see all of this summarized in the first screen shot, "matte_setup.png". Notice how the two mattes thumbnails LOOK the same, but in reality one has a black background and one has a transparent background.

              Now, here's where the experimentation came in...

              First, in VT2's Track Matte Key properties, I chose the "Composite Using Matte Alpha". Then I put my Timeline Indicator over the first matte to see what it looked like, and then the second.

              As you can see in the second screen shot "matte_results.png", when Matte Alpha was chosen, the black background matte (upper left) showed only VT2, whereas the transparent background (upper right) "correctly" showed VT2 fading into VT1.

              Then, the only thing I changed was to select "Composite Using Matte Luma" on VT2's Track Matte Key properties. This time, the two mattes (lower left and right) behaved identically, showing VT2 fading into VT1.

              Now, you'll note the Matte Alpha (upper right) green circle is brighter than the Matte Luma (lower left and right). This is because of what I mentioned earlier, which was that when I created the mattes, I did not use pure white - it was kind of an off white.


              I'd like to say this was on purpose, but it wasn't, although it was actually lucky because it helped explain things even further. Here's what I surmised...

              When Matte Alpha is chosen, the color doesn't matter - it's the degree of transparency. If the color is 100% opaque, the matte shows 100% of VT2. If it's 50%, it shows 50%, etc.


              To prove this, I created another transparent background matte, only instead of off-white, I used pure black. And guess what? The green circle was identical!

              This also explains the anomaly in the upper left. With that matte, every pixel in the matte is 100% opaque - some pixels are pure black, some are pure white, some are a shade of gray in between, but they are all 100% opaque. So since every pixel in the matte is 100% opaque (albeit different colors), Matte Alpha interprets that as 100% VT2.

              However, when Matte Luma is chosen, it's the brightness of the color that matters. Pure white - the brightest possible value - results in 100% of VT2 showing, while pure black - the darkest possible value - results in 0% of VT2 showing. Transparent is also, of course, 0% brightness, so for purposes of Matte Luma, black and transparent are equal.

              So since the color for the circle was not pure white, that means it was somewhat less that the brightest possible value, and hence treated as somewhat less than 100% of VT2 by the Track Matte Key. That means that, for the sake of discussion, that 90% of VT2 shows through and 10% of VT1, so the green is a little less intense that if 100% of VT2 was showing.

              If I had used pure white, the Matte Luma results would have been identical to the Matte Alpha / Transparent background (upper right).


              Looking back, it all seems so easy. Like Steve said.