Have you ever wondered why some videos in the iStockphoto video collection are half black and white (grey scale) and half full color?





The grey scale video is not just for variety, it was created by the artist to be used as a track matte when compositing video.

We’ve made this little tutorial to explore the often-misunderstood track matte. You can download the video files we used in this article yourself for free if you like. The leaves video is the free video of the month for April, and the house and tree video was the free video in March.

Track mattes are essential to video compositing and are fun and easy to use once you understand how to use them. In a nutshell, track mattes are used to manage transparency in a video file. There are several types of track mattes out there, but we are going to focus on the luma matte.
Luma Mattes



Luma mattes store transparency information as luminance (luma) seen by the human eye as a grey scale video or image. The darker the grey or the closer the grey value approaches black the higher the transparency level. Therefore white values are translated as fully opaque and black values are translated as completely transparent.

Using a luma matte requires three layers of video.
  1. One video layer contains the luma matte or the grey scale video that stores the transparency level.
  2. A second video layer contains the fill, the video to which the transparency data is applied.
  3. A third video layer contains the background video, the video that is seen through the transparency of the fill video.


Confusing? This is where a picture is worth a thousand words or a whole bottle of aspirin.



The iStockphoto video collection supports luma mattes due to their flexibility and compatibility. They are typically added as the second half of a computer generated video file. There are many such files on iStock, including more from the very capable member TheMediaSuitcase who created the leaves featured in this tutorial. Here is a lightbox featuring more luma matte footage.

You can separate the file simply by cutting the file in half (or at the point that the action goes to b&w). More on that below.

One of the best features of a luma matte is that they are just a grey scale video, meaning they can be treated just like every other video. This feature opens a whole world of creative possibilities. Effects, filters, blending modes and more can be added to alter the luma matte.

Also since luma mattes are just grey scale videos any full color video can be converted to a luma matte by completely de-saturating the color (converting the color video to a grey scale video).

Ok, let’s get down to looking at using luma mattes with the free files mentioned above with some common software – Adobe After Effects and Final Cut Pro 7. We will assume that you have a basic working knowledge of one of these applications.
After Effects


  1. Download and import the two free files into a new project. The foreground and the background. To better follow along, we re-named the files leaves and background.
  2. Place the leaves video and cut it in half so that you have two files – a color and a greyscale clip.
  3. Place the fill video directly below the matte layer our example is (leaves fill video). This step is very important, if the fill layer is not directly below matte layer the transparency data will not link properly
  4. Place the background video into the layer below the fill layer
  5. In the TrkMat column within the fill layer (leaves fill video) select the drop down (Luma Matte “leaves luma matte”)
  6. After Effects should automatically disable the eyeball on the luma matte layer
  7. The falling leaves nimation will now be composited over the background, resulting in the animated falling leaves over the building.




Final Cut Pro 7


  1. Download and import the two free files into a new project. The foreground and the background. To better follow along, we labeled the files with the following colors: Background is blue and the animation/luma matte is orange.
  2. Place the animation video and cut it in half so that you have two files – a color (fill) and a greyscale (matte) clip.
  3. Place the fill video directly above the matte layer. This step is very important. If the animation fill layer is not directly above the matte layer, the transparency data will not link properly.
  4. Place the background video into the layer below the matte layer.
  5. Click on the top layer (which should be the fill video), then go to ModifyComposite ModeTravel MatteLuma.
  6. FCP will add the Travel MatteLuma to the fill layer, and the animation will now be composited over the background, resulting in the animated falling leaves over the building



References
Christiansen, Mark. Adobe After Effects CS4 Visual Effects and Compositing Studio Techniques. Berkeley: Adobe Press, 2008. Print
Brinkmann, Ron. The Art and Science of Digital Compositing: Techniques for Visual Effects, Animation and Motion Graphics. Burlington: Morgan Kaufmann, 2008. Print
Meyer, Chris and Trish Meyer. Creating Motion Graphics with After Effects: Essential and Advanced Techniques. San Francisco: Focal Press, 2010. Print

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