Post Production: Keying, Alpha Channels and Mattes

Several different functions and tools you can use in video post production include mattes, alpha channels and keying. These features are all part of editing done on the computer but are fairly advanced as they use multi layers. While these features are not necessarily easy to use, using good software will make it a piece of cake to apply the different editing functions. Mattes can be used to drop out background color and render different parts of the image.

Step 1. Load the Project

To apply a matte, you will first need to load your project. It is possible to render some transparent materials and be able to render 2D artwork in the image. If you are new to using mattes then choose an image to test everything out.

Step 2. The Alpha Channel

Most images will have 4 alpha channels. These channels make it possible to save the image in 1 file without altering the different channels. Almost all formats will use an alpha channel. Some good examples of alpha channels include white channels, shadow channels, separate colors or a composite with a transparent channel.

Step 3. Create the Matte

To create an alpha matte you will need to select a color in the image. The software will then assign all of those color pixels to your new alpha matte. Most software will also allow you to select a range of different colors. Normally you will select the color in the image. Most mattes are applied to the background color but this is not necessary. You can choose any color in the image or the shadows and create the matte. It is also necessary to define the transparency in the image as well.

Step 4. The Track Matte

A track matte can be used to show through holes or transparencies you created using a different matte. Normally this is applied to the transparency layer of the alpha channel. This track matte is normally used when you do not want to use an alpha channel. These mattes are used to increase the contrast or soften edges.

Step 5. Traveling Matte

You can then have a matte travel across the screen. This includes using a track matte layer and then a fill layer with the effect you want. These layers are added to your image layer and you will see you texture pattern move across the image. This is great for adding animation to the image. This can get a bit tricky if you have multiple layers as it only gets applied to the layer below the traveling matter layer.

Step 6. Applying them to Video

The easiest way to apply any mattes to a video is through the timeline function when editing. You can drag the matte layers directly where you want them to go. Most software will have predefined mattes to use as well as different transparency mattes. Most software will apply the matte directly to the alpha channel but you can change this by using the channel effect options.

Step 7. Perfecting the Mattes

Once you have created your mattes then it is easy to change them. You can change the colors and transparencies by altering your old matte or creating a new matte. When first using mattes take the time to see what each matte will do to the image. Then pick the one you like the best.