So far we've covered a couple ways to create transparency in your layers: Alpha channels, primarily for stills, and Masks for video. There is a third way which combines some of the best aspects of the two - complex, multilevel transparency like an alpha channel and movement or animation like a mask - the Track Matte.
Track Mattes let you assign one track to act as an alpha channel for another track. You can choose to use either the luminance (brightness) values of the track to create the transparency (just like an alpha channel - black is transparent, white is opaque, greys are partially transparent) or you can use an existing alpha channel in the second layer to create the transparency. If the layer you use as a mask is video or has been animated the transparency it creates will change with the track - this can let you create very sophisticated transparency that would be difficult to achieve with masks or static alpha channels.
There are a couple of limitations to the use of track mattes. The first is that the layer you wish to use as a matte must be directly above the layer you wish to apply the transparency to. This leads to the second limitation - you can only apply your track matte to a single layer at a time. There are ways around this - you can either duplicate the matte layer for each additional layer you wish to apply transparency to, or you can pre-compose all the layers you wish to apply the transparency to first. Either way, just be aware that using a track matte on more than one layer will require some additional planning.
Applying Track Mattes
Applying a track matte is a fairly easy process:
- Place the footage you wish to apply the transparency to into a composition
- Place the footage you wish to use as a track matte into the same composition
- Make sure the matte layer is just above the layer you wish to use it on in the timeline's stacking order.
- If necessary, click on the "Switches/Modes" label at the bottom of the switches column to change it to "Modes" mode (just like we did when working with layer transfer modes).
- On the layer below the matte layer there should be an additional drop down menu next to the layer modes menu - this is for selecting a track matte and is only present when there is a layer above the current layer. It should currently say "None". Click on this and select "Luma Matte '(your matte layer name)'". Notice that the options will all list the name of the layer immediately above the current layer - this is the only one which you can use as a matte.
- Preview your comp - the bottom layer should now have transparency based on the brightness values of the track matte layer. Notice that the layer you used as the track matte is no longer visible in the comp and it's 'eye' icon has been turned off in the timeline - this is because once you use it to create the transparency for the layer below you don't want it visible or it would obscure the other layer and the transparency you've created.
That's it - there's just a few more things to consider...
There are four options when you set a track matte - alpha, alpha inverted, luma, and luma inverted. We've already seen what luma does - it treats the matte layer as if it were black and white and makes the other layer transparent based on it's brightness values. Luma inverted just inverts the image so that the dark areas are bright, bright areas are dark, and the transparency is reversed. The alpha options use the existing alpha channel of the matte layer to apply transparency to the other layer - this only really works if there is an alpha channel present in the matte layer, for instance one you added in Photoshop or that was created by a 3D program. Alpha inverted simply reverses the values of the alpha to swap the transparent and opaque areas.
Manipulating the transparency from your matte
You will probably find yourself using the Luma track matte modes more frequently than the Alpha modes. However, often times the image you use won't create the transparency you want. Because we are often working with color images and video, and luma mode converts the image to greyscale first, many times your transparency will be muddy and indistinct because the image ends up mostly grey with little true white or black.
The solution to this is to use effects on the layer you use as a matte. Image adjustment effects such as levels, curves, brightness and contrast, etc, all will allow you to change the contrast of the image to make the resulting transparency more or less distinct.
You can also use effects such as blur to obscure details in the mask - basically anything you do to manipulate the matte layer will affect the transparency generated. If the layer you are using as a matte is a still image you can give it motion by keyframing it's transform properties, or you can use any of the distort filters to give it some motion within the image itself.
Track mattes can be as simple or complex as you like - experiment with using them in conjunction with effects and animation to create something more than just simple transparency. By combining them with layer modes, masks, alpha channels and color keys you have almost limitless possibilities for blending images and video together to create something new and unique.