Transporter - Mask Generation

Now we're going to get a little complex (and a little more Vegas specific). The basic idea is to use the same mask in two different ways. In the previous section, we made the white areas of the mask transparent to the sparkles. Now we are going to make the black area transparent to a background. In some applications, you will need to duplicate the mask and use it twice. In Vegas, we are going to use a parent/child relationship on the top two tracks to do this. I'm going to walk through this step by step (Download the zipped project with media [257KB].):

1. Click the Make Compositing Child button.

This button toggles Track 2 between being a Parent, or main track, and Child, which means that it is a sort of sub-track to the track immediately above it. This nesting allows the composite we did in the previous step (allowing the sparkles to show through the mask) to function as a unit. This will make it possible to isolate the black and make it transparent to Track 3 which contains our background.

2. Click the Mask FX button.

This opens up the Video Mask FX dialog (pictured below). This dialog will now let you create a new mask (or matte) from the combined Parent/Child of Track 1 and 2. The mask can be based on anything you can isolate in the video. This is commonly referred to as "keying out" the background. The key can be created explicitly from an alpha channel (as we did in the previous step) or you can generate your own transparency based on something else, such as the even hue of a green screen, known as a chromakey.

3. From the Type drop down list, select Alpha.

In our case, we are going to once again take advantage of the alpha channel in the original PNG image matte of the crewman in the Parent track (Track 1).

4. Selecting the Invert option.

Remember that in the previous step that we used the alpha as-is to make the shape of the crewman transparent; here we'll simply invert it.

This is the hardest step to understand, but the project is already looking fairly complete and you should have a good idea of what is going on. Our final step is fairly easy and brings us back to the beginning with the return of our crossfading ghost.

Crossfading the Crewman -->