Playing with video editing

Making the matte
A mask used to hide parts of a clip overlay is called a matte, or in Premiere Elements, a track matte. Mattes in video compositing work like masks in photo editing. They are greyscale images that hide or reveal detail below the masked track depending on the greyscale value of pixels in the mask. Usually, white pixels in the matte display image pixels in the overlay and black pixels mask it, revealing the track below. Grey values in the mask reveal underlying detail in varying degrees of opacity depending on the greyscale value. This is called a Luma matte.

Premiere Elements can also use the alpha transparency in a .psd file as a matte. To create an alpha matte in Photoshop, first create a new document using the appropriate video preset, probably ‘Pal D1/DV, 720x576 (with guides)’ or the widescreen variant, and set the background contents to transparent. Next, make a selection of the area you want to mask; here it’s the right third of the frame occupied by the presenter in front of the wall.

Fill the selection with black (Alt & Delete) and save the file as matte.psd. If you’re working in another application, create a file of the correct pixel dimensions (you don’t need to worry about rectangular pixels as slight distortion will only marginally affect the mask positioning) and save it in a format supported by your video-editing application.

Back in Premiere Elements, import the newly created matte.psd file (click the Get Media button in the Organizer workspace) and drag and drop it at the beginning of the Video 3 track. Drag the right-hand edge of the clip to make it the same duration as the two clips in the tracks below.

Now select the presenter clip in the Video 2 track and click the Edit tab to switch to the Edit workspace. Click the Effects button and select Video Effects and Keying from the two pulldown menus. Then drag and drop the Track Matte Key effect thumbnail onto the clip in the Video 2 Track. Click the Edit Effects button and in the expanded properties area for the Track Matte Key select Video 3 from the Matte pop-up menu and select Matte Alpha from the Composite Using pop-up menu.

You should now be looking at the composited clips in the Monitor. Press Enter to render the work area, then space to play back the timeline, and you’ll see the fast-motion effect on the left with time running normally in the superimposed clip on the right.

Slow it down
What about slowing down time, or stopping it? The technique is the same except you use the Time Stretch tool to expand a clip and slow it down. To make time stand still you’ll need to composite your moving real-time actor over a still frame of the scene.

To export a video frame from Premiere Elements 7, place your background clip in Video 1, drag the current-time indicator on the timeline to the frame you want and click the Freeze Frame button in the Monitor panel. Leave the Freeze Frame duration field at the default five-second setting and click the Export button to save the file to your hard drive and add it to the Organizer.

Delete the background clip from Video 1 and replace it with the single frame clip, dragging the right-hand end to extend it to 10 seconds duration. Now place your live action clip in Video 2, your track matte file in Video 3 and apply the Track Matte filter to the Video 2 clip as before. Now, instead of our presenter standing in a time-accelerated scene, time stands still for everyone but him.

In both examples, I’ve made things easy by composing the scene so the two time frames occupy the left and right halves of the screen. In the second example, where time stands still, you can make things more interesting by having the real-time actor interact with elements of the frozen scene. This requires a bit of planning and more elaborate matting, but the track matting techniques described here form the basis for all such special effects.

It’s an impressive trick that’s made easy by digital-imaging software, but if you can’t pull it off you could always resort to the same method they used back in the 1960s for The Magic Boomerang and get everyone to stand very, very still.