Features

Compositing effects do not, on the whole, come cheap. For serious special effects, you can take your pick: After Effects, Boris Red, Commotion and Shake all provide the tools for adding bespoke effects to your movies, be it an exploding planet, light sabres at dawn, or something more mundane such as a rain shower.

If your budget does not stretch to professional compositing tools, you can still achieve a lot, providing you are prepared to put in the hours. Rotoscoping is the process of painting on individual frames of a movie to create an effect. Using iMovie's ability to export movie frames as individual Photoshop files and to re-import the images and assemble them opens up a whole range of effects possibilities.

In this Masterclass, we will use iMovie 4 and Photoshop Elements 3 to produce a two second-clip simulating biometric identification by a retinal scan machine. Retinal scanning is popular with directors of action and spy movies - this effect has been used in many films, including Mission Impossible, Golden Eye, True Lies and, of course, Minority Report.

There are three stages to the process: exporting the frames from iMovie, applying the effect to each frame image in Elements 3, and re-importing the frames to iMovie.

The effect uses a displacement map to produce the illusion of depth - that is, the retinal scan laser appears to conform to the contours of the subject's face as it passes across the eye.

This technique is not confined to retinal scans: you can use it for all kinds of lighting effects or any situation that requires an overlay image

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to conform to the subject's contours.

Non-square pixels

When you import frames into iMovie that have been edited in Photoshop Elements, the horizontal dimension is squeezed and a black band appears down either side. This is because still images have square pixels, whereas DV uses rectangular ones.

Each frame of a PAL DV clip is composed of 720 x 576 pixels that are wider than they are tall. Photoshop Elements does not know about non-square pixels. As such, when you open an exported video frame in Photoshop Elements 3, the rectangular pixels get squashed into square ones.

You can overcome the problem by stretching the frame back out again. In Photoshop Elements, choose Image>Resize>Image Size and uncheck the Constrain Proportions checkbox. Enter 768 pixels in the width field and click OK. When you re-import the frame into iMovie, it will squeeze the frame back into its original 720 x 576 PAL DV proportions.

The best time to do the resizing is at the start - before you carry out any editing on the frame, as any graphics you add will not be subject to distortion if you do it this way. For example, if you overlay a circle on the frame after it has been resized, it will still be a circle, not an oval, when you export it. Don't worry if things look squashed when you get them into iMovie - when you output your movie either back to you DV camcorder, or to DVD and playback on a TV, everything will look fine.

The only way to be sure everything is as it should be is to connect a video monitor (a portable TV works fine) to your Mac. This will verify that overlay graphics are the correct proportions, and provide early warning of video problems.

Alternative effects

Using displacement maps to achieve depth effects with 2D images provides scope for all sorts of video special effects. You can add projection-lighting effects - to, say, project a movie onto the side of a building. In a previous issue of MacUser, Steve Caplin explained Photoshop displacement map techniques that could be adapted for use with video frames (see Photoshop displacement maps).