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 to conform to the subject's contours.
Non-square pixelsWhen 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 effectsUsing 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).
Rotoscoping can be used for many effects. Exporting frames into Photoshop Elements effectively gives you the same kind of image manipulation options as you would have editing still images. The one drawback is that working on individual frames is incredibly time consuming. Here are a few ideas:
Touch out unwanted detail. Rotoscoping is often used to paint out unwanted detail such as boom microphones. Fast work with the Clone Stamp tool and a relatively detail-free background are prerequisites.
Creative titling. This is an easy one, as once you have created your title using Photoshop Elements text and layer styles, all you have to do is superimpose it on each frame.
Tracking. Programs such as After Effects do the hard work for you, but you can make an object track movement in a video shot by manually placing it on each frame of video.
Light sabres. Popular with Star Wars fans the world over. Film a sequence with your actors holding cardboard tubes, and then paint in the glowing column of light on each frame afterwards.
Motion blurThe Motion Blur filter is one of Photoshop Elements 3's most useful for video editing. Motion blur occurs naturallyin video when objects move during the time it takes to expose one frame of video. We are so accustomed to seeing motion blur that its absence in computer-generated animation looks strange. Motion blur also gives our brain clues about the direction in which an object is moving.
Complementary audioThe video is only half the story. The effectiveness of techniques such as this depend as much on credible audio. It does not have to be authentic, just believable. For the Retinal scan machine, we recorded two-second samples from a number of sources: the most realistic turned out to be an Epson inkjet printer on its cleaning cycle.
Author: Ken McMahon
Rotoscoping using iMovie 4 and Photoshop Elements 3