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50 A Practical Guide to Video and Audio Compression

4.6.1 Aspect ratios Original material could have been shot on larger stock and then reduction-printed down to 35mm for distribution. Aspect ratios broadly fall into three categories:

● Normal aspect ratio that roughly corresponds to 4:3 on a TV set ● Wide screen, which is somewhat equivalent to 16:9 on TV ● Ultra-wide screen, which has to be cropped or letterboxed to fit a TV

Figure 4-4 shows the difference between these aspect ratios. A lot more of the scenery is visible with the wider aspect ratios. In fact, on extremely wide screen presen- tations, the human peripheral vision is included within the viewing angle. This creates an impression of being immersed in the movie. TV does not adequately convey this impres- sion, and it is one reason why we still enjoy going to the cinema to see those big block- buster movies.

Normal

Widescreen

Ultra wide screen

Figure 4-4 Cinema aspect ratios. K80630-Ch04.qxd 3/14/05 10:48 PM Page 51

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The physical imaging area of 35mm is conveniently mapped to 2048 × 1536 for pro- duction work. Anamorphic compression (squeeze prints) allows a wider aspect ratio to be worked on. The final raster is computed digitally or squeezed optically during printing. The normal aspect ratio fits reasonably well on a TV monitor and many TV programs are shot on 35mm or 16mm normal aspect-ratio film stock. Wide screen is accomplished either by shooting on wider film stock and maintaining roughly the same height, or by applying an optical distortion to squeeze a wider image in the horizontal axis into normal aspect-ratio film stock. This is called anamorphic distor- tion, and the same effect happens electronically when normal aspect-ratio video is stretched from 4:3 to fit a 16:9 wide-screen monitor.

4.6.2 Anamorphic Printing Anamorphic lenses were available in the 1960s as a retrofit accessory so that you could attach them to a 16mm movie camera. The same anamorphic lens is designed be attached to the projector. This leads to some distortion if the axis of the lens is not precisely per- pendicular, and the image will appear skewed if it is not fitted correctly. The effect is shown in Figure 4-5. Professional anamorphic lenses are integral to the camera optics and are factory aligned.

4.6.3 IMAX The largest cinema format in current use is the IMAX theatre presentation. This is pro- jected onto a screen that is 215 meters wide by 156 meters high. The audience sits abnor- mally close to this screen, therefore artifacts in the rendering process will be more visible than usual. For this reason the film must be rendered at quite a high resolution. The film is presented on 70mm stock with the picture oriented sideways, and it passes horizontally through the projector. The frames occupy an area that is roughly three times the size of a normal 70mm frame, with a corresponding increase in resolution. Figure 4-6 illustrates a single frame of IMAX film. Note the placement of the small circular holes. This ensures that the operator loads the film the correct side up.

Correctly adjusted Off axis by 15 degrees

Figure 4-5 Incorrect anamorphic stretch. K80630-Ch04.qxd 3/14/05 10:48 PM Page 52

52 A Practical Guide to Video and Audio Compression

IMAX 70 mm format

Imaging area 69.6 mm x 48.5 mm 70 mm

Figure 4-6 IMAX film dimensions.

IMAX programs tend to be quite short. Due to the nature of the projection equipment and the way the film is spooled, older installations were limited to a maximum time of 120 minutes. Upgrades to the projector extend this to 150 minutes. IMAX movies are also pro- jected at 48 frames per second. This will reduce the available run time to 75 minutes. Some IMAX programs are shot in 3D, giving a separate image to the left and right eye. Compression work to display this content on normal TV sets begins with a 50% reduction right off the bat, simply by choosing one or the other image. The 48 fps frame rate is used in IMAX presentations to produce a pair of images for 3D, interleaved, with left and right on alternate frames, effectively presenting a 24 fps frame rate.

4.6.4 VistaVision One format of note is VistaVision. This is shot on 70mm film stock running horizontally, which is the same way IMAX film travels through the projector gate. In this case the for- mat is an ultra-wide aspect ratio. In fact, VistaVision uses one of the largest possible imag- ing areas of any film format (the Todd-AO format is a similar size). While it was used by production companies in the 1960s, all the available cameras were later acquired by Lucasfilm during the production of the early Star Wars movies because they found it to be ideal for use in special effects work. They adapted and modified the format throughout their work on these films. Figure 4-7 shows an example of the VistaVision film format. Working digitally makes most of these formats obsolete, so optical film is usually just 35 or 70mm for projection. The imaging area on 70mm film stock is approximately 48.5mm by 22.1mm. If this were scanned at the maximum 115 pixels per mm, resolution would yield a picture that is 5577 × 2541. The corresponding imaging area on 35mm film is of course much smaller at 21.0 × 15.2mm (2415 × 1748).