Clean aperture ('clap')
An extension that defines the relationship between the pixels in a stored image and a canonical rectangular region of a video system from which it was captured or to which it will be displayed.
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Overview
The clean aperture extension defines the relationship between the pixels in a stored image and a canonical rectangular region of a video system from which it was captured or to which it will be displayed. This can be used to correlate pixel locations in two or more images — possibly recorded using different systems — for accurate compositing. This is necessary because different video digitizer devices can digitize different regions of the incoming video signal, causing pixel misalignment between images. In particular, a stored image may contain “edge” data outside the canonical display area for a given system.
The clean aperture is either coincident with the stored image or a subset of the stored image; if it is a subset, it may be centered on the stored image, or it may be offset positively or negatively from the stored image center.
The clean aperture extension contains a width in pixels, a height in picture lines, and a horizontal and vertical offset between the stored image center and a canonical image center for the given video system. The width is typically the width of the canonical clean aperture for a video system divided by the pixel aspect ratio of the stored data. The offsets also take into account any “overscan” in the stored image. The height and width must be positive values, but the offsets may be positive, negative, or zero.
These values are given as ratios of two 32-bit numbers, so that applications can calculate precise values with minimum roundoff error. For whole values, the value should be stored in the numerator field while the denominator field is set to 1.