Every QuickTime image has a "", which is the horizontal spacing of luma sampling instants vs.the vertical spacing of picture lines when displayed on a display device. Applications must know the pixel aspect ratio in order to draw round circles, lines at a specified angle, etc.
Every QuickTime image has a "", which is a reference rectangle specified relative to the width pixels and height lines of the image by the CleanApertureImageDescriptionExtension structure. The parameters of this structure are represented as a fraction N(umerator)/D(enominator). The fraction may or may not be in reduced terms.
Using the pixel aspect ratio and clean aperture dimensions, we can derive the "picture aspect ratio", which is the horizontal to vertical distance ratio of the clean aperture on a display device. The picture aspect ratio is typically 4:3 or 16:9.
The clean aperture is used to relate locations in two QuickTime images. Given two QuickTime images with identical picture aspect ratio, you can assume that the top left corner of the clean aperture of each image is coincident, and the bottom right corner of the clean aperture of each image is coincident.
The clean aperture also provides a deterministic mapping between a QuickTime image and the region of the video signal (as seen on a display device) from which it was captured or to which it will be played. Each video interface standard (e.g., NTSC, digital 525, PAL, digital 625) also defines a "clean aperture" in terms of its electrical signal.
The QTPlgCleanAperture class provides you with properties and methods that let you whether access or create new instances of CleanApertureImageDescriptionExtension description structure.
The QTPlgCleanAperture class constructors are called when you create a new instance of the class, while the ˜QTPlgCleanAperture class destructor, is called automatically when the class is no more in use. Also, you can invoke the class destructor by setting the instance of the class to nil.
Once a new class instance is returned, it is good practice to check the class property to be sure that the new object can be used. A QTPlgCleanAperture class instance is valid if the value of the property is not 0.
The class provides you with the following initializer methods: