Inside Macintosh: QuickTime
One way to represent an image is with a pixel map, which stores a color for every pixel. For most images, however, a pixel map is an inefficient storage format. For example, a pixel map containing a solid black image would contain the color black stored over and over and over again. By compressing the image, some of this redundant information can be eliminated. The compressed image can occupy much less storage than a pixel map and can be decompressed to a pixel map when necessary.
In addition, human perception of visual images exhibits special qualities that can be exploited to further compress image data. Image-compression algorithms take advantage of these properties to reduce the amount of information required to describe an image well enough to allow a person to see it.
A lossless compression technique can recreate an exact copy of the original image from the compressed form. Small changes in the image are not objectionable in most applications, however, so most compressors sacrifice some accuracy in order to further decrease the size of the compressed data. However, the compressor carefully chooses the data to omit so that the human visual system compensates for the loss and fools the user into seeing what appears to be the original image.
The Image Compression Manager works only with image data. The Image Compression Manager is primarily useful for compressing pictures that have pixel map images, such as those obtained from scanned still images or digitized video images, or from paint or three-dimensional rendering applications. You do not achieve significant compression treating pictures that are stored as groups of graphics primitives, such as those created by drawing, computer-aided design (CAD), or three-dimensional modeling applications. These applications create images in a compact format that precisely states the characteristics of the objects in the image. In fact, if you were to convert such images to pixel map representations and then compress the resulting image with the Image Compression Manager, you would probably end up with a larger, less precise image than the original. If a picture contains both primitives and pixel map image data (such as text or lines drawn over a painted or digitized image) the Image Compression Manager compresses the pixel map data and leaves the graphics primitives unchanged.
The Image Compression Manager also provides services for compressing and decompressing sequences of images or frames (another term for a single visual image in an image sequence). When processing a sequence, compressors may perform temporal compression , compressing the sequence by eliminating information that is redundant from one frame to the next. This temporal compression differs from spatial compression, which is performed on individual images or frames within a sequence. You may use both techniques on a single sequence.
Compressor components perform temporal compression by comparing the current frame in a sequence with the previous frame. The compressor then stores information about only those pixels that change significantly between the two images. When adjacent images contain substantially similar visual information, as is often the case in movies, temporal compression can significantly reduce the amount of data required to describe the images in the sequence. Your application indicates the desired quality level for the compressed image. The compressor uses this value to govern the extent to which it takes advantage of temporal redundancy between images. There is also a spatial quality level that you can use to control the amount of spatial compression applied to each individual image. Both of these quality values govern the amount of accuracy that is lost in the compressed image.
Note that the Image Compression Manager does not maintain any time information for an image sequence. Rather, the Image Compression Manager maintains the order and content of the images in the sequence while the Movie Toolbox handles all timing considerations.