## MMPM/2 Device Driver Reference

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### Luminance Transition Coding

Luminance transition coding (LTC) is based on the reduction of variation in luminance of a region of an image to a single direction. Each region of the image is analyzed to correlate individual luminance values and determine the direction in which the maximum luminance change is occurring. Sample values are then determined by taking the average number of pixels perpendicular to this direction. The luminance values within the region can then be mapped to one of a predetermined set of luminance values based on various functions of increasing luminance. These functions approximate commonly occurring luminance variations in natural images.

The resultant encoding of the region is then compared with the uncompressed image data for the region. The error (in luminance space) is calculated for use in comparing with other encoding types. The selection of an encoding type is based on the following two considerations: (1) comparison of the errors of the different types (where the error as defined above is inversely correlated to the quality of the representation) and (2) the amount of data each type consumes as allowed by the amount of compression to be obtained.

One-Byte Luminance Transition Region Encoding

The data for a one-byte encoding consists of a single luminance value and 2 bits that indicate a direction of increasing luminance of one step, if any. The one-byte encoding is useful because regions that have the same luminance value (known as homogeneous regions), or only a very small change in luminance, are common and should be encoded with as little data as possible.

Two-Byte Luminance Transition Region Encoding

The data for a two-byte encoding consists of the direction of increasing luminance and a transition index value that specifies a base Y value, a delta Y value, and a transition function. The transition index is the index into a table which defines the common set of four byte-transitions. The derivation of this index value is described later.

Four-Byte Luminance Transition Region Encoding

The data for a four-byte encoding consists of a direction of luminance change and four luminance sample values along the specified direction. The four-byte encoding is useful for regions in which the changes in luminance cannot be accurately mapped to the set of predetermined luminance values used in the two-byte encoding. Note that the specified direction (angle) does not include the opposite directions (ñ 180 degrees) because the luminance values need not be increasing (a requirement of the transition index). Thus the opposite directions are implied by the use of the four luminance values.