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Chapter 3 - Image Compression Manager / Image Compression Manager Reference

Data Types

Compression Quality Constants

Compressor components may allow applications to assert some control over the image quality that results from a compression or decompression operation. For example, the CompressSequenceBegin function (described on page 3-110) provides the spatialQuality and temporalQuality parameters so that applications can indicate the level of image accuracy desired within individual frames and across adjacent frames in a sequence, respectively. These quality values become a property of the compressed data and are stored in the image description structure (described on page 3-43) associated with the image or sequence.

For a given compression operation, your application can determine the quality that the component supports by issuing the GetCompressionTime function (described on page 3-64).

The CodecQ data type defines a field that identifies the quality characteristics of a given image or sequence. Note that individual components may not implement all the quality levels shown here. In addition, components may implement other quality levels in the range from codecMinQuality to codecMaxQuality. Relative quality should scale within the defined value range. Values above codecLosslessQuality are reserved for use by individual components.

/* compression quality values */
#define codecMinQuality 0x000L   /* minimum valid value */
#define codecLowQuality 0x100L   /* low-quality reproduction */
#define codecNormalQuality    
                        0x200L   /* normal-quality repro */
#define codecHighQuality
                        0x300L   /* high-quality repro */
#define codecMaxQuality 0x3FFL   /* maximum-quality repro */
#define codecLosslessQuality
                        0x400L   /* lossless-quality repro */
typedef unsigned long CodecQ;

Constant descriptions

codecMinQuality

Specifies the minimum valid value for a CodecQ field.

codecLowQuality

Specifies low-quality image reproduction. This value should correspond to the lowest image quality that still results in acceptable display characteristics.

codecNormalQuality

Specifies image reproduction of normal quality.

codecHighQuality

Specifies high-quality image reproduction. This value should correspond to the highest image quality that can be achieved with reasonable performance.

codecMaxQuality

Specifies the maximum standard value for a CodecQ field.

codecLosslessQuality

Specifies lossless compression or decompression. This special value is valid only for components that can support lossless compression or decompression.