QuickTime Compression and Decompression Guide: Compression Quality Constants

< Previous PageNext Page >

Hide TOC

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 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 “The Image Description Structure” ) 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.

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;

Field	Description
`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.

< Previous PageNext Page >

Hide TOC

Last updated: 2006-01-10

Did this document help you?
Yes: Tell us what works for you. It’s good, but: Report typos, inaccuracies, and so forth. It wasn’t helpful: Tell us what would have helped.




	Get information on Apple products. Visit the Apple Store online or at retail locations. 1-800-MY-APPLE Copyright © 2007 Apple Inc. All rights reserved. \| Terms of use \| Privacy Notice