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Chapter 3 - Image Compression Manager / Image Compression Manager Reference

Image Compression Manager Functions / Changing Sequence-Compression Parameters

SetCSequenceKeyFrameRate

The SetCSequenceKeyFrameRate function adjusts the key frame rate for the current sequence.

pascal OSErr SetCSequenceKeyFrameRate (ImageSequence seqID, 
                                       long keyframerate);

seqID

Contains the unique sequence identifier that was returned by the CompressSequenceBegin function (described on page 3-100).

keyframerate

Specifies the maximum number of frames allowed between key frames. Key frames provide points from which a temporally compressed sequence may be decompressed. Use this parameter to control the frequency at which the compressor places key frames into the compressed sequence.

The compressor determines the optimum placement for key frames based upon the amount of redundancy between adjacent images in the sequence. Consequently, the compressor may insert key frames more frequently than you have requested. However, the compressor will never place fewer key frames than is indicated by the setting of the keyFrameRate parameter. The compressor ignores this parameter if you have not requested temporal compression (that is, you have set the temporalQuality parameter to the CompressSequenceBegin function to 0).

If you set this parameter to 0, the Image Compression Manager only places key frames in the compressed sequence when you call the CompressSequenceFrame function (described on page 3-105) and set the value of the codecFlagForceKeyFrame flag to 1 in the flags parameter. If you pass in any number other than 0, it specifies the number of non-key frames between key frames. Set this parameter to 1 to specify all key frames, to 2 to specify every other frame as a key frame, to 3 to specify every third frame as a key frame, and so forth.

DESCRIPTION

The key frame rate for a sequence specifies the maximum number of frames allowed between key frames. Key frames provide points from which a temporally compressed sequence may be decompressed. The new key frame rate takes effect with the next image in the sequence. See "Defining Key Frame Rates" on page 3-47 for more information about key frames.

RESULT CODES

noErr	0	No error
paramErr	-50	Invalid parameter specified

SEE ALSO

You set the default key frame rate for a sequence with the keyFrameRate parameter to the CompressSequenceBegin function (described on page 3-100).