Inside Macintosh: QuickTime /

Chapter 3 - Image Compression Manager / Image Compression Manager Reference

Image Compression Manager Functions / Getting Information About Compressed Data

GetMaxCompressionSize

The GetMaxCompressionSize function allows your application to determine the maximum size an image will be after compression. You specify the compression characteristics, including compression type and quality, along with the image.

pascal OSErr GetMaxCompressionSize (PixMapHandle src, 
                                    const Rect *srcRect, 
                                    short colorDepth, 
                                    CodecQ quality, 
                                 CodecType cType,
                                 CompressorComponent codec,
                                 long *size);

src

Contains a handle to the source image. The source image must be stored in a pixel map structure. The compressor uses only the image's size and pixel depth to determine the maximum size of the compressed image.

srcRect

Contains a pointer to a rectangle defining the portion of the source image that is to be compressed. You may set this parameter to nil if you are interested only in information about quality settings. GetCompressionTime then uses the bounds of the source pixel map.

colorDepth

Specifies the depth at which the image is to be compressed. If you set this parameter to 0, the Image Compression Manager determines the appropriate value for the source image. Values of 1, 2, 4, 8, 16, 24, and 32 indicate the number of bits per pixel for color images. Values of 34, 36, and 40 indicate 2-bit, 4-bit, and 8-bit grayscale, respectively, for grayscale images. Your program can determine which depths are supported by a given compressor by examining the compressor information structure returned by the GetCodecInfo function (see "Getting Information About Compressor Components" on page 3-56 for more information on the GetCodecInfo function).

quality

Specifies the desired compressed image quality. See "Compression Quality Constants" beginning on page 3-52 for valid values.

cType

Specifies a compressor type. You must set this parameter to a valid compressor type (see Table 3-3 on page 3-59 for a list of the
available compressor types).

codec

Contains a compressor identifier. Specify a particular compressor by setting this parameter to its compressor identifier. Alternatively, you may use one of the special identifiers:

anyCodec

Choose the first compressor of the specified type

bestSpeedCodec

Choose the fastest compressor of the specified type

bestFidelityCodec

Choose the most accurate compressor of the specified type

bestCompressionCodec

Choose the compressor that produces the smallest resulting data

You can also specify a component instance. This may be useful if you have previously set some parameter on a specific instance of a codec field and want to make sure that the specified instance is used for that operation.

size

Contains a pointer to a field to receive the size, in bytes, of the compressed image.

DESCRIPTION

The Image Compression Manager returns the maximum resulting size for the specified image and parameters. Your application may then use this information to allocate memory for the compression operation.

RESULT CODES

noErr	0	No error
paramErr	-50	Invalid parameter specified
noCodecErr	-8961	The Image Compression Manager could not find the specified compressor