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Inside Macintosh: QuickDraw GX Environment and Utilities /

Chapter 7 - QuickDraw GX Stream Format / QuickDraw GX Stream Format Reference

Shape Object Omit Byte Constants and Data Types

Bit Image Omit Byte Masks and Shifts

The gxOmitBitImageMask enumeration defines which bits in an omit byte correspond to the data compression opcode for additional field descriptors. The sequence of data is also defined. The omit byte and its related data sequence is given in the section "New Bit Image Object Data" on page 7-49.

enum gxOmitBitImageMask {
   gxOmitBitImageRowBytesMask = 0xC0,
   gxOmitBitImageHeightMask   = 0x30,
   gxOmitBitImageDataMask     = 0x08
};

Constant descriptions

gxOmitBitImageRowBytesMask

The mask used to select the data compression bits for the rowBytes field descriptor.

gxOmitBitImageHeightMask

The mask used to select the data compression bits for the height.

gxOmitBitImageDataMask

The mask used to select the data compression bits for the image.

Once one of the gxOmitBitImageMask enumeration masks has been used to select a data compression opcode for a field descriptor in the gxBitmap structure, the corresponding bit shift from the gxOmitBitImageShift enumeration can be applied to the selected bits. The selected bits must be moved to the right by the indicated number of bits to isolate the data compression opcode so that it can be compared to other values.

enum gxOmitBitImageShift {
   gxOmitBitImageRowBytesShift   = 6,
   gxOmitBitImageHeightShift     = 4,
   gxOmitBitImageDataShift       = 3
};

Constant descriptions

gxOmitBitImageRowBytesShift

The bit shift required to isolate the compression bits for the rowBytes field descriptor.

gxOmitBitImageHeightShift

The bit shift required to isolate the compression bits for the height.

gxOmitBitImageDataShift

The bit shift required to isolate the compression bits for the image.