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2004.05.19 14:02 "Re: Further doubt on bits per sample", by Bob Friesenhahn
On Wed, 19 May 2004 mayankg@iiitb.ac.in wrote: > I have the ImageMagick software. > Could anybody provide some pointers (which module or file name) as to where i > could find the code for packing/unpacking all kinds of bits per samples cases? In ImageMagick this is done using a combination of code in coders/tiff.c and PushImagePixels()/PopImagePixels(). However, GraphicsMagick (http://www.GraphicsMagick.org/) does things quite differently. The bit stream sample pack/unpack is so short I have attached it to this email (bit_stream.h). See coders/tiff.c in GraphicsMagick 1.1.1 for how these functions are used. /* Copyright (C) 2003 GraphicsMagick Group This program is covered by multiple licenses, which are described in Copyright.txt. You should have received a copy of Copyright.txt with this package; otherwise see http://www.graphicsmagick.org/www/Copyright.html. Interfaces to deal with marshalling quantums to and from a bit-stream. Written by Bob Friesenhahn, September 2003 */ #ifndef _MAGICK_BIT_STREAM_H #define _MAGICK_BIT_STREAM_H #if defined(__cplusplus) || defined(c_plusplus) extern "C" { #endif static const unsigned int BitAndMasks[9] = { /* Same as (~(~0 << retrieve_bits)) */ 0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff }; /* Bit stream reader "handle" */ typedef struct _BitStreamReadHandle { const unsigned char *bytes; int bits_remaining; } BitStreamReadHandle; /* Initialize Bit Stream for reading */ static inline void BitStreamInitializeRead(BitStreamReadHandle *bit_stream, const unsigned char *bytes) { bit_stream->bytes = bytes; bit_stream->bits_remaining = 8; } /* Return the requested number of bits from the current position in a bit stream. Stream is read in most-significant bit/byte "big endian" order. */ static inline unsigned int BitStreamMSBRead(BitStreamReadHandle *bit_stream, const unsigned int requested_bits) { register unsigned int remaining_quantum_bits, quantum; remaining_quantum_bits = requested_bits; quantum = 0; while (remaining_quantum_bits > 0) { register unsigned int octet_bits; octet_bits = remaining_quantum_bits; if (octet_bits > bit_stream->bits_remaining) octet_bits = bit_stream->bits_remaining; remaining_quantum_bits -= octet_bits; bit_stream->bits_remaining -= octet_bits; quantum = (quantum << octet_bits) | ((*bit_stream->bytes >> (bit_stream->bits_remaining)) & BitAndMasks[octet_bits]); if (bit_stream->bits_remaining == 0) { bit_stream->bytes++; bit_stream->bits_remaining=8; } } return quantum; } /* Bit stream writer "handle" */ typedef struct _BitStreamWriteHandle { unsigned char *bytes; int bits_remaining; } BitStreamWriteHandle; /* Initialize Bit Stream for writing */ static inline void BitStreamInitializeWrite(BitStreamWriteHandle *bit_stream, unsigned char *bytes) { bit_stream->bytes = bytes; bit_stream->bits_remaining = 8; } /* Write quantum using the specified number of bits at the current position in the bit stream. Stream is written in most-significant bit/byte "big endian" order. */ static inline void BitStreamMSBWrite(BitStreamWriteHandle *bit_stream, const unsigned int requested_bits, const unsigned int quantum) { register unsigned int remaining_quantum_bits = requested_bits; while (remaining_quantum_bits > 0) { register unsigned int octet_bits; octet_bits = remaining_quantum_bits; if (octet_bits > bit_stream->bits_remaining) octet_bits = bit_stream->bits_remaining; remaining_quantum_bits -= octet_bits; if(bit_stream->bits_remaining == 8) *bit_stream->bytes = 0; bit_stream->bits_remaining -= octet_bits; *bit_stream->bytes |= (((quantum >> (remaining_quantum_bits)) & BitAndMasks[octet_bits]) << (bit_stream->bits_remaining)); if (bit_stream->bits_remaining == 0) { bit_stream->bytes++; bit_stream->bits_remaining=8; } } } #if defined(__cplusplus) || defined(c_plusplus) } #endif #endif /* _MAGICK_BIT_STREAM_H */