PackBits
PackBits Perform RLL byte compression on arbitrary data void PackBits(srcPtr, destPtr, srcLen ); Ptr *srcPtr ; address of a pointer to data to pack Ptr *destPtr ; address of a pointer to a destination buffer short srcLen ; length of data to compress
PackBits compresses up to 32,768 bytes of data by replacing sequences of 3 or more identical bytes with a 2-byte code. It is most often used to compress
image data, as found in a BitMap or a PixMap. The worst case compression can be calculated using the formula: (srcLen + (srcLen + 126) / 127).
srcPtr is the address of a pointer to some data to be compressed. Upon
return, the pointer has been adjusted to just beyond the data that has
been compressed; i.e., ready for the next call.
destPtr is the address of a pointer to a buffer to hold the compressed data.
Upon return, it has been adjusted to just beyond the end of the
compressed data.
srcLen is the size, in bytes, of the data to be compressed.
Notes: Since image data is often full of "white-space", this is ideal for packing
data for long-term RAM storage and before writing it to disk or storing it in
a resource. It is not quite so useful for compressing text or other data. Note
the misnomer: PackBits does not pack bits; it packs sequences of bytes. PackBits was formerly limited to compressing data in 127 bytes blocks. To compress more than 127 bytes, a programmer would have to break the
data up and make multiple calls to PackBits. Starting with System Software version 6.0.2, this restriction has been lifted. If you want your
program to run on Systems prior to version 6.0.2, you must obey the
127-byte limit.
Typical usage is to pack each line of a BitMap or PixMap separately, as in the example below. The example procedure, PackScreen, typically
compresses a black and white screen image from 32K down to about 4K or
5K.
Example
#include <ToolUtils.h>
long mapSize, PackScreen(Ptr); mapSize = (long) screenBits.bounds.bottom * screenBits.rowBytes;
savePtr = NewPtr( mapSize + 1 ); /* allocate storage buffer */ mapSize = PackScreen( savePtr );
SetPtrSize( savePtr, mapSize ); /* shrink to actual size */ UnpackScreen( savePtr );
/* ================== pack and save screen image to caller's buffer */
/* ================== returns size of packed screen */
long PackScreen(Ptr destBuf ) {
short j;
destPtr = destBuf; [TOKEN:12074] make a copy to use */
srcPtr = screenBits.baseAddr;
for ( j=0; j< screenBits.bounds.bottom; j++ ) {
}
return( destPtr-destBuf );
}
/* =================== unpack packed screen image, storing to screen */
void UnpackScreen(Ptr srcPtr ) {
short j;
destPtr = screenBits.baseAddr;
for ( j=0; j< screenBits.bounds.bottom; j++ ) {
}
}