Bload Compressor/1

 BBS: Inland Empire Archive
Date: 12-27-92 (06:19)             Number: 242
From: RICH GELDREICH               Refer#: NONE
  To: ALL                           Recvd: NO  
Subj: Bload Compressor/1             Conf: (2) Quik_Bas
    I'll get directly to the point: if you use BSAVE and BLOAD to
compress SCREEN 13 images, then this program should be a great help. The
purpose of this program is to compress SCREEN 13 (320x200x256) images to
a BLOAD'able file. To decompress the file, you call a VERY FAST assembly
language decompressor that quickly sets the palette and writes the image
to the screen. It used to be small, until I documented it... Ooops.

'Page 1 of ENCODE13.BAS begins here.
'SCREEN 13 (320x200x256) Screen Compressor (SP1.BAS)
'Public Domain By Rich Geldreich, December 26, 1992

'Anyone  may  use this program in anything they want,  as long as the
'original author(me,  duh)  is  given  credit  where credit is due...
'Thanks.   I'd appreciate a little cash on the side if you  make  any
'money  off a product that uses this program...   :-) If you make any
'neat modifications/optimizations to this program or the ASM decoder,
'I would really like to seem them!

'    This simple SCREEN 13  compression  program uses an LZ77 variant
'to compress SCREEN 13 images.  A FAST assembly subroutine is used to
'decompress the image back to the screen.    The  compression  should
'always beat  PCX, and should come fairly close or beat out GIF under
'most cases.
'    The assembly decompressor's  speed  is several magnitudes faster
'than the quickest GIF decoder I've seen, VPIC 5.1. (Look how  simple
'it  is  and  you'll know why!) BTW- The output stage of this program
'was optimized for  decoding  speed,   not for compression.   Several
'optimizations could be added to increase this program's  compression
'performance,   such  as  entropy  encoding  on the distance & length
'tokens(which would slow the decoder down immensely),  increasing the
'sizes of the sliding dictionary and look ahead buffers,  and further
'optimizing the  non-greedy  aspect  of  this  LZ77 implementation to
'choose the best character/match combinations to store in the  output
'    The assembly subroutine is for 286's and  above,   only.    This
'program does NOT work under QuickBASIC,  only PDS and VB/DOS because
'of the use of BYVAL.
'    Any  questions,   cash/and  or  death   threats   call   me   at
'(609)-742-8752  2:30pm  -  11:30pm  eastern  time  or  send  a  self
'addressed stamped evelope (SASE) to:
'                     Rich Geldreich
'                     410 Market St.
'                     Gloucester City, NJ 08030
'Possible  uses of this program:  Use a GIF or PCX converter(or SHELL
'out  to  VPIC)  to  display  the  image  you  want  to  use  in your
'application on SCREEN 13.   Then encode the image with this program.
'You  can  then  instantly  recall  the  image using the fast Decom13
'assembly language subroutine.
'Declaration  for  the assembly decompressor.   If the area of memory
'passed does not start with  "RG",  the compressed image's signature,
'then this routine will just return without  doing  anything.    This
'prevents your machine from hanging when you pass it a bum pointer.
DECLARE SUB Decom13 (BYVAL InSegment, BYVAL InOffset)

CONST True = -1, False = 0
'A larger buffer size would surely increase compression.
CONST BufferSize = 4096, HashSize = 4096
CONST Null = BufferSize, Threshold = 2, MaxMatch = 273
CONST MaxCompares = 300 'Controls compression ratio vs. speed

'Arrays for LZ77 style compression with multiple linked lists
DIM SHARED RingBuffer((BufferSize + MaxMatch - 1) - 1)
DIM SHARED NextCell((BufferSize + HashSize + 1) - 1)
DIM SHARED LastCell((BufferSize + HashSize + 1) - 1)
'Temp. holding buffer for compression tokens
DIM SHARED CodeBuffer(16 * 3 - 1)

'Misc. stuff
DIM SHARED DoneFlag, xp, yp, xl, yl, xh, yh
DIM SHARED Match.Length, Match.Position, Match.Distance
DIM SHARED IOBuffer$, IOPointer
DIM SHARED CodePointer, CodeCounter, OrMask AS LONG, BitAccum AS LONG


FOR a = 1 TO 100  'draw us some garbage
    x = RND * 319: y = RND * 199: c = RND * 255
    CIRCLE (x, y), RND * 60, c: PAINT (x, y), RND * 255, c
FOR a = 1 TO 200: LINE -(RND * 319, RND * 199), RND * 255: NEXT
Compress13 "coolfile.bci" 'compress the screen to coolfile.bci

'Allocate 64,000 bytes for a worst case scenario, decrease this of
'course to match the image's compressed size in bytes...
'Continued on page 2

--- MsgToss 2.0b
 * Origin: Computer Co-Op - Voorhees, NJ | Ted Hare (1:266/29)
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