-----------------------------------------
                   ZCP : ZShell Compressed Pictures v2.0
                        
                      Stephane JANTZEN - February 1996

                     Stephane.Jantzen@scinfo.u-nancy.fr
                        
                        I wish it was a standard ! :)
                 -----------------------------------------



How does it work ?
------------------

I've chosen a RLE such as implemented in PCX format, but adapted to fit B&W
TI-85 pictures. It is the best compromise between the size of the compressed
data and the complexity of the encoding/decoding operations. So let's suppose
we can have :

  76543210
  FBCCCCCC (1) - a 'stream' chunk.
  FDDDDDDD (2) - a 'copy' chunk. 

The two possible byte formats. In both (1) and (2), F represents a flag that
allows to determine what sort of byte it is : if F is zero, then the rest of
the byte conforms to (1), else it conforms to (2).

Here is the problem : PCX-RLE stores the number of data in one byte, then the
data itself in the following byte (this is not _exactly_ how it does, but this
is the idea). But in TI-85, the only data we have to store is 1 or 0, so we
don't need a whole byte to store it, but only a bit. I store it in bit B. Then
the next 6 bits give me the number of contiguous B's in my stream (let's call
that 'count'), until I meet the B's opposite, or the end of the picture, or I
reach count = 63, which is the greater number storable in 6 bits. Here it is !
Just a note : the convention I've used for B is
  
  B = 0 : next pixels are white
  B = 1 : guess what ? :)

It should be enough, but we can make better ...

Then (2) : You can easily see that if 'count' is small, then the compression
ratio for this chunk is low, as I use 8 bits to store 'count' bits. So here is 
the trick (tadaaa !) : if I see that count is smaller or equal than 7, I di-
rectly store the 7 following bit in my byte : this is the byte format (2). The 
local compression ratio for this byte is greater than 1, as I use 8 bits to 
store 7 bits of data. Nevertheless, this is _far_ better than only using the
(1) chunk format. 

Examples
--------

A picture starting with several blank lines should be coded :

    .db  $3F,$3F,$3F, .....

$3F = 00111111, which means : I am in case (1) (given by bit 7), I want to 
store zero's (given by bit 6), and I have 63 of them. (63d = 00111111b)

On the other side, if I meet :

    .db  ...... ,$F3

The most significant bit tells me that I'm in 'copy' mode (2). F3h = 11110011b
so the next seven pixels to be written are 1110011, with the previous conven-
tion.

Related programs
----------------

The first program I've joined uses this algorithm to convert a TI-85 picture 
into a series of db's. Only the executable form is available, as the source   
is quite ugly (written using BP7 - I'm too lazy to rewrite it in C). This
version is a little bit faster and shorter than the previous one. As the com-
pression algorithm hasn't changed, performances are the same as version 1.

The decoder source follows. You'll have to use the encoder to generate a file
containing the db's and concat this file to the ASM program, then recompile
it and send it to your TI. This program has been written for ZShell 4.0 by
Pascal Bouron (Bouron@YOKO.ENS-Cachan.fr) : frenchies on the top :) and is
now fast enough to be fully usable. Its size is actually 97 bytes.

Just one thing about the decoder : as you may have noticed, the data stream 
is zero-terminated. This allows another compression : if your picture ends 
with a series of blank lines, they're just replaced by a $00 : useless data
are not stored.

Ideas
-----

It would be great if a format of this kind was a standard for all Zshell pro-
grammers, and I propose mine to you : I think the compression ratio is good
enough for an efficient use (I had first thought about a LZW compression, but
the decoding time would have been far greater, for a small efficience on the
size of the data). ZCP gives a compression of 50% for 'normal' pictures, which
is rather acceptable. Now that the decoding speed is not a problem anymore
(Thanx God and Pascal), it may be a good alternative to 'normal' picture sto-
rage in a TI-85. Please let me know by mail if you _really_use it.

The PC-based encoder : zcp.exe
------------------------------

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'````````````
`
end

The TI-based decoder source : zcp.asm
-------------------------------------

;---
;--- ZCP decoding routine for ZShell 4.0
;--- original by Stephane Jantzen - 12/16/1995
;--- 
;--- This version by Pascal Bouron - 02/11/96
;---
;--- Please report all bugs to :
;--- Bouron@YOKO.ENS-cachan.fr, or
;--- Stephane.Jantzen@scinfo.u-nancy.fr
;---

#include "ti-85.h"

        .org    $00
        .db     "ZCP decoder v2.0",0

Start:
        DI
        ld      HL,$FFFF
        xor     A
        ld      (HL),A
        ld      DE,$FFFE
        ld      BC,$03FF
        lddr
        exx
        ld      HL,(PROGRAM_ADDR)
        ld      DE,ZCP
        add     HL,DE
        ld      C,$80
Loop:
        ld      A,(HL)
        inc     HL
        ld      D,A
        or      A
        jr      Z,EndDecode
        rlca
        jr      C,Copy
Stream:
        ld      A,D
        and     $3F
        ld      B,A
        ld      A,D
        and     $40
StreamLoop:
        or      A
        jr      Z,SLSuite
        ex      AF,AF'
        ld      A,C
        exx
        or      (HL)
        ld      (HL),A
        exx
        ex      AF,AF'
SLSuite:
        srl     C
        jr      NC,DjnzSL
        exx
        inc     HL
        exx
        ld      C,$80
DjnzSL:
        djnz    StreamLoop
        jr      Loop
Copy:
        ld      B,$07
CopyLoop:
        rlca
        jr      NC,CLSuite
        ex      AF,AF'
        ld      A,C
        exx
        or      (HL)
        ld      (HL),A
        exx
        ex      AF,AF'
CLSuite:
        srl     C
        jr      NC,DjnzCL
        exx
        inc     HL
        exx
        ld      C,$80
DjnzCL:
        djnz    CopyLoop
        jr      Loop
EndDecode:
        call    GET_KEY
        or      A
        jr      Z,EndDecode
        exx
        EI
        ret

        .end

--