gslaplay/README.md

# gslaplay
Apple IIgs GSLA Player

Example of a GS LZB Animation Player

New Apple IIgs Animation file format
GS LZ Byte Compressed Animation
Why?  When there are so many image formats would we need another one?   It<49>s
because $C2/Paintworks animation format is just terribly inefficient.

Care is taken in the encoder, to make sure the 65816 does not have to cross bank
boundaries during any copy.  This is so we can use the MVN instruction, and so
we can reduce the number of bank checks in the code.   Some bank checks will be
required, because we are dealing with data sizes that exceed 64KB at times.

We will have an opcode, that says <20>source data bank has changed<65>

Goals include a good balance between file size, and playback performance
(since one often makes a trade off with the other).

The file is defined as a byte stream.

I will attempt to define the format as

file-offset:   the thing that is at the offset

Header of the File is 20 bytes as follows

File Offset    Data   Commentary
------------------------------------------------------------------
0              0x47   ; <20>G<EFBFBD>  Graphics
1              0x53   ; <20>S<EFBFBD> 
2              0x4C   ; <20>L<EFBFBD>  LZB
3              0x41   ; <20>A<EFBFBD>  Animation

; File Length, is the total length of the file
4              FileLengthLow    ; Low byte, 32-bit file length
5              LengthLowHigh    ; High byte of low word
6              LengthHighLow    ; Low byte, of high word
7              LengthHighHigh   ; High Byte, of high word

; 16 bit word with version #
8              VL     ;  Version # of the file format, currently only version 0
9              VH     ;  Version High Byte
				      ;  %RVVV_VVVV_VVVV_VVVV
				      ;  V is a version #, 0 for now
					  ; R is the MSB,  R = 0 no ring frame
					  ; R = 1, there is a ring frame
					  ; A Ring Frame is a frame that will delta from the last
					  ; frame of the animation, back to the first, for smoother
					  ; looping ,  If a ring frame exists, it<69>s also in the
					  ; frame count

// next is a word, width in bytes (only 160 for now)
0xA				WL    ; Display Width in bytes low byte
0xB				WH    ; Display Width in bytes high byte
// next is a word, height (likely 200 for now, in tiled mode a multiple of 16)
0xC				HL    ; Display Height in bytes, low byte
0xD				HH    ; Display Height in bytes, high byte
// 2 bytes, Frame Size in Bytes, since a <20>Frame<6D> may contain more than just the
// width * height, worth of pixels, for now this is $8000, or 32768
0xE             FBL   ; Frame Buffer Length Low
0xF             FBH   ; Frame Buffer Length High

// 4 byte, 32-bit, Frame Count (includes total frame count, so if there is a
// ring frame, this is included in the total)
0x10            FrameCountLow
0x11            FrameCountLowHigh
0x12            FrameCountHighLow
0x13            FrameCountHigh


After this comes AIFF style chunks of data,  basically a 4 byte chunk name,
followed by a 4 byte length (inclusive of the chunk size).   The idea is that
you can skip chunks you don<6F>t understand.

File Offset:
0x14            First Chunk  (followed by more Chunks, until end of file)

Chunk Definitions
Name:  <20>INIT<49>   -  Initial Frame Chunk, this is the data used to first
                   initialize the playback buffer
0:              0x49  ; <20>I<EFBFBD>
1:              0x4E  ; <20>N<EFBFBD>
2:              0x49  ; <20>I<EFBFBD>
3:              0x54  ; <20>T<EFBFBD>
// 32 bit long, length, little endian, including the 8 byte header
4:              length low low
5:              length low high
6:              length high low
7:              length high high

8:  This is a single frame of data, that decodes/decompresses into frame sized
bytes (right now 0x8000)
This data stream includes, an end of animation opcode, so that the normal
animation decompressor, can be called on this data, and it will emit the initial
frame onto the screen

Name: <20>ANIM<49> - Frames
0:	0x41 <20>A<EFBFBD>
1:	0x4E <20>N<EFBFBD>
2:	0x49 <20>I<EFBFBD>
3:	0x4D <20>M<EFBFBD>
// 32 bit long, length, little endian, including chunk header
4:	length low low
5:	length low high
6:	length high low
7:	length high high

// This is followed by the frames, with the intention of decompressing them at
// 60FPS, which is why no play speed is included, if you need a play-rate slower
// than this, blank frame<6D>s should be inserted into the animation data

// Every attempt is made to delta encode the image,  meaning we just encode
// information about what changed each frame.   We attempt to make the size
// efficient by supporting dictionary copies (where the dictionary is made up of
// existing pixels in the frame buffer).

Command Word, encoded low-high, what the bits mean:

// xxx_xxxx_xxxx_xxx is the number of bytes 1-16384 to follow (0 == 1 byte)

%0xxx_xxxx_xxxx_xxx0 - Copy Bytes - straight copy bytes
%1xxx_xxxx_xxxx_xxx1 - Skip Bytes - skip bytes / move the cursor
%1xxx_xxxx_xxxx_xxx0 - Dictionary Copy Bytes from  frame buffer to frame buffer

%0000_0000_0000_0001- Source Skip -> Source pointer skips to next bank of data
%0000_0000_0000_0011- End of Frame - end of frame
%0000_0000_0000_0111- End of Animation / End of File / no more frames

// other remaining codes, are reserved for future expansion