[A82] Re: the compression

To: assembly-82@lists.ticalc.org
Subject: [A82] Re: the compression
From: "Roger Levy" <trip_n_save@hotmail.com>
Date: Sat, 24 Feb 2001 17:41:32 -0000
Reply-To: assembly-82@lists.ticalc.org
>.db %11111111
>.db %10000000
>.db %10000000
>.db %10000000
>.db %11111111
>.db %00001000
>.db %00001000
>.db %00001000
this would be encoded like this:

the header:
%110  %1 = this tile has redundant bytes
      %10 = there are 2 repeated bytes

then the bytes that are repeated:
%10000000 = byte0
%00001000 = byte1

then the rest:
%101  %1 = encoded
      %01 = $FF
%110,%110,%110  %1 = encoded
                %10 = byte0
%101
%111,%111,%111  %1 = encoded
                %11 = byte1

so the total bits would be 43 or 67% :]

now, for this one....
>.db %11110111 X
>.db %00000111
>.db %01111111 X
>.db %01100011
>.db %01111111 X
>.db %01110000 .db %11110111 X
>.db %00110110
unfortunately it wouldn't be compressable and would be indicated as not 
compressed and thus turn to 67 bits.
if the compresser did attempt to encode the two repeated bytes (marked with 
X's) it would inflate the data to 67 bits as well, ironically. so it isn't 
perfect... this is because of the 16 bits needed to define exactly what 
those repeated bytes are. however with some tweaking of the actual image, 
better compression could be achieved.
im including a table in Word format that shows how it performs on all types 
of tiles. The trick with the format is to make the tiles catored to it. They 
can still look good, and be compressed. For instance, the tiles for Metroid 
on average get 6:8, just because of the way they happen to be, which is with 
lots of %FF's strewn about. Tiles that are sort of like "wallpaper", or a 
pattern of some sort that doesn't have any blank bytes, don't compress well; 
mostly "block"-type tiles or ones with 2 or more blank or filled lines 
compress well.

again, anyone who is able to write the PC-based encoder, even if you don't 
understand the format i included this morning (it really was just a little 
thingy i made for my own reference when making the analysis chart and 
decoder), please e-mail me and I'll explain to you exactly how it should 
work. i want to be able to release this soon so people can start using it in 
their games.

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