AN INTRODUCTION TO UNDERSTANDING/HACKING 8 BIT GRAPHICS
AN INTRODUCTION TO UNDERSTANDING/HACKING 8 BIT GRAPHICS
WRITTEN BY: PREZ prez@execulink.com
http://zenith.lfx.org/prez/
efnets #romhack
x. PREFACE
btw, i'm sorry if i'm not a good text writter, it's just i had a hard time finding a text that explained this, so i sat down for a night and figured it all out myself. by the time your done reading this text, you should understand how the nes's 8 bit graphics work, and how to hack them (manually, for whatever reason you would want to). THIS DOCUMENT IS NOT FOR NEWBIES, the only reason i wrote this text is because i couldn't find anyone else who would explain it, so maybe now they'll know how to when someone else wants to know. i repeat, NOT FOR NEWBIES, this is not the kinda thing your going to want to read if all you want to do is change a sprite in a game. there is software for doing that stuff.. however if you plan on making software that does that kind of thing.. this might be the document for you :)
1. WHAT ARE WE HACKING PREZ?
ok, nes sprites are stored in 8-bit hex strings like the following:
01 02 03 04 05 06 07 08 11 12 13 14 15 16 17 18
by the time your done reading this document, you should understand how that string works, and how to hack it. although this isn't all logic, and you will either have to remember some things, or make a chart.. if you know binary, your all set, that's all there is to it.
2. WHY ARE WE NOT DIVING RIGHT INTO 8-BIT?
another good question, the reason i'm going to start with bpp1 8x8 sprites is because there's no point in trying to understand bpp1 overlays (which is what i call nes sprites) without understanding bpp1. all nes sprites are is two bpp1 sprites put on top of each other.
3. OK, THEN BPP1 WILL DO.. START TEACHING
ok, as i explained above, nes sprites are just two bpp1 sprites put on top of eachother.. if you don't understand what i'm talking about.. good, you don't have to yet. anyways, since a nes sprite is two bpp1 sprites, the hex code we have for a nes sprite:
01 02 03 04 05 06 07 08 11 12 13 14 15 16 17 18
can be separated into two bpp1 sprites:
01 02 03 04 05 06 07 08
and
11 12 13 14 15 16 17 18
and since we're only dealing with one bpp1 sprite, we'll just take the latter one for our examples.
4. OK, SO WHAT DOES THIS HEX STRING DO?
well, obviously the hex string contains the data that will be present in the sprite, but how do we convert the hex to a 8x8 sprite? pretty easy. you see, each hex code is responsible for 1 line of the 8x8 sprite. (see diagram)
11 12 13 14 15 16 17 18
| | | | | | | |
A A A A A A A A -----+ | | | | | | |
A A A A A A A A --------+ | | | | | |
A A A A A A A A -----------+ | | | | |
A A A A A A A A --------------+ | | | |
A A A A A A A A -----------------+ | | |
A A A A A A A A --------------------+ | |
A A A A A A A A -----------------------+ |
A A A A A A A A --------------------------+
to break it down even more, each character in the hex string is responible for one half of the 1 line in the 8x8 string. the first half of the hex code represents the first four pixels, and the second half of the hex code represents the last four pixels.. (see next diagram)
hex code: 13 1 3
A A A A A A A A
now, each letter/number from 0-F (you know your hex, don't you?!) represents a certain sequence of pixels, (see NEXT diagram)
0 . . . .
1 . . . X
2 . . X .
3 . . X X
4 . X . .
5 . X . X
6 . X X .
7 . X X X
8 X . . .
9 X . . X
A X . X .
B X . X X
C X X . .
D X X . X
E X X X .
F X X X X
if you looked very cafefully (or even glanced) at the diagram above, you will notice that the chart is perfect binary ascention.. making this pattern pretty easy to remember.. so you find your first and second half of the hex code's pixel counterpart, and put them together, then you have the line.. do that for all 8 hex codes, and you will have yourself a complete 8x8 bpp1 sprite..
so ok, before you forget all of this, lets do a little example sample :P
5. YAY, an example sample..
take the following string, and use the chart above (or your head, if your name is prez's computer and you have a pentuim based processor for a brain) to 'calculate' the bpp1 sprite.
00 7C 82 80 7C 02 82 7C
now, i expect you to do it.. but if your understanding this pretty solidly, you don't really have to ;P you can just look at what i got.
0 . . . . | . . . . 0 (00)
7 . X X X | X X . . C (7C)
8 X . . . | . . X . 2 (82)
8 X . . . | . . . . 0 (80)
7 . X X X | X X . . C (7C)
0 . . . . | . . X . 2 (02)
8 X . . . | . . X . 2 (82)
7 . X X X | X X . . C (7C)
whoa, it's an "S" from everyones favorite game, Fairy Princess!#%$.. wasn't that fun?
6. uhm. prez.. my nes hacking..
yeah yeah yeah, i'm getting there.. anyways, as we all know, each nes sprite has a 4 color limited pallette.. and we'll call these four colors black, dark, light, and white. you might be catching on, but if your not..
to get four colors, they put two bpp1 sprites on top of eachother.. now, when sprite number 1's "on" pixels are put into sprite number 2's "on" pixels, you'll get "white".. get it? here's a quick chart for really stupid people who haven't caught on..
OFF + OFF = OFF (BLACK)
ON + OFF = 50/50 (DARK)
OFF + ON = 50/50 (LIGHT)
ON + ON = ON (WHITE)
i'm having a hard time explaing this since it's 3:01am.. so i'll use another diagram incase you STILL haven't understood my babblings.
. . . . . . . . X . . X . . . .
. X X X X X . . . X . X . . . .
X . . . . . X . . . X X . . . .
X . . . . . . . . . . X . . . .
. X X X X X . . plus . . . X X . . .
. . . . . . X . . . . X . X . .
X . . . . . X . . . . X . . X .
. X X X X X . . . . . X . . . X
would have overlapping pixels, that make ON/ON, ON/OFF, OFF/ON, and OFF/OFF combinations, until we have ONE sprite.. which would look like:
Y . . X . . . .
. O X O X X . .
X . Y X . . X .
X . . X . . . .
. X X O O X . .
. . . Y . Y X .
X . . Y . . O .
. X X O X X . Y
which, with Y O X . representing the four different colors in the pallette.. would be a NES sprite.. the product of two overlapped bpp1 8x8 sprites..
7. now that i've confused you, lets conclude.
so, the nes sprite hex string:
01 02 03 04 05 06 07 08 11 12 13 14 15 16 17 18
is actually just two bpp1 strings put together.. and that took far too long to explain.
i'm really sorry if i confused anyone, but you can forward any questions my way, and reach me at prez@execulink.com.. or on efnet's #romhack.
8. greetings and thanks..
snowbro, hexposer and tlayer.. the first two
romhacking tools i ever used..
mdw2, hexecute, open source baby:)
ballzy,
braxton,
ff2nce: my favorite romhacking project of all
time boys.. regardless of the out come.
i look forward to more projects from you.
#romhack, ignoring me for so long that i had
to figure stuff out myself :) gotta
love the warm crowd.
