5. A little color

So far everything has been in black and white…let’s add some color.

First, a bit about the NES color palette. There are 32 memory addresses reserved for color ($3f00-3f1f of the PPU). 16 for backgrounds ($3f00-3f0f) and 16 for sprites($3f10-3f1f). Divided into 4 sections of 4, and the first color of each is ignored, and is treated as transparent (for backgrounds, transparent = the default background color will show instead). So really each sprite can use 3 unique colors. And each background tile can use 3 unique colors + the default background color.

($3f00 = the default background color, although I could swear that I’ve changed the background color by writing to $3f10…hmm…the wiki says “Addresses $3F10/$3F14/$3F18/$3F1C are mirrors of $3F00/$3F04/$3F08/$3F0C”…so writing to 3f10 is equivalent to writing to $3f00.)

colorpalettes

That makes 25 colors available on the screen at the same time, out of the 50ish possible colors to choose from.

nes-color-palette2

 

Notes: You can darken/tint the colors by setting various “color emphasis” register bits, but I tend to avoid that. And, a slight warning, don’t use color $0d, it will cause problems with some tvs, just use the f colors for black.

Backgrounds are further limited, because you can only define a palette for a 16×16 section. (even though tiles are actually 8×8).The Nes Screen Tool is very good for seeing the attribute table limitations. Most games just build their backgrounds in 16×16 blocks (metatile = a 2×2 block of tiles). With a little imagination, we can make some backgrounds that aren’t 100% square blocks.

Each background screen (nametable) has 64 bytes devoted to “attributes” …ie which palette to use. Nametable #0 = ppu addresses $2000-23ff. The attribute table is $23c0-23ff. Each byte in the attribute table covers a 32 pixel x 32 pixel area, and is divided into 4 parts. Each 2 bits chooses a color palette for a 16×16 part of the screen. It goes like this (bits vs screen position)

44332211 =

1, 2

3, 4

So, for example, if you want to change the bottom right area (a 16 x 16 block of background) to palette #1…you would change the high 2 bits of its attribute byte to 01. Such as 01xx xxx. If you wanted to change the top left area to palette #3, you would change the low 2 bits to 11. Such as xxxx xx11.

I made a few changes to the code from last time…for one, I changed the palette to be more colorful, and I added a few attribute table bits, to show the size of the smallest unit you can define (16×16). I also put those new bits in a separate file and used an #include, and put the variables in the zeropage with a #pragma (just to show how that can be done). Zeropage variables work faster than non-zeropage variables. (Note, cc65 will also use about 10-20 zeropage variables for various C library functions — leaving you 235 or so to use.)

#pragma bss-name(push, "ZEROPAGE")
blah variables here
#include "PALETTE.c"
#include "CODE.c"

The code looks cleaner if you separate various parts.

const unsigned char PALETTE[]={
0x11, 0x00, 0x00, 0x31, // blues
0x00, 0x00, 0x00, 0x15, // red
0x00, 0x00, 0x00, 0x27, // yellow
0x00, 0x00, 0x00, 0x1a, // green
}; 
// note, 11 is the default background color = blue
// we are only writing the BG palettes (16 bytes total).
const unsigned char Attrib_Table[]={
0x44, // 0100 0100, 
0xbb, // 1011 1011, 
0x44, // 0100 0100, 
0xbb}; // 1011 1011 };
// this is attribute table data, 
// each 2 bits defines a color palette
// for a 16x16 box

 PPU_ADDRESS = 0x23;
 PPU_ADDRESS = 0xda;
 for( index = 0; index < sizeof(Attrib_Table); ++index ){
  PPU_DATA = Attrib_Table[index];
  } // this inserts the Attrib Table data into the PPU

lesson3

Here’s the link to this code…

http://dl.dropboxusercontent.com/s/tqp6s3odgurieep/lesson3.zip

Our next project will show how to draw sprites to the screen. See you next time.

If you’re still confused about attribute tables, here’s a grid of the entire attribute table and its position on the screen.

ATtable

Since the screen is 256 pixels wide, each box represents a 32×32 block of the screen. Each byte is then further divided into 2 bit pieces representing 16×16 blocks of the screen, defining which palette that block gets.

 

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2 thoughts on “5. A little color

  1. Hey thanks a lot for these, i prefer C over ASM so these tutorials have been very helpful! Ive been reading a bit about pattern tables, nametables, and attribute tables to really understand this lesson and also playing a bit with the code so my (really newbie) question is… is it necessary or useful to know how to modify the data in the pattern tables directly in the code? I understand you can use NES Screen Tool to make and edit tiles in pattern tables instead but is there any advantage in doing all of that in the code (which by the way seems a lot more difficult).

    Like

    1. If your background changes, for example Mario breaks a brick and it disappears, you will need to know how to locate those 4 tiles and that 1 attribute byte. And, it is very complicated, yes.

      Like

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