PPU palettes: Difference between revisions

Revision as of 12:17, 26 April 2023

The NES has a limited selection of color outputs. A 6-bit value in the palette memory area corresponds to one of 64 outputs. The emphasis bits of the PPUMASK register ($2001) provide an additional color modifier.

For more information on how the colors are generated on an NTSC NES, see: NTSC video

Memory Map

The palette for the background runs from VRAM $3F00 to $3F0F; the palette for the sprites runs from $3F10 to $3F1F. Each color takes up one byte.

Address	Purpose
$3F00	Universal background color
$3F01-$3F03	Background palette 0
$3F05-$3F07	Background palette 1
$3F09-$3F0B	Background palette 2
$3F0D-$3F0F	Background palette 3
$3F11-$3F13	Sprite palette 0
$3F15-$3F17	Sprite palette 1
$3F19-$3F1B	Sprite palette 2
$3F1D-$3F1F	Sprite palette 3

Each palette has three colors. Each 16x16 pixel area of the background can use the backdrop color and the three colors from one of the four background palettes. The choice of palette for each 16x16 pixel area is controlled by bits in the attribute table at the end of each nametable. Each sprite can use the three colors from one of the sprite palettes. The choice of palette is in attribute 2 of each sprite (see PPU OAM).

Addresses $3F04/$3F08/$3F0C can contain unique data, though these values are not used by the PPU when normally rendering (since the pattern values that would otherwise select those cells select the backdrop color instead). They can still be shown using the background palette hack, explained below.

Addresses $3F10/$3F14/$3F18/$3F1C are mirrors of $3F00/$3F04/$3F08/$3F0C. Note that this goes for writing as well as reading. A symptom of not having implemented this correctly in an emulator is the sky being black in Super Mario Bros., which writes the backdrop color through $3F10.

Thus, indices into the palette are formed as follows:

43210
|||||
|||++- Pixel value from tile data
|++--- Palette number from attribute table or OAM
+----- Background/Sprite select

As in some second-generation game consoles, values in the NES palette are based on hue and brightness:

76543210
||||||||
||||++++- Hue (phase, determines NTSC/PAL chroma)
||++----- Value (voltage, determines NTSC/PAL luma)
++------- Unimplemented, reads back as 0

Hue $0 is light gray, $1-$C are blue to red to green to cyan, $D is dark gray, and $E-$F are mirrors of $1D (black).

It works this way because of the way colors are represented in an NTSC or PAL signal, with the phase of a color subcarrier controlling the hue. For details, see NTSC video, or for a list see Color $0D games.

The canonical code for "black" is $0F or $1D.

Do not use color $0D. It results in a "blacker than black" signal that may cause problems for some TVs.

Further details can be found here

The 2C03 RGB PPU used in the PlayChoice-10 and Famicom Titler renders hue $D as black, not dark gray. The 2C04 PPUs used in many Vs. System arcade games have completely different palettes as a copy protection measure.

Palettes

The 2C02 (NTSC) and 2C07 (PAL) PPU is used to generate an analog composite video signal. These were used in most home consoles.

The 2C03, 2C04, and 2C05, on the other hand, all output analog red, green, blue, and sync (RGBS) signals. The sync signal contains horizontal and vertical sync pulses in the same format as an all-black composite signal. Each of the three video channels uses a 3-bit DAC driven by a look-up table in a 64x9-bit ROM inside the PPU. The look-up tables (one digit for each of red, green, and blue, in order) are given below.

RGB PPUs were used mostly in arcade machines (e.g. Vs. System, Playchoice 10), as well as the Sharp Famicom Titler.

2C02 and 2C07

NES palette generated with Bisqwit's tool

The RF Famicom, AV Famicom, NES (both front- and top-loading), and the North American version of the Sharp Nintendo TV use the 2C02 PPU. Unlike some other consoles' video circuits, the 2C02 does not generate RGB video and then encode that to composite. Instead it generates NTSC video directly in the composite domain, decoded by the television receiver into RGB to drive its picture tube.

Most emulators can use a predefined palette, such as one commonly stored in common .pal format, in which each triplet represents the sRGB color that results from decoding a large flat area with a given palette value. The following table was generated using blargg's Full Palette demo on Nestopia:

 84  84  84    0  30 116    8  16 144   48   0 136   68   0 100   92   0  48   84   4   0   60  24   0   32  42   0    8  58   0    0  64   0    0  60   0    0  50  60    0   0   0
152 150 152    8  76 196   48  50 236   92  30 228  136  20 176  160  20 100  152  34  32  120  60   0   84  90   0   40 114   0    8 124   0    0 118  40    0 102 120    0   0   0
236 238 236   76 154 236  120 124 236  176  98 236  228  84 236  236  88 180  236 106 100  212 136  32  160 170   0  116 196   0   76 208  32   56 204 108   56 180 204   60  60  60
236 238 236  168 204 236  188 188 236  212 178 236  236 174 236  236 174 212  236 180 176  228 196 144  204 210 120  180 222 120  168 226 144  152 226 180  160 214 228  160 162 160

Other tools for generating a palette include one by Bisqwit and one by Drag. These simulate generating a large area of one flat color and then decoding that with the adjustment knobs set to various settings.

The PAL PPU (2C07) has a 15 degree hue shift relative to NTSC.

2C03 and 2C05

This palette is intentionally similar to the NES's standard palette, but notably is missing the greys in entries $2D and $3D. The 2C03 is used in Vs. Duck Hunt, Vs. Tennis, all PlayChoice games, the Famicom Titler, and the Famicom TV. The 2C05 is used in some later Vs. games as a copy protection measure. Both have been used in RGB mods for the NES, as a circuit implementing A0' = A0 xor (A1 nor A2) can swap PPUCTRL and PPUMASK to make a 2C05 behave as a 2C03.

The formula for mapping the DAC integer channel value to 8-bit per channel color is

C = 255 * DAC / 7

	$x0	$x1	$x2	$x3	$x4	$x5	$x6	$x7	$x8	$x9	$xA	$xB	$xC
$0x	333	014	006	326	403	503	510	420	320	120	031	040	022
$1x	555	036	027	407	507	704	700	630	430	140	040	053	044
$2x	777	357	447	637	707	737	740	750	660	360	070	276	077
$3x	777	567	657	757	747	755	764	772	773	572	473	276	467

Note that some of the colors are duplicates: $0B and $1A = 040, $2B and $3B = 276.

Kevtris's dumped palettes imply that the monochrome bit has unique function on the 2C03 and 2C05: instead of just using the leftmost column of the palette (and forcing the lower four bits to zero), instead the top two bits index into a palette with colors: 000, 333, 777, 777. Given that the 2C04 does not do this, this exception feels weird, and corroboration would be appreciated.

2C04

All four 2C04 PPUs contain the same master palette, but in different permutations. It's almost a superset of the 2C03/5 palette, adding four greys, six other colors, and making the bright yellow more pure.

Note that using the greyscale bit in PPUMASK will remap the palette by index, not by color. This means that with the scrambled palettes, each row will remap to the colors in the $0X column for that 2C04 version.

Visualization tool: RGB PPU Palette Converter

No version of the 2C04 was ever made with the below ordering, but it shows the similarity to the 2C03:

	$x0	$x1	$x2	$x3	$x4	$x5	$x6	$x7	$x8	$x9	$xA	$xB	$xC	$xD	$xE	$xF
$0x	333	014	006	326	403	503	510	420	320	120	031	040	022	111	003	020
$1x	555	036	027	407	507	704	700	630	430	140	040	053	044	222	200	310
$2x	777	357	447	637	707	737	740	750	660	360	070	276	077	444	000	000
$3x	777	567	657	757	747	755	764	770	773	572	473	276	467	666	653	760

The PPUMASK monochrome bit has the same implementation as on the 2C02, and so it has an unintuitive effect on the 2C04 PPUs; rather than forcing colors to grayscale, it instead forces them to the first column.

LUT approach

Emulator authors may implement the 2C04 variants as a LUT indexing the "ordered" palette. This has the added advantage of being able to use preexisting .pal files if the end user wishes to do so.

Repeating colors such as 000 and 777 may index into the same entry of the "ordered" palette, as this is functionally identical.

 const unsigned char PaletteLUT_2C04_0001 [64] ={
    0x35,0x23,0x16,0x22,0x1C,0x09,0x1D,0x15,0x20,0x00,0x27,0x05,0x04,0x28,0x08,0x20,
    0x21,0x3E,0x1F,0x29,0x3C,0x32,0x36,0x12,0x3F,0x2B,0x2E,0x1E,0x3D,0x2D,0x24,0x01,
    0x0E,0x31,0x33,0x2A,0x2C,0x0C,0x1B,0x14,0x2E,0x07,0x34,0x06,0x13,0x02,0x26,0x2E,
    0x2E,0x19,0x10,0x0A,0x39,0x03,0x37,0x17,0x0F,0x11,0x0B,0x0D,0x38,0x25,0x18,0x3A
};

const unsigned char PaletteLUT_2C04_0002 [64] ={
    0x2E,0x27,0x18,0x39,0x3A,0x25,0x1C,0x31,0x16,0x13,0x38,0x34,0x20,0x23,0x3C,0x0B,
    0x0F,0x21,0x06,0x3D,0x1B,0x29,0x1E,0x22,0x1D,0x24,0x0E,0x2B,0x32,0x08,0x2E,0x03,
    0x04,0x36,0x26,0x33,0x11,0x1F,0x10,0x02,0x14,0x3F,0x00,0x09,0x12,0x2E,0x28,0x20,
    0x3E,0x0D,0x2A,0x17,0x0C,0x01,0x15,0x19,0x2E,0x2C,0x07,0x37,0x35,0x05,0x0A,0x2D
};

const unsigned char PaletteLUT_2C04_0003 [64] ={
    0x14,0x25,0x3A,0x10,0x0B,0x20,0x31,0x09,0x01,0x2E,0x36,0x08,0x15,0x3D,0x3E,0x3C,
    0x22,0x1C,0x05,0x12,0x19,0x18,0x17,0x1B,0x00,0x03,0x2E,0x02,0x16,0x06,0x34,0x35,
    0x23,0x0F,0x0E,0x37,0x0D,0x27,0x26,0x20,0x29,0x04,0x21,0x24,0x11,0x2D,0x2E,0x1F,
    0x2C,0x1E,0x39,0x33,0x07,0x2A,0x28,0x1D,0x0A,0x2E,0x32,0x38,0x13,0x2B,0x3F,0x0C
};

const unsigned char PaletteLUT_2C04_0004 [64] ={
    0x18,0x03,0x1C,0x28,0x2E,0x35,0x01,0x17,0x10,0x1F,0x2A,0x0E,0x36,0x37,0x0B,0x39,
    0x25,0x1E,0x12,0x34,0x2E,0x1D,0x06,0x26,0x3E,0x1B,0x22,0x19,0x04,0x2E,0x3A,0x21,
    0x05,0x0A,0x07,0x02,0x13,0x14,0x00,0x15,0x0C,0x3D,0x11,0x0F,0x0D,0x38,0x2D,0x24,
    0x33,0x20,0x08,0x16,0x3F,0x2B,0x20,0x3C,0x2E,0x27,0x23,0x31,0x29,0x32,0x2C,0x09
};

RP2C04-0001

MAME's source claims that Baseball, Freedom Force, Gradius, Hogan's Alley, Mach Rider, Pinball, and Platoon require this palette.

755,637,700,447,044,120,222,704,777,333,750,503,403,660,320,777
357,653,310,360,467,657,764,027,760,276,000,200,666,444,707,014
003,567,757,070,077,022,053,507,000,420,747,510,407,006,740,000
000,140,555,031,572,326,770,630,020,036,040,111,773,737,430,473

RP2C04-0002

MAME's source claims that Castlevania, Mach Rider (Endurance Course), Raid on Bungeling Bay, Slalom, Soccer, Stroke & Match Golf (both versions), and Wrecking Crew require this palette.

000,750,430,572,473,737,044,567,700,407,773,747,777,637,467,040
020,357,510,666,053,360,200,447,222,707,003,276,657,320,000,326
403,764,740,757,036,310,555,006,507,760,333,120,027,000,660,777
653,111,070,630,022,014,704,140,000,077,420,770,755,503,031,444

RP2C04-0003

MAME's source claims that Balloon Fight, Dr. Mario, Excitebike (US), Goonies, and Soccer require this palette.

507,737,473,555,040,777,567,120,014,000,764,320,704,666,653,467
447,044,503,027,140,430,630,053,333,326,000,006,700,510,747,755
637,020,003,770,111,750,740,777,360,403,357,707,036,444,000,310
077,200,572,757,420,070,660,222,031,000,657,773,407,276,760,022

RP2C04-0004

MAME's source claims that Clu Clu Land, Excitebike (Japan), Ice Climber (both versions), and Super Mario Bros. require this palette.

430,326,044,660,000,755,014,630,555,310,070,003,764,770,040,572
737,200,027,747,000,222,510,740,653,053,447,140,403,000,473,357
503,031,420,006,407,507,333,704,022,666,036,020,111,773,444,707
757,777,320,700,760,276,777,467,000,750,637,567,360,657,077,120

Games compatible with multiple different PPUs

Some games don't require that arcade owners have the correct physical PPU.

At the very least, the following games use some of the DIP switches to support multiple different PPUs:

Atari RBI Baseball
Battle City
Star Luster
Super Sky Kid
Super Xevious
Tetris (Tengen)
TKO Boxing

Backdrop color (palette index 0) uses

During forced blanking, when neither background nor sprites are enabled in PPUMASK ($2001), the picture will show the backdrop color. If only the background or sprites are disabled, or if the left 8 pixels are clipped off, the PPU continues its normal video memory access pattern but uses the backdrop color for anything disabled.

The background palette hack

If the current VRAM address points in the range $3F00-$3FFF during forced blanking, the color indicated by this palette location will be shown on screen instead of the backdrop color. (looking at the relevant circuitry in Visual 2C02, this is because the palette RAM's output is not disconnected from the video output circuitry when not rendering) This can be used to display colors from the normally unused $3F04/$3F08/$3F0C palette locations. A loop that fills the palette will cause each color in turn to be shown on the screen, so to avoid horizontal rainbow bar glitches while loading the palette, wait for a real vertical blank first using an NMI technique.

Color names

When programmers and artists are communicating, it's often useful to have human-readable names for colors. Many graphic designers who have done web or game work will be familiar with HTML color names.

Luma

$0F: Black
$00: Dark gray
$10: Light gray or silver
$20: White
$01-$0C: Dark colors, medium mixed with black
$11-$1C: Medium colors, similar brightness to dark gray
$21-$2C: Light colors, similar brightness to light gray
$31-$3C: Pale colors, light mixed with white

Chroma

Names for hues:

$x0: Gray
$x1: Azure
$x2: Blue
$x3: Violet
$x4: Magenta
$x5: Rose
$x6: Red or maroon
$x7: Orange
$x8: Yellow or olive
$x9: Chartreuse
$xA: Green
$xB: Spring
$xC: Cyan

RGBI

These NES colors approximate colors in 16-color RGBI palettes, such as the CGA, EGA, or classic Windows palette, though the NES doesn't really have particularly good approximations:

$0F: 0/Black
$02: 1/Navy
$1A: 2/Green
$1C: 3/Teal
$06: 4/Maroon
$14: 5/Purple
$18: 6/Olive ($17 for the brown in CGA/EGA in RGB)
$10: 7/Silver
$00: 8/Gray
$12: 9/Blue
$2A: 10/Lime
$2C: 11/Aqua/Cyan
$16: 12/Red
$24: 13/Fuchsia/Magenta
$28: 14/Yellow
$30: 15/White

References

Re: Various questions about the color palette and emulators - 2012 collection of 2C03, 2C04, 2C05 palettes.

@@ Line 385: / Line 385: @@
 If the current VRAM address points in the range $3F00-$3FFF during forced blanking, the color indicated by this palette location will be shown on screen instead of the backdrop color.
-(Looking at the relevant circuitry in [[Visual 2C02]], this is an intentional feature of the PPU and not merely a side effect of how rendering works.)
+(looking at the relevant circuitry in [[Visual 2C02]], this is because the palette RAM's output is not disconnected from the video output circuitry when not rendering)
 This can be used to display colors from the normally unused $3F04/$3F08/$3F0C palette locations.
 A loop that fills the palette will cause each color in turn to be shown on the screen, so to avoid horizontal rainbow bar glitches while loading the palette, wait for a real vertical blank first using an [[NMI]] technique.