main.c File Reference

Backgrounds — Mode 4: a 256-colour layer with offset-per-tile. More...

#include <snes.h>
#include <snes/tile.h>
#include <snes/math.h>

Macros
#define	BG1_CHR   0x0000
	VRAM word addresses: BG1 8bpp chars, BG1 map, BG3 offset map.
#define	BG1_MAP   0x2000
#define	BG3_MAP   0x3000
#define	OPT_BG1   0x2000
#define	OPT_VERTICAL   0x8000
	Mode 4 offset word bits.

Functions
static u16	hue (u16 i)
	Colour i of a 256-step hue ramp, 32 steps per sextant pair.
int	main (void)

Variables
static u16	maprow [32]
	One tilemap row being built.
u8	opt_vertical
static u16	owords [32]
	The single offset row, rebuilt every frame.
static u16	pal [256]
	The 256-colour palette, built at boot.
static u8	px [64]
	One 8bpp tile's pixels and its encoding.
static u8	tilebuf [64]
u16	wave_phase
	Probe oracles: wave phase, 1 while the words are vertical.

Detailed Description

Backgrounds — Mode 4: a 256-colour layer with offset-per-tile.

Mode 4 is the cartridge-game mode of offset-per-tile: an 8bpp (256 colour) BG1, a 2bpp BG2, and BG3 as an offset table like Mode 2. The difference is in that table: Mode 4 reads only one row of it, so each column gets one offset word that is either horizontal or vertical, chosen by bit 15 of the word.

Here BG1 is a diagonal 256-colour gradient. A sine wave is written into the single offset row each frame: as vertical words, the columns ripple up and down; press A and the same wave is written as horizontal words, and the columns slide sideways instead (8 pixels at a time — the low 3 bits of a horizontal offset are ignored).

SNES Concepts

• Mode 4: setMode(BG_MODE4, 0), BG1 8bpp + BG2 2bpp, BG3 = offset map
• The Mode 4 offset word: bit 15 = 1 vertical / 0 horizontal, bit 13 = apply to BG1 (bit 14 = BG2), value in bits 0-9; one row of 32 words
• Column 0 is never offset: word 0 applies to the second visible column
• 8bpp tiles built at run time (tileEncode8bpp), a 256-entry palette

What to Observe

• The gradient's columns ripple vertically as a travelling wave; the leftmost column stays put.
• Press A: the columns now shift horizontally, in 8-pixel steps.

Modules Used

console, dma, background, input, math, tile

See also

backgrounds/mode2 (the two-row table), docs/tutorials/graphics.md

Macro Definition Documentation

BG1_CHR

#define BG1_CHR   0x0000

VRAM word addresses: BG1 8bpp chars, BG1 map, BG3 offset map.

BG1_MAP

#define BG1_MAP   0x2000

BG3_MAP

#define BG3_MAP   0x3000

OPT_BG1

#define OPT_BG1   0x2000

apply this word to BG1

OPT_VERTICAL

#define OPT_VERTICAL   0x8000

Mode 4 offset word bits.

1 = vertical offset, 0 = horizontal

Function Documentation

hue()

u16 hue ( u16 i )

static

Colour i of a 256-step hue ramp, 32 steps per sextant pair.

main()

int main

(

void

)

< 1 = vertical offset, 0 = horizontal

< apply this word to BG1

< apply this word to BG1

Variable Documentation

maprow

u16 maprow[32]

static

One tilemap row being built.

opt_vertical

u8 opt_vertical

owords

u16 owords[32]

static

The single offset row, rebuilt every frame.

pal

u16 pal[256]

static

The 256-colour palette, built at boot.

px

u8 px[64]

static

One 8bpp tile's pixels and its encoding.

tilebuf

u8 tilebuf[64]

static

wave_phase

u16 wave_phase

Probe oracles: wave phase, 1 while the words are vertical.