main.c File Reference

Backgrounds — Mode 2 offset-per-tile: per-column scroll from BG3. More...

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

Macros
#define	BG1_CHR   0x0000
#define	BG1_MAP   0x2000
#define	BG3_MAP   0x3000 /* BG3 = the OPT offset table */
#define	OPT_BG1   0x2000 /* apply this offset word to BG1 */

Functions
static void	encode_4bpp (void)
int	main (void)

Variables
static u8	px [64]
static u8	tilebuf [32]
static u16	vwords [32]
u16	wave_phase
	Probe oracle: wave phase, advances each frame.

Detailed Description

Backgrounds — Mode 2 offset-per-tile: per-column scroll from BG3.

Modes 2, 4 and 6 have a trick no other mode does: offset-per-tile (OPT). BG3 stops being a drawable layer and becomes an offset table — for each 8-pixel column of BG1/BG2 the PPU reads a scroll offset out of BG3's tilemap. That lets each column scroll independently, which is how you get a flag rippling in the wind, heat-haze, or per-column parallax without a scanline of HDMA.

Here BG1 is a stack of horizontal colour bands; a sine table written into BG3 gives every column its own vertical offset, so the bands ripple. The phase advances each frame. Only BG1 is shown — BG3 is pure offset data.

ROM mode: LoROM (project default).

SNES Concepts

• Mode 2 OPT: setMode(BG_MODE2, 0); BG3's tilemap is the offset source, not a picture
• Per column N≥1 the PPU reads BG3 tile-row 0 for the H offset word and tile-row 1 for the V offset word; column 0 cannot be offset
• Offset word: bit 0x2000 = apply to BG1 (0x4000 = BG2); V value in bits 0-9, H value in bits 3-9. This example writes only V words
• Modes 4 and 6 reuse this exact data path (mode 4 packs H/V into one word)

What to Observe

Horizontal colour bands ripple as a travelling sine wave — each column at a different vertical offset, all from one small BG3 table. wave_phase is the probe oracle.

Modules Used

console, dma, background, math

See also

lib/include/snes/video.h — setMode(BG_MODE2)

Macro Definition Documentation

BG1_CHR

#define BG1_CHR   0x0000

BG1_MAP

#define BG1_MAP   0x2000

BG3_MAP

#define BG3_MAP   0x3000 /* BG3 = the OPT offset table */

OPT_BG1

#define OPT_BG1   0x2000 /* apply this offset word to BG1 */

Function Documentation

encode_4bpp()

void encode_4bpp ( void )

static

main()

int main

(

void

)

Variable Documentation

px

u8 px[64]

static

tilebuf

u8 tilebuf[32]

static

vwords

u16 vwords[32]

static

wave_phase

u16 wave_phase

Probe oracle: wave phase, advances each frame.