main.c File Reference

HDMA-driven sinusoidal wave distortion on background. More...

#include <snes.h>
#include <snes/console.h>
#include <snes/input.h>
#include <snes/hdma.h>

Macros
#define	AMP_DEFAULT   3
#define	AMP_LEVELS   7
#define	SINE_PERIOD   112
#define	TABLE_ENTRIES   335
#define	TABLE_SIZE   1006

Functions
int	main (void)
	Entry point – HDMA-driven sinusoidal wave distortion on background.

Variables
static const u16	amp_offsets [7]
	Pre-computed byte offsets into hdma_tables[] for each amplitude level.
static const u8	hdma_tables [7 *1006]
	Pre-computed HDMA sine wave tables for 7 amplitude levels.
static const u8	tiles []
	Minimal 4bpp tile data: one empty tile and one solid tile.
Wave Effect State
Runtime state variables for the wave distortion effect. These are static (file-scope) variables, which the compiler places in WRAM via RAMSECTION + .data_init. They persist across frames and are initialized explicitly in main() rather than relying on C static init.
static u8	amp_idx
static u8	animating
static u8	phase
static u8	wave_on

Detailed Description

HDMA-driven sinusoidal wave distortion on background.

Creates a wavy horizontal distortion effect by using HDMA to write per-scanline BG1 horizontal scroll offsets (BG1HOFS, $210D). A set of 7 pre-computed sine tables at increasing amplitudes (0 to 24 pixels) are stored in ROM, each containing 335 entries (224 visible scanlines + 111 wrap entries for smooth phase animation). HDMA channel 6 is configured in write-twice mode (1REG_2X) to write both the low and high bytes of BG1HOFS each scanline. When animation is enabled, the table pointer advances by 3 bytes per frame (one HDMA entry), cycling through the wrap portion to create continuous wave motion. The background consists of simple alternating solid/empty tiles to make the distortion clearly visible.

SNES Concepts

• HDMA write-twice mode (1REG_2X) targeting BG1HOFS ($210D)
• Per-scanline horizontal scroll offset for wave distortion
• Repeat-mode HDMA entries (bit 7 set = write every scanline)
• Pre-computed sine tables with wrap region for seamless animation
• Bare-metal VRAM and CGRAM writes (no library DMA helpers)

What to Observe

• Press A to toggle the wave effect on/off
• Press LEFT/RIGHT to decrease/increase wave amplitude (7 levels)
• Press UP to start wave animation (the wave scrolls vertically)
• Press DOWN to stop animation (wave freezes in place)

Modules Used

console, dma, sprite, input, hdma

See also

hdma.h, input.h, video.h

Macro Definition Documentation

AMP_DEFAULT

#define AMP_DEFAULT   3

Default amplitude index (amplitude level 12 pixels)

AMP_LEVELS

#define AMP_LEVELS   7

Number of amplitude levels (0, 4, 8, 12, 16, 20, 24 pixels)

SINE_PERIOD

#define SINE_PERIOD   112

Sine wave period in scanlines (half the visible screen height)

TABLE_ENTRIES

#define TABLE_ENTRIES   335

Entries per table: 224 visible scanlines + 111 wrap entries for smooth animation

TABLE_SIZE

#define TABLE_SIZE   1006

Bytes per amplitude table: 335 entries x 3 bytes + 1 end marker (0x00)

Function Documentation

main()

int main

(

void

)

Entry point – HDMA-driven sinusoidal wave distortion on background.

Sets up a simple striped background using bare-metal VRAM and CGRAM writes (no library DMA helpers), then configures HDMA channel 6 to apply per-scanline horizontal scroll offsets from pre-computed sine tables.

Controls:

• A: toggle wave on/off
• LEFT/RIGHT: decrease/increase amplitude
• UP: start animation (wave scrolls vertically)
• DOWN: stop animation (wave freezes)

Returns

Does not return (infinite loop).

< Default amplitude index (amplitude level 12 pixels)

Variable Documentation

amp_idx

u8 amp_idx

static

Current amplitude index (0-6, selects which table to use)

amp_offsets

const u16 amp_offsets[7]

static

Initial value:

= {
    0, 1006, 2012, 3018, 4024, 5030, 6036
}

Pre-computed byte offsets into hdma_tables[] for each amplitude level.

Since each amplitude table is TABLE_SIZE (1006) bytes, the offset for level N is N * 1006. These are pre-computed to avoid runtime multiplication (the 65816 has no hardware multiply for 16-bit values, so amp_idx * 1006 would require an expensive software multiply routine).

animating

u8 animating

static

Animation enabled flag (0=frozen, 1=scrolling)

hdma_tables

const u8 hdma_tables[7 * 1006]

static

Pre-computed HDMA sine wave tables for 7 amplitude levels.

Each amplitude level has TABLE_SIZE (1006) bytes containing 335 HDMA entries plus a 0x00 terminator. Each entry is 3 bytes:

• Byte 0: 0x81 = repeat flag (bit 7) + 1 scanline count (bits 6-0). The repeat flag tells HDMA to re-read data bytes for every scanline in the group (here, just 1 scanline), which is required for smooth per-line distortion.
• Bytes 1-2: BG1HOFS low byte and high byte (16-bit scroll offset). Positive values shift the BG right, negative (two's complement, e.g. 0xFF,0xFF = -1) shift left.

The 335 entries cover 224 visible scanlines plus 111 extra entries. By advancing the HDMA start pointer by 3 bytes (one entry) per frame, the wave pattern appears to scroll vertically. After 112 entries (one full sine period), the pattern repeats seamlessly due to the wrap region.

Amplitude levels: 0, 4, 8, 12, 16, 20, 24 pixels peak displacement.

phase

u8 phase

static

Current animation phase (0-111, index into sine period)

tiles

const u8 tiles[]

static

Initial value:

= {
    
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
}

Minimal 4bpp tile data: one empty tile and one solid tile.

SNES 4bpp tiles are 32 bytes each (8x8 pixels, 4 bitplanes). The first 16 bytes contain interleaved bitplanes 0 and 1, the next 16 contain bitplanes 2 and 3. For a "solid color 1" tile, only bitplane 0 needs to be set (all 0xFF), with all other bitplanes zero. The alternating empty/solid pattern in the tilemap creates vertical stripes that make the wave distortion clearly visible.

wave_on

u8 wave_on

static

Wave effect enabled flag (0=off, 1=on)