ChocolateBird/shaders/audio_renderer.gdshader

// ============================================================= BOILERPLATE =============================================================
// While most of the data we are working with is integral, GPU conversion overheads mean almost all of this will be floats.
// Unfortunately, this loses type-checking on [0.0, 1.0] vs [0,255] etc. so a lot of this will involve comments declaring ranges.
shader_type canvas_item;
render_mode blend_premul_alpha;
const float TEX_SIZE = 4096.0;
const float UV_QUANTIZE = TEX_SIZE;
// I feel like these magic numbers are a bit more intuitive in hex
const float x00FF  = float(0x00FF);   //   255.0
const float x0100  = float(0x0100);   //   256.0
const float x7FFF  = float(0x7FFF);   // 32767.0
const float x8000  = float(0x8000);   // 32768.0
const float xFF00  = float(0xFF00);   // 65280.0
const float xFFFF  = float(0xFFFF);   // 65535.0
const float x10000 = float(0x10000);  // 65536.0

const vec2 INT16_DOT_BE = vec2(xFF00, x00FF);
const vec2 INT16_DOT_LE = vec2(x00FF, xFF00);

uniform sampler2D tex : hint_normal;

float unpack_uint16(vec2 uint16) {
	// Convert packed 2byte integer, sampled as two [0.0, 1.0] range floats, to the original int value [0, 65535] in float32
	return dot(uint16, INT16_DOT_LE);
}

float unpack_int16(vec2 int16) {
	// Convert packed 2byte integer, sampled as two [0.0, 1.0] range floats, to the original int value [-32768, 32767] in float32
	float unsigned = dot(int16, INT16_DOT_LE);
	return unsigned - (unsigned < x7FFF ? 0.0 : x10000);
}

float rescale_int16(float int16) {
	// Rescale from [-32768, 32767] to [-1.0, 1.0)
	return int16 / x8000;
}

vec2 pack_float_to_int16(float value) {
	// Convert a float in range [-1.0, 1.0) to a signed 2byte integer [-32768, 32767] packed into two [0.0, 1.0] floats
	float scaled = value * x8000;
	float unsigned = scaled + (scaled < 0.0 ? x10000 : 0.0);
	float unsigned_div_256 = unsigned / x0100;
	float MSB = trunc(unsigned_div_256)         / x00FF;
	float LSB = fract(unsigned_div_256) * x0100 / x00FF;
	return vec2(LSB, MSB);
}

vec4 test_writeback(vec2 uv) {
	// Test importing and exporting the samples,
	// and exporting a value derived from the UV
	vec4 output;
	float sample_1 = rescale_int16(unpack_int16(texture(tex, uv).xw));
	float sample_2 = rescale_int16(dot(trunc(uv*TEX_SIZE), vec2(1.0, TEX_SIZE)));
	output.xy = pack_float_to_int16(sample_1);
	output.zw = pack_float_to_int16(sample_2);
	return output;
}


// =============================================================    LOGIC    =============================================================
// We have around 200k frames across 35 instrument samples
// 35 instrument samples and 8 sfx samples = 43 samples
// 2048x128 texture maybe? at 2bytes per texel, that's 512KiB of VRAM
// We start the texture with a bunch of same-size headers
//     uint16 sample_start       // The true start, after the prepended 3 frames of silence
//     uint16 sample_length      // 3 frames after the true end, because of how we loop
//     uint16 sample_loop_begin  // 3 frames after the true loop point
//     uint16 mixrate
//     2*uint8 AD of ADSR ([0.0, 1.0] is fine)
//     2*uint8 SR of ADSR ([0.0, 1.0] is fine)
// So six texture() calls spent on header information, and one on the final lookup.
// Alternatively, sample length could be omitted and fetched as the start of the next entry to save redundant entries.
//
// To accomodate filtering, every sample must begin with 3 frames of silence, and end with 6 frames of the beginning of the loop.
// Looped playback will go from the first 3 of 6 frames at the end, to the third frame after the loop start point, to avoid filter bleeding.
// If a sample does not loop, it must have 6 frames of silence at the end, not including the subsequent next sample's 3 frames of silence prefix.
// As such, every sample will have an additional 9 frames, 3 before, 6 after.
// Additionally, every row of the texture must have 3 redundant frames on either side - i.e., we only sample from [3, 2045) on any given row.
// So the payload of a 2048-wide texture will be 2042 per row, excluding the initial header.
// So for 43 samples, a header of 43*6 = 258 texels starts the first row,
//   after which the first sample's 3 frames of silence (3 texels of (0.0, 0.0), 6 bytes of 0x00) may begin.
// A 2048x128 texture would have a payload of 2042x128 = 261376 frames (texels) excluding header
// With the 258 texel header, which uses 3 texels of margin, 255 would be subtracted from the above payload,
//   leaving 261121 texels for the sample data.

const float HEADER_LENGTH_TEXELS = 6.0;
uniform sampler2D instrument_samples;
uniform vec2 instrument_samples_size = vec2(2048.0, 128.0);
uniform float instrument_row_padding = 3.0;  // In case we want to go to cubic filtering
uniform float instrument_row_payload = 2042.0;  // 2048-3-3 Make sure to set with instrument_samples_size and instrument_row_padding!
uniform float reference_note = 71.0;  // [0, 255], possibly [0, 127]
uniform float output_mixrate = 32000.0;  // SNES SPC output is 32kHz

float get_pitch_scale(float note) {
	// return pow(2.0, (note - reference_note)/12.0);
	return exp2((note - reference_note)/12.0);
}

vec2 get_inst_texel(vec2 xy) {
	return texture(instrument_samples, xy/instrument_samples_size).xw;
}

float get_instrument_sample(float instrument_index, float pitch_scale, float t, float t_end) {
	// t_end is for ADSR purposes
	float header_offset = instrument_index * HEADER_LENGTH_TEXELS;
	float sample_start = unpack_uint16(get_inst_texel(vec2(header_offset, 0.0)));  // The true start, after the prepended 3 frames of silence
	float sample_length = unpack_uint16(get_inst_texel(vec2(header_offset + 1.0, 0.0)));  // 3 frames after the true end, because of how we loop
	float sample_loop_begin = unpack_uint16(get_inst_texel(vec2(header_offset + 2.0, 0.0)));  // 3 frames after the true loop point
	float sample_mixrate = unpack_uint16(get_inst_texel(vec2(header_offset + 3.0, 0.0)));
	vec2 attack_decay = get_inst_texel(vec2(header_offset + 4.0, 0.0));
	vec2 sustain_release = get_inst_texel(vec2(header_offset + 5.0, 0.0));
	// Calculate the point we want to sample in linear space
	float mixrate = sample_mixrate * pitch_scale;
	float target_frame = t * mixrate;
	// If we're past the end of the sample, we need to wrap it back to within the loop range
	float loop_length = sample_length - sample_loop_begin;
	float overshoot = max(target_frame - sample_length, 0.0);
	float overshoot_loops = ceil(overshoot/loop_length);
	target_frame -= overshoot_loops*loop_length;
	// Now we need to identify the sampling point since our frames are spread across multiple rows for GPU reasons
	// We only sample from texel 4 onwards on a given row - texel 0 is the header, texels 1,2,3 are lead-in for filtering
	// Note that y should be integral, but x should be continuous, as that's what applies the filtering!
	target_frame += sample_start;
	vec2 sample_xy = vec2(instrument_row_padding + mod(target_frame, instrument_row_payload), trunc(target_frame/instrument_row_payload));
	return rescale_int16(unpack_int16(get_inst_texel(sample_xy)));
}


void fragment() {
	// GLES2
	vec2 uv = vec2(UV.x, 1.0-UV.y);
	uv = (trunc(uv*UV_QUANTIZE)+0.5)/UV_QUANTIZE;

	COLOR.xyzw = test_writeback(uv);
}