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edge relax go brr

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This commit is contained in:
Andrew Cohn 2025-11-29 16:30:47 -08:00
parent 8305dcf10d
commit c8bf647014
9 changed files with 519 additions and 64 deletions
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fallingCand
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52
main.c
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@ -8,18 +8,32 @@
#define GRID_H 2048
static int g_fbWidth = 800;
static int g_fbHeight = 800;
static double g_cursorX = 0.0;
static double g_cursorY = 0.0;
SandSim sim;
static void cursor_pos_callback(GLFWwindow* window, double xpos, double ypos) {
(void)window;
g_cursorX = xpos;
g_cursorY = ypos;
}
static void render_sand(SandSim* sim,
GLuint program,
GLuint vao,
int fbW,
int fbH) {
int fbH,
float brushX,
float brushY,
float brushRadius) {
glUseProgram(program);
GLint uResLoc = glGetUniformLocation(program, "u_resolution");
GLint uGridLoc = glGetUniformLocation(program, "u_gridSize");
GLint uStateLoc = glGetUniformLocation(program, "u_state");
GLint uBrushPosLoc = glGetUniformLocation(program, "u_brushPos");
GLint uBrushRadLoc = glGetUniformLocation(program, "u_brushRadius");
GLint uShowBrushLoc = glGetUniformLocation(program, "u_showBrush");
if (uResLoc >= 0) {
glUniform2f(uResLoc, (float)fbW, (float)fbH);
@ -27,13 +41,22 @@ static void render_sand(SandSim* sim,
if (uGridLoc >= 0) {
glUniform2i(uGridLoc, sim->gridW, sim->gridH);
}
if (uStateLoc >= 0) {
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, sim->tex_curr);
glUniform1i(uStateLoc, 0);
}
if (uBrushPosLoc >= 0) {
glUniform2f(uBrushPosLoc, brushX, brushY);
}
if (uBrushRadLoc >= 0) {
glUniform1f(uBrushRadLoc, brushRadius);
}
if (uShowBrushLoc >= 0) {
glUniform1i(uShowBrushLoc, 1); // always show for now
}
glBindVertexArray(vao);
glDrawArrays(GL_TRIANGLES, 0, 3);
}
@ -43,6 +66,7 @@ int main(void) {
if (!window) {
return EXIT_FAILURE;
}
glfwSetCursorPosCallback(window, cursor_pos_callback);
glfwGetFramebufferSize(window, &g_fbWidth, &g_fbHeight);
glViewport(0, 0, g_fbWidth, g_fbHeight);
printf("Renderer: %s\n", (const char*)glGetString(GL_RENDERER));
@ -91,6 +115,8 @@ int main(void) {
double sim_dt = 1.0 / 360.0;
double currentTime = glfwGetTime();
double accumulator = 0.0;
float brushX = 0.0f;
float brushY = 0.0f;
// --- Main loop ---
while (!glfwWindowShouldClose(window)) {
@ -100,13 +126,18 @@ int main(void) {
if (frameTime > 0.25) frameTime = 0.25;
accumulator += frameTime;
while (accumulator >= sim_dt) {
sand_step_gpu(&sim); // one discrete CA step on the GPU (commented bc unimplemeted)
sand_step_gpu(&sim); // one discrete CA step on the GPU
sand_relax_gpu(&sim);
accumulator -= sim_dt;
}
int paintMode = 0;
glfwPollEvents();
if (glfwGetMouseButton(window, GLFW_MOUSE_BUTTON_LEFT) == GLFW_PRESS) {
paintMode = 1;
} else if (glfwGetMouseButton(window, GLFW_MOUSE_BUTTON_RIGHT) == GLFW_PRESS) {
paintMode = -1;
}
// Re-query framebuffer size each frame (cheap and simple)
int fbw, fbh;
glfwGetFramebufferSize(window, &fbw, &fbh);
@ -115,9 +146,18 @@ int main(void) {
g_fbHeight = fbh;
glViewport(0, 0, g_fbWidth, g_fbHeight);
}
if (g_fbWidth > 0 && g_fbHeight > 0) {
float uvx = (float)g_cursorX / (float)g_fbWidth;
float uvy = (float)g_cursorY / (float)g_fbHeight;
brushX = uvx * (float)sim.gridW;
brushY = uvy * (float)sim.gridH;
}
float brushRadius = 60.0f;
sand_paint_gpu(&sim, brushX, brushY, brushRadius, paintMode);
glClear(GL_COLOR_BUFFER_BIT);
render_sand(&sim, program, vao, g_fbWidth, g_fbHeight);
render_sand(&sim, program, vao, g_fbWidth, g_fbHeight, brushX, brushY, brushRadius);
glfwSwapBuffers(window);
}

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sand_hello
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188
sand_sim.c
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@ -1,11 +1,45 @@
// sand_sim.c
#include "sand_sim.h"
#include "gl_utils.h" // for create_compute_program_from_file
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "gl_utils.h" // for create_compute_program_from_file
void sand_paint_gpu(SandSim* sim,
float brushX,
float brushY,
float brushRadius,
int mode) {
if (!sim || !sim->prog_paint) return;
if (mode == 0) return; // nothing to do
glUseProgram(sim->prog_paint);
GLint locGrid = glGetUniformLocation(sim->prog_paint, "u_gridSize");
GLint locPos = glGetUniformLocation(sim->prog_paint, "u_brushPos");
GLint locRad = glGetUniformLocation(sim->prog_paint, "u_brushRadius");
GLint locMode = glGetUniformLocation(sim->prog_paint, "u_mode");
if (locGrid >= 0) glUniform2i(locGrid, sim->gridW, sim->gridH);
if (locPos >= 0) glUniform2f(locPos, brushX, brushY);
if (locRad >= 0) glUniform1f(locRad, brushRadius);
if (locMode >= 0) glUniform1i(locMode, mode);
// Bind current state as read-write image
glBindImageTexture(0, sim->tex_curr,
0, GL_FALSE, 0,
GL_READ_WRITE, GL_R8UI);
// Dispatch over entire grid; 16x16 workgroup like your sim shader
GLuint groupsX = (sim->gridW + 16 - 1) / 16;
GLuint groupsY = (sim->gridH + 16 - 1) / 16;
glDispatchCompute(groupsX, groupsY, 1);
// Ensure writes visible to subsequent rendering/compute
glMemoryBarrier(GL_SHADER_IMAGE_ACCESS_BARRIER_BIT);
}
static void init_textures(SandSim* sim) {
// tex_curr
glGenTextures(1, &sim->tex_curr);
@ -25,6 +59,15 @@ static void init_textures(SandSim* sim) {
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
// NEW: tex_claim for the atomic “ownership” buffer
glGenTextures(1, &sim->tex_claim);
glBindTexture(GL_TEXTURE_2D, sim->tex_claim);
glTexStorage2D(GL_TEXTURE_2D, 1, GL_R32UI, sim->gridW, sim->gridH);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
// Unbind
glBindTexture(GL_TEXTURE_2D, 0);
}
@ -75,14 +118,26 @@ bool sand_init(SandSim* sim, int gridW, int gridH, const char* computeShaderPath
sim->gridH = gridH;
sim->tex_curr = 0;
sim->tex_next = 0;
sim->tex_claim = 0;
sim->prog_sim = 0;
sim->prog_paint = 0;
init_textures(sim);
upload_initial_state(sim);
sim->prog_sim = create_compute_program_from_file(computeShaderPath);
if (!sim->prog_sim) {
fprintf(stderr, "[sand_init] Failed to create compute program\n");
fprintf(stderr, "[sand_init] Failed to create compute cell tick program\n");
return false;
}
sim->prog_paint = create_compute_program_from_file("./shaders/sand_paint.comp");
if (!sim->prog_paint) {
fprintf(stderr, "[sand_init] Failed to create compute paint program\n");
return false;
}
sim->prog_relax = create_compute_program_from_file("./shaders/sand_relax.comp");
if (!sim->prog_relax) {
fprintf(stderr, "[sand_init] Failed to create compute relax program\n");
return false;
}
@ -90,27 +145,110 @@ bool sand_init(SandSim* sim, int gridW, int gridH, const char* computeShaderPath
}
void sand_step_gpu(SandSim* sim) {
if (!sim || !sim->prog_sim) return;
glUseProgram(sim->prog_sim);
static unsigned int frameCounter = 0;
frameCounter++;
// Set grid size uniform
GLint uGridLoc = glGetUniformLocation(sim->prog_sim, "u_gridSize");
if (uGridLoc >= 0) {
glUniform2i(uGridLoc, sim->gridW, sim->gridH);
GLint locFrame = glGetUniformLocation(sim->prog_sim, "u_frame");
if (locFrame >= 0) {
glUniform1ui(locFrame, frameCounter);
}
GLint locGrid = glGetUniformLocation(sim->prog_sim, "u_gridSize");
if (locGrid >= 0) {
glUniform2i(locGrid, sim->gridW, sim->gridH);
}
// Bind images (must match bindings in sand_step.comp)
glBindImageTexture(0, sim->tex_curr, 0, GL_FALSE, 0, GL_READ_ONLY, GL_R8UI);
glBindImageTexture(1, sim->tex_next, 0, GL_FALSE, 0, GL_WRITE_ONLY, GL_R8UI);
// Clear next state to AIR (0)
GLuint groupsX = (sim->gridW + 15) / 16;
GLuint groupsY = (sim->gridH + 15) / 16;
const GLubyte zeroByte[1] = {0};
glClearTexImage(sim->tex_next,
0,
GL_RED_INTEGER,
GL_UNSIGNED_BYTE,
zeroByte);
// Clear claim buffer to 0
const GLuint zero1[1] = {0};
glClearTexImage(sim->tex_claim,
0,
GL_RED_INTEGER, // format doesn't really matter; data is uint
GL_UNSIGNED_INT,
zero1);
// Bind images
glBindImageTexture(0, sim->tex_curr,
0, GL_FALSE, 0,
GL_READ_ONLY, GL_R8UI);
glBindImageTexture(1, sim->tex_next,
0, GL_FALSE, 0,
GL_WRITE_ONLY, GL_R8UI);
glBindImageTexture(2, sim->tex_claim,
0, GL_FALSE, 0,
GL_READ_WRITE, GL_R32UI);
GLuint groupsX = (sim->gridW + 16 - 1) / 16;
GLuint groupsY = (sim->gridH + 16 - 1) / 16;
glDispatchCompute(groupsX, groupsY, 1);
glMemoryBarrier(GL_SHADER_IMAGE_ACCESS_BARRIER_BIT);
// Ping-pong
// ping-pong: curr <-> next
GLuint tmp = sim->tex_curr;
sim->tex_curr = sim->tex_next;
sim->tex_next = tmp;
}
void sand_relax_gpu(SandSim* sim) {
if (!sim || !sim->prog_relax) return;
glUseProgram(sim->prog_relax);
static unsigned int relaxFrame = 0;
relaxFrame++;
GLint locFrame = glGetUniformLocation(sim->prog_relax, "u_frame");
if (locFrame >= 0) {
glUniform1ui(locFrame, relaxFrame);
}
GLint locGrid = glGetUniformLocation(sim->prog_relax, "u_gridSize");
if (locGrid >= 0) {
glUniform2i(locGrid, sim->gridW, sim->gridH);
}
// Clear next state to AIR (0) – tex_next is GL_R8UI
const GLubyte zeroByte[1] = {0};
glClearTexImage(sim->tex_next,
0,
GL_RED_INTEGER,
GL_UNSIGNED_BYTE,
zeroByte);
// Clear claim buffer (GL_R32UI) to 0
const GLuint zero1[1] = {0};
glClearTexImage(sim->tex_claim,
0,
GL_RED_INTEGER,
GL_UNSIGNED_INT,
zero1);
// Bind images: curr = read, next = write, claim = read/write
glBindImageTexture(0, sim->tex_curr,
0, GL_FALSE, 0,
GL_READ_ONLY, GL_R8UI);
glBindImageTexture(1, sim->tex_next,
0, GL_FALSE, 0,
GL_WRITE_ONLY, GL_R8UI);
glBindImageTexture(2, sim->tex_claim,
0, GL_FALSE, 0,
GL_READ_WRITE, GL_R32UI);
GLuint groupsX = (sim->gridW + 16 - 1) / 16;
GLuint groupsY = (sim->gridH + 16 - 1) / 16;
glDispatchCompute(groupsX, groupsY, 1);
glMemoryBarrier(GL_SHADER_IMAGE_ACCESS_BARRIER_BIT);
// ping-pong: curr <-> next
GLuint tmp = sim->tex_curr;
sim->tex_curr = sim->tex_next;
sim->tex_next = tmp;
@ -119,16 +257,14 @@ void sand_step_gpu(SandSim* sim) {
void sand_destroy(SandSim* sim) {
if (!sim) return;
if (sim->tex_curr) {
glDeleteTextures(1, &sim->tex_curr);
sim->tex_curr = 0;
}
if (sim->tex_next) {
glDeleteTextures(1, &sim->tex_next);
sim->tex_next = 0;
}
if (sim->prog_sim) {
glDeleteProgram(sim->prog_sim);
sim->prog_sim = 0;
}
if (sim->tex_curr) glDeleteTextures(1, &sim->tex_curr);
if (sim->tex_next) glDeleteTextures(1, &sim->tex_next);
if (sim->tex_claim) glDeleteTextures(1, &sim->tex_claim);
sim->tex_curr = sim->tex_next = sim->tex_claim = 0;
if (sim->prog_sim) glDeleteProgram(sim->prog_sim);
if (sim->prog_paint) glDeleteProgram(sim->prog_paint);
if (sim->prog_relax) glDeleteProgram(sim->prog_relax);
sim->prog_sim = sim->prog_paint = sim->prog_relax = 0;
}

18
sand_sim.h
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@ -7,21 +7,29 @@
// Simple binary sand vs air simulation
typedef struct {
GLuint tex_curr; // R8UI; 0 = air, 1 = sand
GLuint tex_next; // R8UI; ping-pong target
GLuint prog_sim; // compute shader program
int gridW;
int gridH;
GLuint tex_curr;
GLuint tex_next;
GLuint tex_claim; // NEW: claim buffer
GLuint prog_sim;
GLuint prog_paint;
GLuint prog_relax;
} SandSim;
// Initialize sim, allocate textures, upload initial state, load compute shader.
// Returns true on success, false on failure.
bool sand_init(SandSim* sim, int gridW, int gridH, const char* computeShaderPath);
// Advance simulation by 1 discrete tick (one CA step).
void sand_step_gpu(SandSim* sim);
void sand_relax_gpu(SandSim* sim);
// Destroy all GL objects owned by the sim.
void sand_destroy(SandSim* sim);
void sand_paint_gpu(SandSim* sim,
float brushX,
float brushY,
float brushRadius,
int mode);
#endif // SAND_SIM_H

21
shaders/sand_display.frag
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@ -5,7 +5,9 @@ out vec4 FragColor;
uniform vec2 u_resolution; // framebuffer size in pixels
uniform ivec2 u_gridSize; // sand grid size
uniform usampler2D u_state; // GL_R8UI texture
uniform vec2 u_brushPos;
uniform float u_brushRadius;
uniform int u_showBrush;
void main() {
vec2 fragCoord = gl_FragCoord.xy;
@ -21,6 +23,23 @@ void main() {
vec3 color = (v == 1u)
? vec3(0.9, 0.8, 0.1) // sand
: vec3(0.05, 0.05, 0.10); // background
if (u_showBrush != 0) {
// center of this cell in grid coordinates
vec2 cellCenter = vec2(cell) + vec2(0.5);
float dist = length(cellCenter - u_brushPos);
// how thick the ring is, in cells
float thickness = 1.0; // 1-cell-thick ring
// abs(dist - radius) < thickness => inside ring
float d = abs(dist - u_brushRadius);
// Smooth edge: edge = 1 at center of ring, 0 outside
float edge = smoothstep(thickness, 0.0, d);
vec3 ringColor = vec3(1.0); // white ring
color = mix(color, ringColor, edge);
}
FragColor = vec4(color, 1.0);
}

42
shaders/sand_paint.comp Normal file
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@ -0,0 +1,42 @@
#version 430 core
layout(local_size_x = 16, local_size_y = 16) in;
// Read-write access to the current grid
layout(r8ui, binding = 0) coherent uniform uimage2D state;
uniform ivec2 u_gridSize;
uniform vec2 u_brushPos; // in grid-space (cells)
uniform float u_brushRadius; // in cells
uniform int u_mode; // 0 = none, 1 = add sand, -1 = erase
void main()
{
ivec2 pos = ivec2(gl_GlobalInvocationID.xy);
if (pos.x >= u_gridSize.x || pos.y >= u_gridSize.y) {
return;
}
if (u_mode == 0) {
return;
}
// Center of this cell in grid coords
vec2 cellCenter = vec2(pos) + vec2(0.5);
float dist = length(cellCenter - u_brushPos);
if (dist > u_brushRadius) {
return;
}
const uint AIR = 0u;
const uint SAND = 1u;
if (u_mode > 0) {
// left click: fill with sand
imageStore(state, pos, uvec4(SAND, 0u, 0u, 0u));
} else if (u_mode < 0) {
// right click: erase sand
imageStore(state, pos, uvec4(AIR, 0u, 0u, 0u));
}
}

135
shaders/sand_relax.comp Normal file
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@ -0,0 +1,135 @@
#version 430 core
layout(local_size_x = 16, local_size_y = 16) in;
// state_curr: read-only; state_next: write-only
layout(r8ui, binding = 0) readonly uniform uimage2D state_curr;
layout(r8ui, binding = 1) writeonly uniform uimage2D state_next;
// claim buffer to resolve conflicts
layout(r32ui, binding = 2) coherent uniform uimage2D claim;
uniform ivec2 u_gridSize;
uniform uint u_frame;
const uint AIR = 0u;
const uint SAND = 1u;
// How far down we scan to approximate column thickness
const int MAX_SCAN = 6;
// How much “taller” this column must be vs neighbor before we slide
// Smaller -> shallower, more flowy piles
const int SLOPE_THRESHOLD = 2;
uint hash(uvec2 p, uint seed) {
p = p * 1664525u + 1013904223u + seed;
p.x ^= p.y >> 16;
p.y ^= p.x >> 16;
return p.x ^ p.y;
}
// Approximate thickness of the sand column under (x, y)
int columnThickness(int x, int y) {
int t = 0;
for (int dy = 0; dy < MAX_SCAN; ++dy) {
int yy = y + dy;
if (yy >= u_gridSize.y) break;
uint v = imageLoad(state_curr, ivec2(x, yy)).r;
if (v == SAND) {
t++;
} else {
break;
}
}
return t;
}
void main() {
ivec2 pos = ivec2(gl_GlobalInvocationID.xy);
if (pos.x >= u_gridSize.x || pos.y >= u_gridSize.y) return;
uint self = imageLoad(state_curr, pos).r;
if (self != SAND) return;
// Lock bottom row in place
if (pos.y == u_gridSize.y - 1) {
imageStore(state_next, pos, uvec4(SAND, 0u, 0u, 0u));
return;
}
ivec2 dest = pos;
// Only relax grains that are resting on something
ivec2 down = pos + ivec2(0, 1);
uint below = imageLoad(state_curr, down).r;
if (below == SAND) {
int hSelf = columnThickness(pos.x, pos.y);
bool canLeft = false;
bool canRight = false;
ivec2 left = pos + ivec2(-1, 0);
ivec2 right = pos + ivec2( 1, 0);
int hLeft = 0;
int hRight = 0;
// Check left side
if (left.x >= 0) {
uint leftCell = imageLoad(state_curr, left).r;
if (leftCell == AIR) {
hLeft = columnThickness(left.x, pos.y);
int slopeL = hSelf - hLeft;
if (slopeL > SLOPE_THRESHOLD) {
canLeft = true;
}
}
}
// Check right side
if (right.x < u_gridSize.x) {
uint rightCell = imageLoad(state_curr, right).r;
if (rightCell == AIR) {
hRight = columnThickness(right.x, pos.y);
int slopeR = hSelf - hRight;
if (slopeR > SLOPE_THRESHOLD) {
canRight = true;
}
}
}
if (canLeft || canRight) {
if (canLeft && canRight) {
// Prefer the *lower* column; randomize if equal
if (hLeft < hRight) {
dest = left;
} else if (hRight < hLeft) {
dest = right;
} else {
uint h = hash(uvec2(pos), u_frame);
bool chooseLeft = (h & 1u) != 0u;
dest = chooseLeft ? left : right;
}
} else if (canLeft) {
dest = left;
} else { // canRight
dest = right;
}
}
}
bool staying = (dest.x == pos.x && dest.y == pos.y);
if (staying) {
imageStore(state_next, pos, uvec4(SAND, 0u, 0u, 0u));
} else {
// Moving to new cell; resolve conflicts via claim buffer
uint old = imageAtomicCompSwap(claim, dest, 0u, 1u);
if (old == 0u) {
imageStore(state_next, dest, uvec4(SAND, 0u, 0u, 0u));
} else {
// lost: stay put
imageStore(state_next, pos, uvec4(SAND, 0u, 0u, 0u));
}
}
}

125
shaders/sand_step.comp
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@ -2,10 +2,25 @@
layout(local_size_x = 16, local_size_y = 16) in;
layout(r8ui, binding = 0) readonly uniform uimage2D state_curr;
layout(r8ui, binding = 1) writeonly uniform uimage2D state_next;
// state_curr: read-only; state_next: write-only
layout(r8ui, binding = 0) readonly uniform uimage2D state_curr;
layout(r8ui, binding = 1) writeonly uniform uimage2D state_next;
// claim buffer to resolve conflicts
layout(r32ui, binding = 2) coherent uniform uimage2D claim;
uniform ivec2 u_gridSize;
uniform uint u_frame;
const uint AIR = 0u;
const uint SAND = 1u;
uint hash(uvec2 p, uint seed) {
p = p * 1664525u + 1013904223u + seed;
p.x ^= p.y >> 16;
p.y ^= p.x >> 16;
return p.x ^ p.y;
}
void main() {
ivec2 pos = ivec2(gl_GlobalInvocationID.xy);
@ -13,30 +28,90 @@ void main() {
return;
}
const uint AIR = 0u;
const uint SAND = 1u;
uint self = imageLoad(state_curr, pos).r;
bool canFallDown =
(self == SAND) &&
(pos.y + 1 < u_gridSize.y) &&
(imageLoad(state_curr, pos + ivec2(0, 1)).r == AIR);
bool filledFromAbove =
(self == AIR) &&
(pos.y > 0) &&
(imageLoad(state_curr, pos + ivec2(0, -1)).r == SAND);
uint next;
if (canFallDown) {
next = AIR;
} else if (filledFromAbove) {
next = SAND;
} else {
next = self;
if (self != SAND) {
// AIR cells: do nothing (next is cleared to AIR on CPU)
return;
}
imageStore(state_next, pos, uvec4(next, 0u, 0u, 0u));
ivec2 dest = pos;
// Hard floor: bottom row does not move down
if (pos.y < u_gridSize.y - 1) {
// 1) Try straight down
ivec2 down = pos + ivec2(0, 1);
bool canDown = (imageLoad(state_curr, down).r == AIR);
if (canDown) {
dest = down;
} else {
// 2) Try diagonals
bool canLeft = false;
bool canRight = false;
ivec2 downLeft = pos + ivec2(-1, 1);
ivec2 downRight = pos + ivec2( 1, 1);
if (downLeft.x >= 0 && downLeft.y < u_gridSize.y) {
if (imageLoad(state_curr, downLeft).r == AIR) {
canLeft = true;
}
}
if (downRight.x < u_gridSize.x && downRight.y < u_gridSize.y) {
if (imageLoad(state_curr, downRight).r == AIR) {
canRight = true;
}
}
// Flip which side is "first" based on frame parity
bool flip = ((u_frame & 1u) != 0u);
ivec2 firstDest, secondDest;
bool canFirst, canSecond;
if (!flip) {
// even frames: left is "first"
firstDest = downLeft;
secondDest = downRight;
canFirst = canLeft;
canSecond = canRight;
} else {
// odd frames: right is "first"
firstDest = downRight;
secondDest = downLeft;
canFirst = canRight;
canSecond = canLeft;
}
if (canFirst && canSecond) {
// both free: pick randomly, but "first"/"second" swap each frame
uint h = hash(uvec2(pos), u_frame);
bool useFirst = (h & 1u) != 0u;
dest = useFirst ? firstDest : secondDest;
} else if (canFirst) {
dest = firstDest;
} else if (canSecond) {
dest = secondDest;
}
// else: no diagonal move, dest stays = pos
}
}
// Write into next state with claim resolution
if (dest.x == pos.x && dest.y == pos.y) {
// Not moving; just keep sand here
imageStore(state_next, pos, uvec4(SAND, 0u, 0u, 0u));
} else {
// Moving to a new cell; try to claim it
uint old = imageAtomicCompSwap(claim, dest, 0u, 1u);
if (old == 0u) {
// Won the slot: move
imageStore(state_next, dest, uvec4(SAND, 0u, 0u, 0u));
// Original pos stays AIR because state_next was cleared
} else {
// Lost the slot: stay put
imageStore(state_next, pos, uvec4(SAND, 0u, 0u, 0u));
}
}
}