Game of life shader

game_of_life.glsl

#version 330 core

#ifdef GL_ES
precision highp float;
#endif

uniform vec2 resolution;   // screen size in pixels
uniform float time;
uniform vec2 mouse;        // normalized (0‑1) mouse position
uniform sampler2D prevBuffer;

layout(location = 0) out vec4 diffuseColor;

// ------------------------------------------------------------
// Helper: simple random based on integer cell coordinates
float random (in ivec2 cell) {
    // convert to float, keep the same formula as before
    vec2 p = vec2(cell);
    return fract(100.0 * sin(p.x + fract(100.0 * sin(p.y))));
}

// ------------------------------------------------------------
void main() {
    // ----- 1.  Map fragment to a cell index -----
    const ivec2 GRID = ivec2(192, 108);               // grid size
    vec2 cellSize = resolution / vec2(GRID);         // size of one cell in pixels
    ivec2 cell = ivec2(floor(gl_FragCoord.xy / cellSize));
    
    // ----- 2.  Determine if this cell is alive in the previous frame -----
    // Sample the previous buffer at the centre of the cell
    vec2 uv = (vec2(cell) + 0.5) / vec2(GRID);        // normalized coordinates (0‑1)
    bool self = texture(prevBuffer, uv).g > 0.5;

    // ----- 3.  Count alive neighbours (Moore neighbourhood) -----
    int n = 0;
    for (int dx = -1; dx <= 1; ++dx) {
        for (int dy = -1; dy <= 1; ++dy) {
            if (dx == 0 && dy == 0) continue;

            ivec2 neigh = cell + ivec2(dx, dy);
            // wrap around the edges (toroidal grid)
            neigh = ivec2(mod(vec2(neigh), vec2(GRID)));

            vec2 nUV = (vec2(neigh) + 0.5) / vec2(GRID);
            float col = texture(prevBuffer, nUV).y;
            n += (col > 0.5) ? 1 : 0;
        }
    }

    // ----- 4.  Apply Conway’s Game of Life rules -----
    bool alive = ((n == 2 || n == 3) && self) || (n == 3 && !self);

    // ----- 5.  Mouse interaction (highlight cells near the cursor) -----
    // mouse is supplied in normalized coordinates (0‑1)
    vec2 mouseCell = mouse * vec2(GRID);
    bool nearMouse = distance(vec2(cell), mouseCell) <= 3.0; // 3‑cell radius

    // ----- 6.  Determine final colour -----
    vec3 green = vec3(0.0, 1.0, 0.0);
    vec3 black = vec3(0.0);
    vec3 color = (nearMouse || alive) ? green : black;

    // ----- 7.  Initial random fill (first 0.1 s) -----
    if (time <= 0.1) {
        color = random(cell) > 0.5 ? green : black;
    }

    diffuseColor = vec4(color, 1.0);
}