#version 430 core

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

void main() {
    vec2 fragCoord = gl_FragCoord.xy;

    // Normalized coordinates
    vec2 uv = fragCoord / u_resolution;
    uv.y = 1.0 - uv.y;
    // Map to grid coordinates
    ivec2 cell = ivec2(uv * vec2(u_gridSize));
    cell = clamp(cell, ivec2(0), u_gridSize - ivec2(1));

    uint v = texelFetch(u_state, cell, 0).r;

    vec3 color = (v == 1u)
        ? vec3(0.9, 0.8, 0.1)        // sand
        : vec3(0.05, 0.05, 0.10);    // background

    FragColor = vec4(color, 1.0);
}