/*{
  "DESCRIPTION": "A virtual Chladni plate: two vibration modes crossfade over time and grains of sand migrate to the nodal lines, accumulating in a persistent feedback buffer like a real resonating plate. The mode field itself loops exactly; the sand keeps quietly settling and never resets.",
  "CREDIT": "glslop",
  "CATEGORIES": ["Simulation", "Feedback", "Generative"],
  "INPUTS": [
    { "NAME": "rate",  "TYPE": "float", "DEFAULT": 0.05, "MIN": 0.0, "MAX": 0.15 },
    { "NAME": "grain", "TYPE": "color", "DEFAULT": [1.0, 0.9, 0.72, 1.0] },
    { "NAME": "plateHue", "TYPE": "color", "DEFAULT": [0.06, 0.09, 0.2, 1.0] }
  ],
  "PASSES": [
    { "TARGET": "sand", "PERSISTENT": true },
    { }
  ]
}*/

#define PI 3.14159265359

float chladni(vec2 p, float m, float n){
  return sin(PI*m*p.x) * sin(PI*n*p.y) - sin(PI*n*p.x) * sin(PI*m*p.y);
}

float field(vec2 p, float ph){
  // two mode pairs crossfaded by a periodic weight -> the FIELD loops exactly
  float w = 0.5 + 0.5 * cos(2.0 * PI * ph);
  float m = mix(3.0, 5.0, w);
  float n = mix(5.0, 8.0, w);
  return chladni(p, m, n);
}

void main(){
  vec2 uv = isf_FragNormCoord;
  vec2 p = uv * 2.0 - 1.0;
  p.x *= RENDERSIZE.x / RENDERSIZE.y;

  float ph = fract(TIME / 9.0);
  float A = field(p, ph);

  if (PASSINDEX == 0){
    vec3 prev = IMG_THIS_PIXEL(sand).rgb;
    float dt = min(TIMEDELTA * 60.0, 2.0);

    float nodal = 1.0 - smoothstep(0.0, 0.05, abs(A));
    // grains only settle where the plate is actually moving little elsewhere too --
    // bias deposit by a touch of node sharpness so lines stay thin instead of filling
    float sharp = 1.0 - smoothstep(0.0, 0.22, abs(A));
    float deposit = nodal * mix(0.35, 1.0, sharp) * rate * dt;

    vec3 s = prev * (1.0 - 0.02 * dt) + vec3(deposit);

    // soft-knee compress highlights, hard clamp as backstop (float feedback can diverge)
    vec3 hot = max(s - 1.0, 0.0);
    s = min(s, 1.0) + hot / (1.0 + hot);
    s = clamp(s, 0.0, 4.0);

    gl_FragColor = vec4(s, 1.0);
  } else {
    vec3 sandCol = IMG_THIS_PIXEL(sand).rgb;

    float shimmer = 0.5 + 0.5 * sin(A * 5.0 - TIME * 0.6);
    vec3 plate = mix(plateHue.rgb * 0.5, plateHue.rgb * 1.6, shimmer);

    vec3 grains = sandCol * grain.rgb * 1.35;
    vec3 col = plate + grains;

    col = col / (1.0 + col);
    col = pow(col, vec3(0.4545));

    gl_FragColor = vec4(col, 1.0);
  }
}