FORM / FUNCTION —
HIỆN THỰC HÓA THỦ CÔNG SỐ

// Procedural GLSL Fragment Shader: Micro-tactile Paper & Concrete Normal Map
precision highp float;

uniform vec2 u_resolution;
uniform float u_time;
uniform vec2 u_mouse;
uniform float u_scroll_velocity;

// Simplex 2D noise generator for microscopic paper grain
vec3 mod289(vec3 x) { return x - floor(x * (1.0 / 289.0)) * 289.0; }
vec2 mod289(vec2 x) { return x - floor(x * (1.0 / 289.0)) * 289.0; }
vec3 permute(vec3 x) { return mod289(((x*34.0)+1.0)*x); }

float snoise(vec2 v) {
  const vec4 C = vec4(0.211324865405187, 0.366025403784439,
                     -0.577350269189626, 0.024390243902439);
  vec2 i  = floor(v + dot(v, C.yy));
  vec2 x0 = v -   i + dot(i, C.xx);
  vec2 i1 = (x0.x > x0.y) ? vec2(1.0, 0.0) : vec2(0.0, 1.0);
  vec4 x12 = x0.xyxy + C.xxzz;
  x12.xy -= i1;
  i = mod289(i);
  vec3 p = permute(permute(i.y + vec3(0.0, i1.y, 1.0)) + i.x + vec3(0.0, i1.x, 1.0));
  vec3 m = max(0.5 - vec3(dot(x0,x0), dot(x12.xy,x12.xy), dot(x12.zw,x12.zw)), 0.0);
  m = m*m ; m = m*m ;
  vec3 x = 2.0 * fract(p * C.www) - 1.0;
  vec3 h = abs(x) - 0.5;
  vec3 ox = floor(x + 0.5);
  vec3 a0 = x - ox;
  m *= 1.79284291400159 - 0.85373472095314 * (a0*a0 + h*h);
  vec3 g;
  g.x  = a0.x  * x0.x  + h.x  * x0.y;
  g.yz = a0.yz * x12.xz + h.yz * x12.yw;
  return 130.0 * dot(m, g);
}

void main() {
  vec2 uv = gl_FragCoord.xy / u_resolution.xy;
  
  // Optical page curvature driven by fluid velocity
  float bend = sin(uv.x * 3.14159) * (u_scroll_velocity * 0.04);
  uv.y += bend;

  // Micro-tactile fiber grain pass (Zero raster textures downloaded)
  float grain = snoise(uv * 1280.0) * 0.038;
  vec3 paperBase = vec3(0.941, 0.933, 0.925) - grain;
  
  // Specular light sweep across printed foil typography
  vec2 lightPos = u_mouse / u_resolution.xy;
  float dist = length(uv - lightPos);
  float specular = pow(max(1.0 - dist * 1.8, 0.0), 32.0) * 0.22;

  gl_FragColor = vec4(paperBase + specular, 1.0);
}