Seascape 海面

#version 300 es
precision highp float;

uniform float iTime;
uniform vec2 iResolution;
uniform float u_waveHeight;
uniform float u_waveSpeed;
uniform float u_sunHeight;

out vec4 fragColor;

#define uv (gl_FragCoord.xy / iResolution.xy)

// ---- octave 海面高度场（三角波叠加，Seascape 同款思路）----
// 每层频率翻倍、振幅减半，方向旋转，叠加成水面起伏
float seaHeight(vec2 p) {
  float freq = 0.16;
  float amp = 1.0;
  float t = iTime * u_waveSpeed;
  mat2 rot = mat2(0.8, 0.6, -0.6, 0.8);
  float h = 0.0;
  for (int i = 0; i < 4; i++) {
    // 三角波：|fract(x)-0.| 的变体， Seascape 用 sin 的叠加正弦近似
    vec2 q = p * freq + vec2(t * 0.5, 0.0);
    float w = sin(q.x * 2.0 + sin(q.y * 3.0 + t)) * 0.5
            + sin(q.y * 2.7 - t * 0.7) * 0.3;
    h += w * amp;
    p = rot * p;
    freq *= 2.0;
    amp *= 0.5;
  }
  return h * u_waveHeight;
}

// 海面法线（差分）
vec3 seaNormal(vec2 p) {
  vec2 e = vec2(0.01, 0.0);
  float hL = seaHeight(p - e.xy);
  float hR = seaHeight(p + e.xy);
  float hD = seaHeight(p - e.yx);
  float hU = seaHeight(p + e.yx);
  return normalize(vec3(hL - hR, 2.0, hD - hU));
}

void main() {
  vec2 uv2 = uv * 2.0 - 1.0;
  uv2.x *= iResolution.x / iResolution.y;

  // 相机在水面以上，向前看
  vec3 ro = vec3(0.0, 1.2, -2.5);
  vec3 rd = normalize(vec3(uv2, -1.5));

  // raymarch 海面：水面 y = seaHeight(xz)
  float t = 0.0;
  bool hit = false;
  vec3 hitPos = vec3(0.0);
  for (int i = 0; i < RAYMARCH_STEPS; i++) {
    vec3 p = ro + rd * t;
    float h = seaHeight(p.xz);
    if (p.y < h) { hit = true; hitPos = p; break; }
    t += 0.06;
    if (t > 20.0) break;
  }

  vec3 col;
  if (hit) {
    vec3 n = seaNormal(hitPos.xz);
    vec3 v = -rd;
    vec3 l = normalize(vec3(0.4, u_sunHeight, -0.5));

    // 天空色：高处青绿，地平线琥珀（落日）
    vec3 sky = mix(
      vec3(0.31, 0.82, 0.78),   // 青绿天
      vec3(0.91, 0.69, 0.29),   // 琥珀地平线
      smoothstep(0.5, 0.0, l.y)
    );
    // 深水色
    vec3 deep = vec3(0.02, 0.08, 0.10);
    vec3 shallow = vec3(0.05, 0.18, 0.20);

    // Fresnel：越平视角反射越强
    float fres = pow(1.0 - max(dot(n, v), 0.0), 3.0);
    // 漫反射水色（背光面深、向光面浅）
    float ndl = max(dot(n, l), 0.0);
    vec3 water = mix(deep, shallow, ndl);

    // 太阳高光：视线反射方向接近太阳
    vec3 r = reflect(-v, n);
    float sun = pow(max(dot(r, l), 0.0), 60.0);

    col = mix(water, sky, fres);
    col += vec3(0.91, 0.69, 0.29) * sun * 1.5;  // 琥珀落日反光

    // 距离雾：远处淡化到天空
    float fog = smoothstep(5.0, 18.0, t);
    col = mix(col, sky * 0.8, fog);
  } else {
    // 未命中：天空
    col = mix(
      vec3(0.31, 0.82, 0.78),
      vec3(0.91, 0.69, 0.29),
      smoothstep(0.5, 0.0, uv2.y * 0.5 + 0.3)
    );
    col = mix(col, vec3(0.02, 0.025, 0.03), smoothstep(0.6, 1.0, uv2.y));
  }

  // gamma + tonemap
  col = col / (col + 1.0);
  col = pow(col, vec3(1.0 / 2.2));
  fragColor = vec4(col, 1.0);
}