gl_FragDepth：用平面模拟球体

uniform mat4 projectionMatrix;   // Three.js 只在顶点 shader 里自动声明它
uniform float uRadius;
uniform float uWriteDepth;
in vec2 vUv;
in vec3 vViewPos;
out vec4 fragColor;

void main() {
  vec2 p = vUv * 2.0 - 1.0;
  float r2 = dot(p, p);
  if (r2 > 1.0) discard;                            // 圆外的部分丢掉
  vec3 N = vec3(p, sqrt(1.0 - r2));                 // 把这里当作球面
  vec3 surface = vViewPos + vec3(0.0, 0.0, N.z * uRadius);
  vec4 clip = projectionMatrix * vec4(surface, 1.0);
  float sphereDepth = clip.z / clip.w * 0.5 + 0.5;  // NDC 的 z → 深度缓冲的 0～1
  gl_FragDepth = mix(gl_FragCoord.z, sphereDepth, uWriteDepth);
  float light = max(dot(N, normalize(vec3(-0.5, 0.7, 0.8))), 0.0);
  fragColor = vec4(vec3(0.48, 0.83, 0.69) * (0.2 + 0.8 * light), 1.0);
}