#version 330 core
out vec4 FragColor;

in vec3 TexCoords;

uniform sampler2D u_CloudTexture;
uniform vec3 u_LightDir;

const float PI = 3.14159265359;

void main() {
  /* Convert 3D texture coords to 2D using spherical mapping. */
  float u = atan(TexCoords.z, TexCoords.x) / (2.0 * PI) + 0.5;
  float v = asin(TexCoords.y) / PI + 0.5;

  /* Sample noise value from cloud texture. */
  float noise = texture(u_CloudTexture, vec2(u, v)).r;

  /* Create soft-edged clouds from the noise. */
  float cloudAlpha = smoothstep(0.5, 0.8, noise);

  /* Some simple lighting. */
  vec3 normal = normalize(TexCoords);
  float diffuse = max(dot(normal, -u_LightDir), 0.0);
  vec3 cloudColor = vec3(1.0) * (diffuse * 0.5 + 0.5); /* Add some ambient light. */

  /* Cloud colour is white. it's transparency is determined by alpha. */
  FragColor = vec4(cloudColor, cloudAlpha);
}