improve planet rings shaders

data/shaders/opengl/lib.glsl

@@ -12,6 +12,25 @@ struct Surface {
  float ambientOcclusion;
 };
 
+// Phase functions
+// https://www.scratchapixel.com/lessons/procedural-generation-virtual-worlds/simulating-sky/simulating-colors-of-the-sky.html
+float miePhaseFunction(const float g, const float mu)
+{
+ /*
+ * Mie phase function:
+ */
+ return 3.f / (8.f * 3.141592) * ((1.f - g * g) * (1.f + mu * mu)) / ((2.f + g * g) * pow(1.f + g * g - 2.f * g * mu, 1.5f));
+}
+
+float rayleighPhaseFunction(const float mu)
+{
+ /*
+ * Rayleigh phase function:
+ */
+ return 3.f / (16.f * 3.141592) * (1 + mu * mu);
+}
+
+
 // Currently used by: hopefully everything
 // Evaluates a standard blinn-phong diffuse+specular, with the addition of a
 // light intensity term to scale the lighting contribution based on (pre-calculated)

data/shaders/opengl/planetrings.frag

@@ -7,6 +7,7 @@
 uniform sampler2D texture0;
 in vec2 texCoord0;
 in vec4 texCoord1;
+in vec4 varyingEyepos;
 
 out vec4 frag_color;
 
@@ -16,10 +17,24 @@ void main(void)
  vec4 col = vec4(0.0);
  vec4 texCol = texture(texture0, texCoord0);
 
+ vec3 eyenorm = normalize(varyingEyepos.xyz);
+
  for (int i=0; i<NUM_LIGHTS; ++i) {
  float l = findSphereEyeRayEntryDistance(-vec3(texCoord1), vec3(uViewMatrixInverse * uLight[i].position), 1.0);
  if (l <= 0.0) {
- col = col + texCol*uLight[i].diffuse;
+ // first term: diffuse light phase (like in full/new moon)
+ float mu = dot(normalize(vec3(uLight[i].position)), eyenorm);
+ float diffuse = sqrt((1 - mu) / 2);
+
+ // second term: diffuse mie (scattering through ring)
+ float g = 0.76f;
+ float phaseThrough = miePhaseFunction(g, mu);
+
+ // third term: reflect mie (imitate albedo >= 1.0)
+ float muRev = dot(-normalize(vec3(uLight[i].position)), eyenorm);
+ float phaseReflect = miePhaseFunction(g, muRev);
+
+ col = col + texCol * (diffuse + phaseThrough + phaseReflect) * uLight[i].diffuse;
  }
  }
  col.a = texCol.a;

data/shaders/opengl/planetrings.vert

@@ -6,11 +6,13 @@
 
 out vec2 texCoord0;
 out vec4 texCoord1;
+out vec4 varyingEyepos;
 
 void main(void)
 {
  gl_Position = matrixTransform();
 
  texCoord0 = a_uv0.xy;
  texCoord1 = a_vertex;
+ varyingEyepos = uViewMatrix * a_vertex;
 }

src/Planet.cpp

@@ -156,9 +156,9 @@ void Planet::GenerateRings(Graphics::Renderer *renderer)
  {
  Color *row;
  row = buf.get();
- std::fill_n(row, RING_TEXTURE_WIDTH, Color::BLACK);
+ std::fill_n(row, RING_TEXTURE_WIDTH, Color::BLANK);
  row = buf.get() + (RING_TEXTURE_LENGTH - 1) * RING_TEXTURE_WIDTH;
- std::fill_n(row, RING_TEXTURE_WIDTH, Color::BLACK);
+ std::fill_n(row, RING_TEXTURE_WIDTH, Color::BLANK);
  }
 
  const vector3f texSize(RING_TEXTURE_WIDTH, RING_TEXTURE_LENGTH, 0.0f);