Add LOD for planet rings

src/celengine/body.h

@@ -23,6 +23,7 @@
 #include <GL/glew.h>
 #include <string>
 #include <vector>
+#include <array>
 #include <map>
 #include <list>
 
@@ -87,7 +88,7 @@ class RingSystem
     float outerRadius;
     Color color;
     MultiResTexture texture;
-    GLuint vboId{ 0 };
+    std::array<GLuint, 4> vboId {{ 0, 0, 0, 0 }};
 
     RingSystem(float inner, float outer) :
         innerRadius(inner), outerRadius(outer),

src/celengine/render.cpp

@@ -4783,15 +4783,20 @@ void Renderer::renderObject(const Vector3f& pos,
         }
     }
 
-    if (obj.rings != nullptr &&
-        (renderFlags & ShowPlanetRings) != 0 &&
-        distance <= obj.rings->innerRadius)
+    float segmentSizeInPixels = 0.0f;
+    if (obj.rings != nullptr && (renderFlags & ShowPlanetRings) != 0)
     {
-        renderRings_GLSL(*obj.rings, ri, ls,
-                         radius, 1.0f - obj.semiAxes.y(),
-                         textureResolution,
-                         (renderFlags & ShowRingShadows) != 0 && lit,
-                         this);
+        // calculate ring segment size in pixels, actual size is segmentSizeInPixels * tan(segmentAngle)
+        segmentSizeInPixels = 2.0f * obj.rings->outerRadius / (max(nearPlaneDistance, altitude) * pixelSize);
+        if (distance <= obj.rings->innerRadius)
+        {
+            renderRings_GLSL(*obj.rings, ri, ls,
+                             radius, 1.0f - obj.semiAxes.y(),
+                             textureResolution,
+                             (renderFlags & ShowRingShadows) != 0 && lit,
+                             segmentSizeInPixels,
+                             this);
+        }
     }
 
     if (obj.atmosphere != nullptr)
@@ -4958,6 +4963,7 @@ void Renderer::renderObject(const Vector3f& pos,
                              radius, 1.0f - obj.semiAxes.y(),
                              textureResolution,
                              (renderFlags & ShowRingShadows) != 0 && lit,
+                             segmentSizeInPixels,
                              this);
         }
     }

src/celengine/renderglsl.cpp

@@ -588,7 +588,8 @@ renderAtmosphere_GLSL(const RenderInfo& ri,
 
 static void renderRingSystem(GLuint *vboId,
                              float innerRadius,
-                             float outerRadius)
+                             float outerRadius,
+                             unsigned int nSections = 180)
 {
     struct RingVertex
     {
@@ -597,12 +598,12 @@ static void renderRingSystem(GLuint *vboId,
     };
 
     constexpr const float angle = 2*static_cast<float>(PI);
-    constexpr const unsigned int nSections = 180;
 
     if (*vboId == 0)
     {
         struct RingVertex vertex;
         vector<struct RingVertex> ringCoord;
+        ringCoord.reserve(2 * nSections);
         for (unsigned i = 0; i <= nSections; i++)
         {
             float t = (float) i / (float) nSections;
@@ -669,6 +670,7 @@ void renderRings_GLSL(RingSystem& rings,
                       float planetOblateness,
                       unsigned int textureResolution,
                       bool renderShadow,
+                      float segmentSizeInPixels,
                       const Renderer* renderer)
 {
     float inner = rings.innerRadius / planetRadius;
@@ -773,7 +775,16 @@ void renderRings_GLSL(RingSystem& rings,
     else
         glDisable(GL_TEXTURE_2D);
 
-    renderRingSystem(&rings.vboId, inner, outer);
+    unsigned nSections = 180;
+    size_t i = 0;
+    for (i = 0; i < rings.vboId.size() - 1; i++)
+    {
+        float s = segmentSizeInPixels * tan(PI / nSections);
+        if (s < 30.0f) // TODO: make configurable
+            break;
+        nSections <<= 1;
+    }
+    renderRingSystem(&rings.vboId[i], inner, outer, nSections);
 
     glBlendFunc(GL_SRC_ALPHA, GL_ONE);
 

src/celengine/renderglsl.h

@@ -68,6 +68,7 @@ void renderRings_GLSL(RingSystem& rings,
                       float planetOblateness,
                       unsigned int textureResolution,
                       bool renderShadow,
+                      float segmentSizeInPixels,
                       const Renderer* renderer);
 
 void renderGeometry_GLSL_Unlit(Geometry* geometry,