improved shadow volume for mobile

Source/Shaders/Builtin/Functions/depthClamp.glsl

@@ -1,16 +1,32 @@
 // emulated noperspective
-#ifndef LOG_DEPTH
+#if defined(GL_EXT_frag_depth) && !defined(LOG_DEPTH)
 varying float v_WindowZ;
 #endif
 
 /**
- * Clamps a vertex to the near and far planes.
+ * Emulates GL_DEPTH_CLAMP, which is not available in WebGL 1 or 2.
+ * GL_DEPTH_CLAMP clamps geometry that is outside the near and far planes, 
+ * capping the shadow volume. More information here: 
+ * https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_depth_clamp.txt.
+ *
+ * When GL_EXT_frag_depth is available we emulate GL_DEPTH_CLAMP by ensuring 
+ * no geometry gets clipped by setting the clip space z value to 0.0 and then
+ * sending the unaltered screen space z value (using emulated noperspective
+ * interpolation) to the frag shader where it is clamped to [0,1] and then
+ * written with gl_FragDepth (see czm_writeDepthClamp). This technique is based on:
+ * https://stackoverflow.com/questions/5960757/how-to-emulate-gl-depth-clamp-nv.
+ *
+ * When GL_EXT_frag_depth is not available, which is the case on some mobile 
+ * devices, we must attempt to fix this only in the vertex shader. 
+ * The approach is to clamp the z value to the far plane, which closes the 
+ * shadow volume but also distorts the geometry, so there can still be artifacts
+ * on frustum seams.
  *
  * @name czm_depthClamp
  * @glslFunction
  *
  * @param {vec4} coords The vertex in clip coordinates.
- * @returns {vec4} The vertex clipped to the near and far planes.
+ * @returns {vec4} The modified vertex.
  *
  * @example
  * gl_Position = czm_depthClamp(czm_modelViewProjection * vec4(position, 1.0));
@@ -20,8 +36,12 @@ varying float v_WindowZ;
 vec4 czm_depthClamp(vec4 coords)
 {
 #ifndef LOG_DEPTH
+#ifdef GL_EXT_frag_depth
     v_WindowZ = (0.5 * (coords.z / coords.w) + 0.5) * coords.w;
-    coords.z = clamp(coords.z, -coords.w, +coords.w);
+    coords.z = 0.0;
+#else
+    coords.z = min(coords.z, coords.w);
+#endif
 #endif
     return coords;
 }

Source/Shaders/Builtin/Functions/writeDepthClamp.glsl

@@ -1,9 +1,11 @@
 // emulated noperspective
-#ifndef LOG_DEPTH
+#if defined(GL_EXT_frag_depth) && !defined(LOG_DEPTH)
 varying float v_WindowZ;
 #endif
+
 /**
- * Clamps a vertex to the far plane by writing the fragments depth.
+ * Emulates GL_DEPTH_CLAMP. Clamps a fragment to the near and far plane
+ * by writing the fragment's depth. See czm_depthClamp for more details.
  * <p>
  * The shader must enable the GL_EXT_frag_depth extension.
  * </p>