Improve Skybox eyeDirection precision (#7685)

filament/src/materials/skybox.mat

@@ -58,6 +58,15 @@ fragment {
 
 vertex {
     void materialVertex(inout MaterialVertexInputs material) {
-        material.eyeDirection.xyz = material.worldPosition.xyz;
+        // This code is taken from computeWorldPosition and assumes the vertex domain is 'device'.
+        vec4 p = getPosition();
+        // GL convention to inverted DX convention
+        p.z = p.z * -0.5 + 0.5;
+        vec4 worldPosition = getWorldFromClipMatrix() * p;
+        // Getting the true world position would require dividing by w, but since this is a skybox
+        // at inifinity, this results in very large numbers for material.eyeDirection.
+        // Since the eyeDirection is only used as a direction vector in the fragment shader, we can
+        // skip that step to improve precision.
+        material.eyeDirection.xyz = worldPosition.xyz;
     }
 }