GLSL Programming/Unity/Skyboxes

This tutorial covers the rendering of environment maps as backgrounds with the help of cube maps.

It is based on Section “Reflecting Surfaces”. If you haven't read that tutorial, this would be a very good time to read it.

Rendering a Skybox in the Background

As explained in Section “Reflecting Surfaces”, a skybox can be thought of as an infinitely large, textured box that surrounds a scene. Sometimes, skyboxes (or skydomes) are implemented by sufficiently large textured models, which approximate an infinitely large box (or dome). However, Section “Reflecting Surfaces” introduced the concept of a cube map, which actually represents an infinitely large box; thus, we don't need the approximation of a box or a dome of limited size. Instead, we can render any screen-filling model (it doesn't matter whether it is a box, a dome, or an apple tree as long as it covers the whole background), compute the view vector from the camera to the rasterized surface point in the vertex shader (as we did in Section “Reflecting Surfaces”) and then perform a lookup in the cube map with this view vector (instead of the reflected view vector in Section “Reflecting Surfaces”) in the fragment shader:

         #ifdef FRAGMENT
 
         void main()
         {
            gl_FragColor = textureCube(_Cube, viewDirection);
         }
 
         #endif

For best performance we should, of course, render a model with only a few vertices and each pixel should be rasterized only once. Thus, rendering the inside of a cube that surrounds the camera (or the whole scene) is fine.

Complete Shader Code

The shader should be attached to a material, which should be attached to a cube that surrounds the camera. In the shader code, we deactivate writing to the depth buffer with ZWrite Off such that no objects are occluded by the skybox. (See the description of the depth test in Section “Per-Fragment Operations”.) Front-face culling is activated with Cull Front such that only the “inside” of the cube is rasterized. (See Section “Cutaways”.) The line Tags { "Queue" = "Background" } instructs Unity to render this pass before other objects are rendered.

Shader "GLSL shader for skybox" {
   Properties {
      _Cube ("Environment Map", Cube) = "" {}
   }
   SubShader {
      Tags { "Queue" = "Background" }
      
      Pass {   
         ZWrite Off
         Cull Front

         GLSLPROGRAM
 
         // User-specified uniform
         uniform samplerCube _Cube;   
 
         // The following built-in uniforms 
         // are also defined in "UnityCG.glslinc", 
         // i.e. one could #include "UnityCG.glslinc" 
         uniform vec3 _WorldSpaceCameraPos; 
            // camera position in world space
         uniform mat4 _Object2World; // model matrix
 
         // Varying
         varying vec3 viewDirection;
 
         #ifdef VERTEX
 
         void main()
         {            
            mat4 modelMatrix = _Object2World;
 
            viewDirection = vec3(modelMatrix * gl_Vertex 
               - vec4(_WorldSpaceCameraPos, 1.0));
 
            gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
         }
 
         #endif
 
 
         #ifdef FRAGMENT
 
         void main()
         {
            gl_FragColor = textureCube(_Cube, viewDirection);
         }
 
         #endif
 
         ENDGLSL
      }
   }
}

Shader Code for Unity's Skybox System

The shader above illustrates how to render a skybox by rendering a cube around the camera with a specific shader. This is a very general approach. Unity, however, has its own skybox system that doesn't require any game object: you just specify the material with the skybox shader in the main menu Edit > Render Settings > Skybox Material and Unity takes care of the rest. Unfortunately, we cannot use our shader for this system since we have to do the lookup in the cube texture at the position that is specified by the vertex texture coordinates. This is actually easier than computing the view direction. Here is the code:

Shader "GLSL shader for skybox" {
   Properties {
      _Cube ("Environment Map", Cube) = "" {}
   }
   SubShader {
      Tags { "Queue" = "Background" }
      
      Pass {   
         ZWrite Off
         Cull Off

         GLSLPROGRAM
 
         #extension GL_NV_shadow_samplers_cube : enable

         // User-specified uniform
         uniform samplerCube _Cube;   
  
         // Varying
         varying vec3 texCoords;
 
         #ifdef VERTEX
 
         void main()
         {            
            texCoords = vec3(gl_MultiTexCoord0);
            gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
         }
 
         #endif
 
 
         #ifdef FRAGMENT
 
         void main()
         {
            gl_FragColor = textureCube(_Cube, texCoords);
         }
 
         #endif
 
         ENDGLSL
      }
   }
}

As mentioned above, you should create a material with this shader and drag the material to Edit > Render Settings > Skybox Material. There is no need to attach the material to any game object.

Summary

Congratulations, you have reached the end of another tutorial! We have seen:

How to render skyboxes.
How to render skyboxes in Unity without game objects.

Rendering a Skybox in the Background

Complete Shader Code

Shader Code for Unity's Skybox System

Summary

Further Reading