GLSL Programming/Blender/Reflecting Surfaces



This tutorial introduces reflection mapping (and cube maps to implement it).

It's the second in a small series of tutorials about advanced texture mapping techniques; specifically about environment mapping using cube maps in Blender. The tutorial is based on the per-pixel lighting described in the tutorial on smooth specular highlights and on the concept of texture mapping, which was introduced in the tutorial on textured spheres.



Reflection Mapping with a Skybox
The illustration to the left depicts the concept of reflection mapping with a static skybox: a view ray is reflected at a point on the surface of an object and the reflected ray is intersected with the skybox to determine the color of the corresponding pixel. The skybox is just a large cube with textured faces surrounding the whole scene. It should be noted that skyboxes are usually static and don't include any dynamic objects of the scene. However, “skyboxes” for reflection mapping are often rendered to include the scene from a certain point of view. This is, however, beyond the scope of this tutorial. Moreover, this tutorial covers only the computation of the reflection, it doesn't cover the rendering of the skybox.

For the reflection of a skybox in an object, we have to render the object and reflect the rays from the camera to the surface points at the surface normal vectors. The mathematics of this reflection is the same as for the reflection of a light ray at a surface normal vector, which was discussed in the tutorial on specular highlights.

Once we have the reflected ray, its intersection with a large skybox has to be computed. This computation actually becomes easier if the skybox is infinitely large: in that case the position of the surface point doesn't matter at all since its distance from the origin of the coordinate system is infinitely small compared to the size of the skybox; thus, only the direction of the reflected ray matters but not its position. Therefore, we can actually also think of a ray that starts in the center of a small skybox instead of a ray that starts somewhere in an infinitely large skybox. (If you are not familiar with this idea, you probably need a bit of time to accept it.) Depending on the direction of the reflected ray, it will intersect one of the six faces of the textured skybox. We could compute, which face is intersected and where the face is intersected and then do a texture lookup (see the tutorial on textures spheres) in the texture image for the specific face. However, GLSL offers cube maps, which support exactly this kind of texture lookups in the six faces of a cube using a direction vector. Thus, all we need to do, is to provide a cube map for the environment as a shader property and use the  instruction with the reflected direction to get the color at the corresponding position in the cube map.



Cube Maps
The image to the left shows a Blender cube map, which is discussed in the wikibook “Blender 3D Noob to Pro”. Actually, it is a scaled version of the cube map that is described in the Blender wikibook because the dimensions of faces of cube maps for GLSL shaders have to be powers of two, e.g. 128 × 128.

In order to use the cube map to the left for the reflection in a sphere, add a UV sphere mesh, give it a material and create a new texture as in the tutorial on textures spheres. However, in the Properties window > Textures tab choose Type: Environment Map and click on Environment Map > Image File. Then download the image and open it in Blender with the Environment Map > Open button. (If you select Mapping > Coordinates: Reflection, the Preview > Material should show the applied reflection map.)

The vertex shader has to compute the view direction  and the normal direction   in world space. We saw how to compute them in view space in the tutorial on specular highlights. Therefore, we use this code to go to world space and then back to view space by transforming with the inverse view matrix as discussed in the tutorial on shading in view space. An additional complication is that the normal vector has to be transformed with the transposed, inverse matrix (see “Applying Matrix Transformations”). In this case, the inverse matrix is the inverse of the inverse view matrix, which is just the view matrix itself. Thus the vertex shader could be:

Furthermore, we have to set the uniforms for  and   in the Python script:

Now to the fragment shader. A cube map is defined similarly to a regular 2D texture in a GLSL shader but with  instead of  : The uniform is set in the Python script just like any other texture sampler: Where the  specifies the position in the list of textures.

To reflect the view direction in the fragment shader, we can use the GLSL function  as discussed in the tutorial on specular highlights: And to perform the texture lookup in the cube map and store the resulting color in the fragment color, we would usually simply use: However, Blender appears to reshuffle the cube faces; thus, we have to adapt the coordinates of : This produces a reflection that is consistent with the material preview.

Complete Shader Code
The shader code then becomes:

Summary
Congratulations! You have reached the end of the first tutorial on environment maps. We have seen:
 * How to compute the reflection of a skybox in an object.
 * How to import suitable cube maps in Blender and how to use them for reflection mapping.