mib_bent_normal_env

Category: mental ray > Texture

Shader Family: Texture

Output: Color

"Bent normal" is a term used for the average unoccluded direction vector from a surface point. For completely unoccluded surfaces, this is the same as the normal vector, but for surfaces occluded by other geometry, this points in the direction in which the least amount of occluding geometry is found. Bent normals are used as an acceleration technique for ambient occlusion, allowing for rendering that looks like global illumination or final gathering lit by an environment, but at a small fraction of the rendering time.

Ambient occlusion as done by the mib_amb_occlusion shader is a raytracing technique that casts potentially large numbers of probe rays to determine to which extent a surface point is occluded. The speed of this operation depends on the number of rays (samples), the reach of the rays (max_distance), and the complexity of the scene.

When rendering an animation (or multiple views of the same scene), any object that does not move, does not change shape, or has no moving occluding object nearby yields the same result for every frame. Therefore, you can "bake" (render to a file) the ambient occlusion solution once in a first rendering pass, and then reuse this result in subsequent rendering passes for any number of frames, with potentially huge performance gains. If you also bake the average unoccluded direction (the bent normal) to a texture, the entire process of lighting the object based on an environment is moved to this second rendering pass, without having to trace a single ray.

By setting the output_mode parameter to 2, 3, or 4 on the mib_amb_occlusion shader, bent normals are returned, with the vector being encoded as a color where X is red, Y is green, and Z is blue. If occlusion_in_alpha is enabled, the scalar occlusion value is returned in the alpha channel. This color can be baked into a texture, such as by connecting mib_amb_occlusion shader to the mib_lightmap_write's input parameter and then rendering it.

Once the texture file is generated, you can assign the mib_bent_normal_env shader to a surface shader, such as to the ambient parameter of mib_illum_phong, to look up an environment to light the object based on the baked bent normal texture, which is connected to the bent_normals parameter in the mib_bent_normal_env shader.

This allows for a very "low-cost" simulation of global illumination-like effects for rigid objects and is especially suitable for animations. The technique does not work on deformable objects since their occlusion, shape, and normals can vary from frame to frame. For deformable objects, it is better to apply the occlusion shader directly, without baking its output.