RSL
What is a Displacement Shader?

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Displacement Shader Algorithm

Displacement shaders alter the smoothness of a surface, however, unlike bump mapping which mimics the appearance of bumpiness by reorientating surface normals, displacement shading genuinly effects the geometry of a surface. In the case of Pixars prman renderer, each object in a 3D scene is sub-divided into a fine mesh of micro-polygons after which, if a displacement shader has been assigned to an object, they are "pushed" or "pulled" in a direction that is parallel to the original surface normal of the micro-polygon. After displacing the micro-polygon the orientation of the local surface normal(N) is recalculated.

Figure 1

The following algorithm lists the four basic steps that a displacement shader generally follows in order to set the position (P) and normal (N) of the micro-polygon being shaded.

To make a meaningful decision about the distance, if any, a micro-polygon should be displaced, a shader will make reference to the micro-polygon's,

• 2D surface position s, t, u, v,
• 3D xyz position P,
• orientation N,
• camera distance L.

plus other less obvious attributes of a micro-polygon. Such information is either directly or indirectly available in data the renderer makes available to a shader through the use of global variables.

Displacement Shading & Global Variables

The following table lists the global variables accessible to a displacement shader. [List of global variables available to surface shaders].