Pick the right tool for the job

In general, subdivision surfaces have many advantages but sometimes you actually need a polygon mesh.

Subdivision surfaces

• Pros
  • Perfectly smooth at any resolution.
  • Can have sharp or semi-sharp features with creases and corners.
  • Can be very lightweight in memory.
  • Less displacement cracks (watertight)
• Cons
  • Different modeling techniques/skills
  • Need to tag sharp edges and vertices
  • Some topological constraints (no holes or non-manifold geometry)

Polygon meshes

• Pros
  • Great for heavy geometry like 3D scanned mesh
  • Best for geometry that will be fractured / simmed.
  • Good for models with only sharp edges that don't need displacement
  • No microploygon generation if not displaced.
• Cons
  • Heavy models use a lot of memory, however small they may be in the image.
  • Shadow terminator may be visible on low resolution smoothed objects
  • Displacement cracks may be visible

NURBs

• Pros
  • Perfectly smooth at any resolution.
  • Lightweight in memory
• Cons
  • Quad-only modeling
  • Not supported in XPU
  • Not very popular nowadays

Suggested reading:

• Geometry
• Subdivision Surfaces
• Polygons

Subdivision modeling

• Always model the simplest control cage possible.
• Always view your model with smoothing or render it using PxrVisualizer.
  • You can even model in a live PxrVisualizer render !
• Use creases to make sharp or semi-sharp edges without adding extra edge loops.
  
• Corners work like creases for vertices.
  
• There are 3 type of subdivision surfaces.
  
  • Catmull-Clark is the most flexible and popular one.
  • Loop is a triangle-based subdivision surface.
    • No creases or corners
    • useful for cloth simulations
  • Bilinear is like a catmull-clark with infinitely hard creases on all edges
    

Tesselation

If you are coming from another renderer where you have to set subdivision levels, forget about it: tesselation of surfaces is automatic in RenderMan (we call it dicing).

The main control is the micropolygon length.

• By default, it is expressed in pixels and a micropolygonlength of 1 (the default) means that every micropolygon will be approximately 1 pixel wide.
• If you don't want the tesselation to change dynamically, you can set it in object or world space or define a dicing camera.

Model detailing

Often, it is better to rely on displacement and bump mapping to add details.

• Displacement allows you to add larger geometric features that impact your object's outline and shadowing.
  • Displacement impacts time to first pixel (micropolygon generation) but doesn't slow down shading.
• Bump or normal maps will add small scale details
  • Bump mapping add small cost during shading (recomputed at each ray hit).
  • Note that our bump mapping nodes will NOT compute bump if the incoming ray is too "blurry", which means an absence of bump will go un-noticed.

If you have modeled a highly detailed object in ZBrush, you should consider:

• retopologizing to create a lightweight model

• extracting a displacement map from the high-resolution mesh.

  Our displacement nodes have a "model displacement" input to connect the extracted displacement map. 

Memory usage and performance

Polygon meshes

• Shortest time to first pixel (TTFP) if not displaced.
• When displaced, the mesh will be diced and micropolygons generated.
• Un-displaced heavy meshes use the same amount of RAM, whether they are full screen or 2 pixels high.

Subdivision surfaces

• Slightly longer TTFP but dicing is multi-threaded and efficient.
• Use a bit more memory than meshes for the same number of vertices.
• A sparsely modeled subdiv will only contain the base mesh faces when 2 pixels high.
  • This adaptivity lowers memory usage and speed up raytracing.