---
title: "Modeling Guidelines"
canonical: "https://rmanwiki-27.pixar.com/space/RU/631308308/Modeling%20Guidelines"
format: markdown
---
# Picking the right tool for the job
In general, [subdivision surfaces](https://rmanwiki-27.pixar.com/space/REN27/542213016/Subdivision+Surfaces) have many advantages, and at Pixar, the vast majority of modeling is created using that approach, but sometimes you actually need a polygonal mesh. This page helps you decide which modelling technique is right for your desired look or effect.
### Subdivision surfaces
- **Pros**
- Perfectly smooth at any resolution.
- Can have sharp or semi-sharp features with creases and corners.
- They can be very lightweight in memory.
- Fewer displacement cracks (watertight)
- **Cons**
- Different modeling techniques/skills
- Need to tag sharp edges and vertices
- Some topological constraints (quads or tris, no non-manifold geometry)
### Polygon meshes
- **Pros**
- Great for heavy geometry like 3D scanned mesh
- Best for geometry that will be fractured / simmed.
- Suitable for models with only sharp edges that don't need displacement
- No micropolygon generation if not displaced.
- **Cons**
- Heavy models use a lot of memory, even when small in the image.
- Shadow terminator may be visible on low-resolution, smoothed objects
- Displacement cracks may be visible
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# Subdivision Modeling Tips
- Always model the simplest possible control cage.
- Always view your model with smoothing or render it using [PxrVisualizer](https://rmanwiki-27.pixar.com/space/REN27/542232400/PxrVisualizer).
- You can even model in a live PxrVisualizer render!
- Use creases to make sharp or semi-sharp edges without adding extra edge loops.
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# Tesselation
If you are coming from another renderer where you have to set subdivision levels, forget about it: **surface tesselation is automatic** in RenderMan (we call it **dicing**).
The main control is the **micropolygon length**.
- By default, it is expressed in pixels, and a micropolygon length 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.
> 📝 We recommend disabling "raster-oriented dicing" unless you see some artefacts.
Increasing the micropolygon length is equivalent to making the object's size smaller in the image.
- When the object is large, it is finely tessellated into millions of micropolygons, showing fine displacement detail.
- When the object becomes smaller than a pixel, only the base mesh remains.
**Micropolygon Length** works a bit like an "automatic level of detail". It preserves memory when an object is so small on the screen that it doesn't need a fully detailed representation.
> 📝 Notice that the subdiv degrades to a much lower number of faces, preserving memory and speeding up raytracing.
This is what we call **Data Amplification**: a compact geometric representation that can be augmented to create more detail on demand.
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# 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 adds a small cost during shading (recomputed at each ray hit).
- Note that our bump-mapping nodes will NOT compute a bump if the incoming ray is too "blurry", so an absence of a bump will go unnoticed.
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.
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# 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.