Working with Meshes in Unreal Engine

Static Mesh (SM) vs Instance SM (ISM)

• Static Mesh Components
  • Every single mesh contains:
    • Transform
    • Mesh, materials
    • Physics, collision
    • Lighting
  • Rendering: at least 1 draw call per movable SM

  Result: Heavy to render

• Single Instance
  • An instance contains:
    • Transform
  • Other properties (mesh, material, ...) are common for the whole group
  • Rendering: all instances drawn at once

  Result: Lightweight to render

Why rendering separate objects is slow?

• GPUs are crazy fast at rendering polygons ... but it needs to be fed with data by the (slow) CPU
• OpenGL and DirectX (Up to 11) have a global state - it points to a buffer of vertices and the material to draw with
• Want to draw another mesh or change the material? The CPU will issue a new command, a draw call

Drawing 100 meshes

• Single meshes
  • Apply properties - 100x
  • Set material - 100x
  • Draw - 100x.

  Draw call is slow. It goes also through the graphics driver, which isn't helping

• Partial solution
  • Combine pieces into a single big mesh
    • + Only a few draw calls
    • - Culling hides entire object
    • - Huge weight, LODs are hard to work with

    → Worth to use only for groups of nearby objects

• 100 instances
  • Upload an array of 100 transforms
  • Set material
  • Draw (in one call)

Instanced Static mesh

• One template mesh for all copies
• It does not work with Level of Detail (LoD)

Hierarhical Instanced Static mesh

Like Instanced Static Mesh, but contains multiple instances groups - one group for each LOD (Level Of Detail). The instances are then moved between the groups based on the distance.