Impostors and billboards

An impostor is a flat card that shows a pre-rendered picture of a 3D object instead of drawing its geometry. Far away, the player cannot tell the difference, and a tree of thousands of triangles costs two. SideFX describes its impostor tool as creating "texture sheets to fake 3D objects inside a game engine" (Houdini 21.0 documentation: Labs Impostor Texture render node).

Impostors are usually the last step of a level of detail chain, after the reduced meshes. That page introduces the idea; this one covers the types, the atlas maths and the trade-offs.

#Billboards and octahedral impostors

A billboard is the simplest form: a card that turns to face the camera and shows one view, or a few fixed views. Ryan Brucks's Impostor Baker for Unreal included a "Traditional Billboards" mode with "a 3×3 layout of frames with 8 side views and 1 top view" on a cross of cards (80 Level: Impostor Baker for UE4). Its limitation is that more views mean more cards, and "the 9 card cross is already 72-81 verts" (80 Level: Impostor Baker for UE4).

An octahedral impostor stores many views of the object in one texture, using "a layout based on octahedra", and renders "without needing geometry for each card" (80 Level: Impostor Baker for UE4). The shader picks the stored views closest to the current camera direction. Brucks's tool offered three capture modes: "Full Sphere, Upper Hemisphere, and Traditional Billboards" (80 Level: Impostor Baker for UE4). Upper hemisphere suits objects that are never seen from below, such as trees on the ground, and spends all its frames on the views that matter.

#The atlas maths

An impostor is a texture-memory trade: you save triangles and pay in pixels. SideFX's documentation gives the basic sum: 8 × 8 frames at 128 × 128 pixels each make a "1024×1024 pixel texture" (Houdini 21.0 documentation: Labs Impostor Texture render node).

#Where they are used

• Forests and open worlds. The 80 Level article reports that Brucks's tool "was used for the creation of Fortnite Battle Royale", with tree impostors "built into the LOD actors" (80 Level: Impostor Baker for UE4).
• HLOD. Unreal's World Partition hierarchical LOD has an Instancing layer type that replaces meshes with instanced static meshes, which Epic calls "ideal for imposter meshes, such as trees and foliage" (Unreal Engine documentation: World Partition - Hierarchical Level of Detail).
• Distant vistas. Buildings and rock formations on the horizon can be impostors, as long as the player never gets close.

#Related terms

level of detail · hierarchical LOD · instancing · foliage and vegetation · texture atlases · draw calls

#Further reading and watching