Practical Volume rendering for realtime applications

Stoyan Nikolov

VIDEO

Who am I

Stoyan Nikolovstoyan@coherent-labs.com@stoyannk

Co-Founder & Tech lead @ Coherent Labs

Senior Graphics dev @ Masthead Studios

Senior dev @ Eutelia

Why use Volume rendering at all?

What is Volume rendering?

Types of Volume rendering techniques

Practical (today) rendering techniques

Overview

Why Volume rendering

• Allows us to see „inside“ an object

• Medical imaging

• Industry

• Biology

Why Volume rendering - Games

Freedom of form

Why Volume rendering - Games

Destructability

EverQuest Next by Sony Online Entertainment

Why Volume rendering - Games

Procedural generation

Volume rendering - Input

Usually discrete volume elements — voxels

Direct rendering in RT

• Ray-marching distance fields• Gigavoxel• Sparse voxel octrees (SVO)• SVO for Global Illumination

slisesix demo by rgba

Unreal Engine 4 by Epic Games

• Create a polygon mesh• GPUs are designed for polygons• Other subsystems usually expect polygons

Indirect rendering for RT

Basic algorithms

Marching cubes / tetrahedra / MC 33

Realtime requirements

• We want to support very large meshes

• Level of Detail (LOD) required

• Difficult with MC

• Transvoxel designed to solve this

Transvoxel

• Outline in Eric Lengyel’s PhD thesis

• Designed for RT applications

• Originally implemented in C4 Engine

Transvoxel overview

• Based on MC

• Extend equivalence classes

Basic Algorithm

• Subdivide grid in blocks (once)• Polygonize blocks (once)• Decide blocks to draw and LOD (per-frame)• Draw relevant blocks (per-frame)

The LOD solution

• Introduce “transition cells” between levels• Restricted octree for rendering• Identify neighbors and draw correct transitions

Transition cells

Important detail

Surface shifting

Texturing

• Per-voxel material Id → Texture Ids• Triplanar projection• Two sets of textures are blended for better transitions

Parallelism

• Embarrassingly parallel

• Relatively easy for GPGPU

• Parts can be re-computed independently

DEMO

Dual contouring

• Solves the sharp edges• Works on hermite data sets

VoxelFarm engine by Voxel Farm Inc.

Comparison

• MC-based algorithms

faster

require less memory

• Dual algorithms support more geometries

Potential headaches

• Memory consumption

• Correct implementations sometimes tricky

• Texturing

THANK YOU

stoyannk@gmail.com@stoyannkstoyannk.wordpress.com