Advances in Real - Time Rendering in Games

GPU-Driven Rendering Pipelines
Ulrich Haar, Lead Programmer 3D, Ubisoft Montreal
Sebastian Aaltonen, Senior

Topics

Motivation
Mesh Cluster Rendering
Rendering Pipeline Overview
Occlusion Depth Generation
Results and future work
SIGGRAPH 2015:

GPU-Driven Rendering?

GPU controls what objects are actually rendered
“draw scene” GPU-command
n viewports/frustums
GPU

Motivation (RedLynx)
Modular construction using in-game level editor
High draw distance. Background built

Massive amounts of geometry: architecture

Motivation Assassin’s Creed Unity
SIGGRAPH 2015: Advances

Motivation Assassin’s Creed Unity

Massive amounts of geometry: seamless interiors
SIGGRAPH 2015:

Motivation Assassin’s Creed Unity

Massive amounts of geometry: crowds
SIGGRAPH 2015: Advances

Motivation Assassin’s Creed Unity

Modular construction (partially automated)
~10x instances compared to

Mesh Cluster Rendering

Fixed topology (64 vertex strip)
Split & rearrange all meshes

Mesh Cluster Rendering

Arbitrary number of meshes in single drawcall
GPU-culled by cluster

Mesh Cluster Rendering (ACU)

Problems with triangle strips:
Memory increase due to degenerate

Rendering Pipeline Overview

MULTI-DRAW

COARSE FRUSTUM CULLING

BATCH DRAWCALLS

INSTANCE CULLING (FRUSTUM/OCCLUSION)

CLUSTER CHUNK EXPANSION

CLUSTER CULLING

Rendering pipeline overview

CPU quad tree culling
Per instance data:
E.g. transform, LOD factor...
Updated

Transform
Bounds
Mesh

Rendering Pipeline Overview

MULTI-DRAW

INSTANCE CULLING (FRUSTUM/OCCLUSION)

CLUSTER

Rendering Pipeline Overview

Instance Idx
Chunk Idx

MULTI-DRAW

INSTANCE

Instance Idx
Cluster Idx

Rendering Pipeline Overview

MULTI-DRAW

INSTANCE

Triangle Mask
Read/Write Offsets

Rendering Pipeline Overview

MULTI-DRAW

INSTANCE

Rendering Pipeline Overview

MULTI-DRAW

INSTANCE CULLING (FRUSTUM/OCCLUSION)

CLUSTER

Rendering Pipeline Overview

0

1

0

1

0

1

2

MULTI-DRAW

INSTANCE CULLING (FRUSTUM/OCCLUSION)

CLUSTER

Rendering Pipeline Overview

MULTI-DRAW

INSTANCE CULLING (FRUSTUM/OCCLUSION)

CLUSTER CHUNK EXPANSION

CLUSTER CULLING (FRUSTUM/OCCLUSION/TRIANGLE BACKFACE)

INDEX BUFFER

Static Triangle Backface Culling

Bake triangle visibility for pixel frustums of cluster

Static Triangle Backface Culling
SIGGRAPH 2015: Advances in Real-Time Rendering course

Static Triangle Backface Culling

Only one pixel per cubemap face (6 bits

Occlusion Depth Generation
SIGGRAPH 2015: Advances in Real-Time Rendering course

Occlusion Depth Generation

Hierarchy

Depth pre-pass with best occluders
Rendered in full resolution for

Occlusion Depth Generation

Hierarchy

300 best occluders (~600us)
Rendered in full resolution for High-Z

Shadow Occlusion Depth Generation

For each cascade
Camera depth reprojection (~70us)
Combine with shadow

Camera Depth Reprojection

SIGGRAPH 2015: Advances in Real-Time Rendering course

Camera Depth Reprojection

SIGGRAPH 2015: Advances in Real-Time Rendering course

Camera Depth Reprojection

SIGGRAPH 2015: Advances in Real-Time Rendering course

Camera Depth Reprojection

SIGGRAPH 2015: Advances in Real-Time Rendering course

Camera Depth Reprojection

SIGGRAPH 2015: Advances in Real-Time Rendering course

Camera Depth Reprojection

Light Space Reprojection
SIGGRAPH 2015: Advances in Real-Time Rendering

Camera Depth Reprojection

Reprojection “shadow” of the building
SIGGRAPH 2015: Advances in

Camera Depth Reprojection

Similar to [Silvennoinen12]
But, mask not effective because of

Results

CPU:
1-2 Orders of magnitude less drawcalls
~75% of previous AC, with ~10x

Future

Bindless textures
GPU-driven vs. DX12/Vulkan
SIGGRAPH 2015: Advances in Real-Time Rendering course

RedLynx Topics

Virtual Texturing in GPU-Driven Rendering
Virtual Deferred Texturing
MSAA Trick
Two-Phase Occlusion Culling
Virtual

Virtual Texturing

Key idea: Keep only the visible texture data in memory

GPU-Driven Rendering with VT

Virtual texturing is the biggest difference between our

Single Draw Call Rendering

Viewport = single draw call (x2)
Dynamic branching for

Additional VT Advantages

Complex material blends and decal rendering results are stored

Virtual Deferred Texturing

Old Idea: Store UVs to the G-buffer instead of

Gradients and Tangent Frame

Calculate pixel gradients in screen space. UV distance

Recap & Advantages

64 bits. Full fill rate. No MRT.
Overdraw is dirt

Gradient reconstruction quality

Ground truth

Reconstructed

Difference (x4)
SIGGRAPH 2015: Advances in Real-Time Rendering course

MSAA Trick

Key Observation: UV and tangent can be interpolated
Idea: Render the

1080p Reconstruction

Reconstruct 1080p into LDS
Edge pixels are perfectly reconstructed. MSAA runs

8xMSAA Trick Benchmark

128 bpp G-Buffer
One pixel is a 2x2 tile of

Two-Phase Occlusion Culling

No extra occlusion pass with low poly proxy geometry
Precise

Two-Phase Occlusion Culling

1st phase
Cull objects

Benchmark

“Torture” unit test scene
250,000 separate moving objects
1 GB of mesh data

Benchmark Results

CPU time: 0.2 milliseconds (single Jaguar CPU core)

Xbox One, 1080p
SIGGRAPH

Virtual Shadow Mapping

128k2 virtual shadow map
2562 texel pages
Identify needed shadow pages

VTSM Quality and Performance

Close to 1:1 shadow-to-screen resolution in all areas
Measured:

GPU-Driven Rendering + DX12

NEW DX12 (PC) FEATURES
ExecuteIndirect
Asynchronous Compute
VS RT index (GS

References

[SBOT08] Shopf, J., Barczak, J., Oat, C., Tatarchuk, N. March of

Acknowledgements

Stephen Hill
Roland Kindermann
Jussi Knuuttila
Jalal Eddine El Mansouri
Tiago Rodrigues
Lionel Berenguier
Stephen McAuley
Ivan Nevraev