The document discusses screen space reflections implemented in the game The Surge. It describes using screen space ray marching against the depth buffer to find reflection points, convolving the scene to accumulate multiple bounces, and using asynchronous compute to overlap rendering passes and improve performance. Key techniques included interleaved rendering, temporal reprojection, and using local data storage. Performance gains were achieved through optimizations like lower resolution rendering and computing mip chains in-place.

1. Screen Space Reflections in
Michele Giacalone
Graphics Programmer @ Deck13

2. ▶ Current working title
▶ Sci-Fi third person action RPG
▶ Exoskeletons!
▶ In-house engine “Fledge”
▶ Multiplatform
▶ PC (D3D11)
▶ Xbox One
▶ PS4
Deck13 - The Surge

3. What this talk is NOT about:
▶ Novel rendering technique
▶ Accurate physically based approach
▶ Heavy math formulas
Disclaimer

4. What this talk IS about:
▶ What worked for us
▶ How we approached the problem
▶ Share ideas that can be used for other techniques
Disclaimer

5. SSR OFF

6. SSR ON

7. SSR ON

8. ▶ Overview
▶ Rendering
▶ Async Compute
▶ Conclusions
Agenda

9. Overview

10. ▶ Performance
▶ Not that much frame time to spare
▶ Maximum budget allowed < 2 ms in worst case
▶ Still other features to implement
▶ Particularly true on Xbox One platform
▶ Quality
▶ Plausible BRDF match with our IBL
▶ Contact hardening reflections
▶ Ambient specular occlusion approximation
▶ No aggressive masking based on roughness
Overview - What we wanted

11. ▶ Compute reflection vector from view direction
▶ Use GBuffer normals
▶ Ray marching against depth buffer
▶ Iterate until ray ‘intersects’ the depth buffer
▶ Use hit coordinate to resolve reflection color
▶ Reproject from previous frame
Overview - Screen Space Reflections
Hit Point

12. Rendering

13. ▶ Tile classification
▶ Ray Marching
▶ Convolve Scene
▶ Resolve Reflections
▶ Deinterleave and Reproject
▶ Async Compute
Rendering - Overview

14. Rendering - Tile Classification
▶ Some texels are not contributing
▶ Other texels might require extra marching steps
▶ Divide screen in 16x16 texel tiles
▶ Fast ray march
▶ Sparse ray distribution [Wronski14]
▶ 64 rays in 16x16 texel
▶ Non-uniform jittered
▶ Different each frame to maximize coverage
▶ Estimate tile ray hit variance
▶ Discard non contributing tiles
▶ Produce GPU job queue
▶ Encode tile data into uint32
▶ Append to GPU job queue
▶ Consume later on with DispatchIndirect
(0, 4) (0,5) (0,6) (0, 8) ...
GPU job queue

15. ▶ Naive approach is simple but it is also slow
▶ Hi-Z is sexy but might have too much overhead
▶ Depth sample distribution is a serious thing [McGuire14]
▶ Don’t forget you’re bound to screen space data
▶ What about depth thickness?
▶ And sampling coherency?
▶ What else?
▶ (ノಠ益ಠ)ノ彡┻━┻
Rendering - Ray Marching Overview

16. ▶ Ray march at lower resolution (720p, 900p)
▶ Interleaved rendering
▶ Even/Odd checkerboard pattern [El Mansouri16]
▶ Successive passes work with interleaved data
▶ Use low resolution depth buffer
▶ Less bandwidth, better cache usage
▶ No big impact on quality
▶ Importance sampling (GGX distributed rays)
▶ Fixed ray step count
▶ Line segment intersection [Valient14][Timonen15]
▶ Jitter ray start time, reduce banding artifacts
▶ Noise filtered out with temporal reprojection
▶ Process 4 depth values at time to hide VMEM latency (GCN)
▶ Output hit coordinate in a R10G10B10A2_UNORM target
Rendering - Ray Marching
A B C D
E F G H
I J K L
M N O P
B D
E G
J L
M O
A C
F H
I K
N P
Odd Frame
Checkerboard Pattern
Even Frame

17. Ray Hit Point (Interleaved) Attenuation mask (Interleaved)

18. ▶ Based on “Screen-Space Cone-Traced Reflections” [Uludag14]
▶ Create convolved scene buffer mip chain
▶ Use previous frame buffer
▶ Includes reflections
▶ Accumulate multiple bounces
▶ 7x7 separable blur in a single dispatch
▶ Derive cone angle from roughness
▶ Best fit to match IBL
▶ Accumulate samples
▶ Use roughness as weight factor
▶ On Consoles
▶ Compute mip chain on same resource
▶ Avoid unnecessary copies
▶ Saves ~0.1 ms
Rendering - Convolve Scene And Resolve Reflections

19. MIP 0 MIP 1 MIP 2
MIP 3 MIP 4 MIP 5

20. Resolved Reflections (Interleaved)

21. ▶ Based on “Screen-Space Cone-Traced Reflections” [Uludag14]
▶ Create convolved scene buffer mip chain
▶ Use previous frame buffer
▶ Includes reflections
▶ Accumulate multiple bounces
▶ 7x7 separable blur in a single dispatch
▶ Derive cone angle from roughness
▶ Best fit to match IBL
▶ Accumulate samples
▶ Use roughness as weight factor
▶ On Consoles
▶ Compute mip chain on same resource
▶ Avoid unnecessary copies
▶ Saves ~0.1 ms
Rendering - Convolve Scene And Resolve Reflections

22. ▶ Deinterleave samples into LDS (Local Data Share)
▶ Load samples into LDS
▶ Extra samples required for reconstruct neighbour data
▶ Combine reads with gather
▶ Reconstruct missing samples using neighbors
▶ Temporal Reprojection
▶ Neighbors color data already available in LDS ☺
▶ Clamp history with 3x3 neighborhood AABB [Karis14]
▶ Use reversible tone map operator to reduce fireflies [Karis13]
▶ Local Data Storage (Grandma's Home Remedy)
▶ "Careful With That Axe, Eugene"
▶ Store separate RGB channels
▶ Pack two color channel into a single slot
Rendering - Deinterleave and Reproject
Loaded Samples into LDS

23. Final Reflections (Deinterleaved + Temporal Reprojection)

24. Async Compute

25. Async Compute - Dependencies
Tile Classification
Convolve Scene
Depth Buffer
Prev Frame Buffer
Deinterleave And Reproject Resolve Reflections
Ray Marching
Main dependencies:
▶ Depth Buffer
▶ Available after GBuffer
▶ Previous Frame Buffer
▶ Available after scene combine

26. ▶ Start computing data in previous frame directly ☺
▶ Async dispatch Convolve Scene right after scene is resolved
▶ Overlaps mostly SAT and Post Process
▶ Bandwidth intensive, limit occupancy
▶ Async dispatch Tile Classification right after GBuffer
▶ Overlaps Decal Rendering
▶ Helps filling the holes in the pipeline
▶ Async dispatch Ray Marching
▶ Remaining Passes
▶ Async Dispatch while Shadow Rendering
▶ Find the right balance with Compute Lighting
▶ Do not use CS if you can use PS instead!
▶ On PC D3D11, no async dispatch available
▶ On GCN, going through CB cache is generally faster [Persson14]
Async Compute - Dispatch

27. Conclusions

28. ▶ Usually few depth samples are enough
▶ Line segment intersection works great!
▶ Thin objects require more samples
▶ Use hybrid tracing algorithms [Stachowiak15]
▶ Interleaved rendering is awesome!
▶ Easy to use with other passes (e.g. SSAO)
▶ GPU work queues can be useful
▶ Dispatch only required threads
▶ Can overlap other Compute jobs (Console, D3D12, Vulkan, etc.)
▶ Reality check!
▶ Screen space data inherited problems
▶ Extremely easy to break
▶ Maybe invest GPU time in something else? [Pettineo11]
Conclusions - What we learnt

29. Conclusions - Performance Table
Tile
Classification
Ray Marching Convolve
Scene
Resolve
Reflections
Deinterleave
and Reproject
Total
0.07 ms 0.21 ms 0.43 ms 0.41 ms 0.27 ms 1.39 ms
Xbox One, SSR @ 720p, (no ESRAM, No Async Compute)

30. References
[ElMonsouri16] Jalal El Mansouri, “Rendering Rainbow Six Siege”, GDC, 2016
[Stachowiak15] Tomasz Stachowiak, “Stochastic Screen-Space Reflections”, SIGGRAPH, 2015
[Timonen15] Ari Silvennoinen and Ville Timonen, “Multi-Scale Global Illumination in Quantum Break”, SIGGRAPH, 2015
[McGuire14] Morgan McGuire and Michael Mara, “Efficient GPU Screen-Space Ray Tracing”, JCGT, 2014
[Uludag14] Yasin Uludag, “Hi-Z Screen-Space Cone-Traced Reflections”, In GPU Pro 5, 2014
[Valiant14] Michal Valient, “Reflections and Volumetrics of Killzone: Shadow Fall”, SIGGRAPH, 2014
[Karis14] Brian Karis, “High-Quality Temporal Supersampling”, SIGGRAPH, 2014
[Wronski14] Bart Wronski, “Assassin’s Creed 4: Road to Next-gen Graphics”, GDC, 2014
[Persson14] Emil Persson, “Low-Level Shader Optimization for Next-Gen and DX11”, GDC, 2014
[Pettineo11] Matt Pettineo, “10 Things that need to die for Next-Gen”,
https://mynameismjp.wordpress.com/2011/12/06/things-that-need-to-die/
[Karis13] Brian Karis, “Tone Mapping”, http://graphicrants.blogspot.de/2013/12/tone-mapping.html

31. Thank You!
Email: mgiacalone@deck13.com
Twitter: miccode