Presentation at KGC2013 about the techniques developed for COH2 to reproduce the harsh winter conditions of the eastern front of World War 2. It covers the technology developed for dynamic snow and ice rendering, what worked what didn't. It covers as well the lighting and conversion of the COH1 engine from forward to a deferred renderer.

1. Company of Heroes 2 Rendering Tech:
The cold facts of recreating the
hardest winter conditions of World
War 2
Daniel Barrero PhD
Senior Graphics Programmer
Relic Entertainment / SEGA
Korea Game Conference – KGC 2013 – September 25 2013

2. Introduction

3. Presentation Index
• Implementing winter tech, what was tried, what worked,
what didn't:
– Rendering realistic and stylized snow and ice
• Procedurally generated snow
• Automatic snow edge fading
• Ice cracking
– Generating terrain snow tracks and procedural snow
– Solution for fast volumetric outdoor ambient occlusion
– Blizzard FX optimization
• Seamlessly upgrading the COH1 rendering engine from forward to
deferred

4. COH2 Winter Tech Requirements
• Procedural Snow on world objects
• Fully dynamic terrain snow and persistent snow foot prints and
tracks
• Blizzards
• Fully dynamic Ice, can break and refreeze based on player
interaction
• Improved lighting:
• Ambient occlusion is key for snowy enviroments.
• Possibility to add many dynamic lights to the world

5. Rendering Snow on World Objects
• Mostly visual representation of snow accumulation on objects,
a.k.a snow caps:

6. Rendering Snow on World Objects
• Possible solutions:
– Snow prepainted on the texture by artists:
✔Just an art change no change to the shaders or engine
✗Every model has to have a new set of textures with snow
painted on ( lots of work for the artists)
• Can't change the amount of snow dynamically in a realistic
way

7. Procedural Snow on World Objects
Observation: Snow accumulates on surfaces that tend towards
horizontal
It means we can just filter faces whose normals are closer than
a given to the up world vector and apply snow to them!
✔Simple shader change to check up for vertical normals
✔Can use normal map information to get accurate snow
✔Can control the snow amount by checking how far the pixel
normal is from the vertical direction
✔Blending object material with snow material based on
normal up component is fast and looks good.

8. Procedural Snow on World Objects
Observation:
• Snow accumulates on surfaces that tend towards horizontal
• It means we can just filter faces whose normals are closer than
a given to the up world vector and apply snow to them!
Snow covered faces
Faces without snow
False positive for snow
covered faces

9. Procedural Snow on World Objects
Observation: Snow accumulates on surfaces that tend towards
horizontal
It means we can just filter faces whose normals are closer than
a given to the up world vector and apply snow to them!
More Observations:
• Only the topmost surfaces should get snow
• Snow accumulating on top of objects changes the visible shape of
the mesh.
• Snow is thinner and tends to melt towards the edges of surfaces
first.

10. Getting rid of snow inside objects
Problem: Only the topmost surfaces should get snow
Solution: Generate a “snow shadow” map (a.k.a. Top down map)
1. Set a camera above the world it looking down
2. Render a depth map of closest surfaces with it
3. In the normal render check that world height against
the current pixel world height if is less then don't
Put snow on it
Top Down Map
Snow Surface Selection
Top Down
Camera
View
Top Down MapTop Down Map

11. Generating snow caps geometry
Problem: Snow accumulating on top of objects changes the visible
shape of the mesh.
Possible solution: Extrude the geometry of the horizontal faces upward.
Extrude
Direction
Extruded Geometry
Original top faces

12. Generating snow caps: edge fading
Problem: Snow is thinner and tends to melt towards the edges of
surfaces first
Possible solution: If we are extruding the mesh to generate geometry,
then to make snow caps thinner just push the corners of the surface
back in the opposite direction of the edge surface normal.
Extrude
Direction
Push direction
Original top faces
Original non biased
extruded geometry
Extruded Geometry
Average Edge Normal

13. Generating snow caps
Problems with geometry extrusion:
✗ Need to know the edges of surfaces on a mesh and the normal on
the edge. E.g. edges between wall and ceilings
✔Can be solved by offline tool easily by processing the mesh and
looking for big changes in normal direction.
Surface 1
Surface 2
Surface 3
Surface Edge
Edge Normal
Triangle Normal

14. Generating snow caps
Problems with geometry extrusion:
✗ Need to know the edges of surfaces on a mesh and the normal on
the edge and pass it to the shaders. E.g. edges between wall and
ceilings
✔Can be solved by offline tool easily by processing the mesh and
looking for big changes in normal direction.
✗ Need to generate geometry in the GPU
✗ Performance issues on Dx10 hardware
✗How to pass the information to the shaders
•There are lots of special cases

15. Snow caps optimization: virtual
extrusion
Observation: From the standard game camera view just “painting” over
the snow and modifying the perpixel normals it looks almost the same
as with snow caps since we can't see the extruded geometry close by.
Problem: Still need to fade snow towards the edge of surfaces.
• Need to pass edge information to the shaders
Solution: Do a “virtual extrusion”in the shaders:
Vertex Shader:
1. Compute the vertex extruded point
2. Project point into the triangle surface
3. Compute distance to original vertex
4. Pass value into pixel shader
Pixel Shader:
4. Use interpolated “extruded”
Value as snow depth and fade
Snow out.

16. Snow caps optimization: virtual
extrusion
Observation: From the standard game camera view just “painting” over
the snow and modifying the per pixel normal it looks almost the same as
with snow caps since we can't see the extruded geometry close by.
Problem: Still need to fade snow towards the edge of surfaces.
• Need to pass edge information to the shaders
Solution: Do a “virtual extrusion” in the shaders

17. Improving Snow edge fading
Problem: Virtual extrusion produces some odd artifacts on many corner
cases due to different mesh topologies.

18. Snow edge fading on damaged
buildings
Problem: In COH it is possible to damage and destroy parts of a
builings in many ways.
• Buildings are composed of panels
• Panels get damaged by punching “holes” in the texture and
revealing underlying substance
• Shape of the hole is based on damaged neighbors
• Geometry is added on the edges of the damaged hole

19. Improving Snow edge fading
Problem: Virtual extrusion produces some odd artifacts on many corner
cases due to different mesh topologies and use cases
Solution: Take advantage of the “snow shadow” map using image
processing to improve snow edge fading, by finding the min/max
Top Down Map
(world height)
Apply min height
Filter (erode)
Apply top height
Filter (dilate)
Actual used top down map
Red
Channel
Green
Channel
Final snow
Use the two channels to
Determine edge fading.
Add noise to break the edge

20. Improving Snow edge fading
Problem: Virtual extrusion produces some odd artifacts on many corner
cases due to different mesh topologies and use cases
Solution: Take advantage of the “snow shadow” map using image
processing to improve snow edge fading, by finding the min/max
Sample of noise to break
the edge

21. Improving Snow edge fading
Problem: Virtual extrusion produces some odd artifacts on many corner
cases due to different mesh topologies and use cases
Solution: Take advantage of the “snow shadow” map using image
processing to improve snow edge fading, by finding the min/max
Pros:
✔Reuses “snow shadow” which is required anyway
✔ Solves the damaged buildings problem
✔ Mesh geometry doesn't matter
Cons:
✗ Needs a high resolution texture to look good
✗ Snow map costly to generate every frame

22. After all that: final look

23. Terrain Snow
• Artist driven
– Extra terrain layer
– Affects terrain height

24. Terrain Snow
• Artist driven
– Extra terrain layer
– Affects terrain height
– Snow edges procedurally
distorted to reproduce snow
behavior

25. Terrain Snow
• Artist driven
– Extra terrain layer
– Affects terrain height
– Snow edges procedurally
distorted to reproduce snow
behavior
– Use GPU tessellation in
D3D11

26. Terrain Snow affecting Objects
Problem: consistency between terrain snow and object
snow
Solution: Allow objects to “suck in” the terrain snow
material.
✔ It allows the snow to look consistent between terrain and objects
✔ Side effect that artists can tune edge fading more on objects
placed on a map by painting the snow under it
✔ Side effect that damaging terrain removes snow also removes
parts of snow from objects improving the melting snow caused
by damage effect.

27. Terrain Snow affecting Objects
Problem: consistency between terrain snow and object
snow
Solution: Allow objects to “suck in” the terrain snow
material.
Undamaged Terrain Snow Damaged Terrain Snow

28. Terrain Snow Tracks
Extra pass:
– Render objects that moved into a texture with a top down camera view
and clipped at max snow height using an additive shader
Track Map
Top Down
Camera
View
Clip Plane

29. Terrain Snow Tracks
● Extra pass:
– Render objects that moved into a texture with a top down
camera view and clipped at max snow height using an
additive shader
– Just visual, uses bump mapping in D3D10 HW and GPU
tessellation on D3D11

30. Terrain Snow Tracks
Extra pass:
– Render objects that moved into a texture with a top down camera view
and clipped at max snow height using an additive shader
– Pros:
✔Easy way to get precise snow tracks and foot prints
– Cons:
✗Requires a high texel density per terrain area to get accurate foot
prints.
✗Since it persists over multiple frames it is not very SLI/Crossfire
friendly.
✗Requires high frame rate to get smooth continuous vehicle tracks.

31. Muck map
- Material based on snow track depth to make the tracks stand
out more and reveal mud

32. Ice
Requirements:
- Fully dynamic
- Can break and refreeze
- Should look realistic
- Edge breaking should look
good
- Easy to tune by artists

33. Ice
- World has an Ice cell map that controls ice health
- The sim controls breaking and refreezing looking at
ice health of neighboring cells

34. Ice
- World has an Ice cell map that controls ice health
- The simulation controls breaking and refreezing looking at ice
health of neighboring cells
- Ice Shader takes the Ice health map as a texture and uses the
health information to determine how to blend between textures
that represent each ice state:
- Solid
- Broken
- Slush
- Water

35. Ice
Problem: Grid is very visible on the edges of broken ice
Solution: Use an artist defined texture to offset the uv sampling
to break the edges, use noise to break the edges even more:
Sample UV Offset TextureNormal UV
Use UVTexture Value
To sample Ice textures
Final look

36. Outdoor Volumetric Ambient Occlusion
- On snowy weather, ambient occlusion is what gives variety to
the environment and makes snow variation stand out and be
more visible.
- COH1 used fake splat based ambient occlusion.
✗ Not good enough to depict light occlusion on snowy
environments.
✗ Not accurate.
✗ Looks like sets of blobs

37. Outdoor Volumetric Ambient Occlusion
Solution: Take advantage of the top down map used for the
edge fading and “snow shadow” map to compute ambient
occlusion:
- use the height information from the topdown map as a height
map and do quick ray casting in the GPU to compute the
ambient occlusion
TopDownMap Computed Ambient Occlusion

38. Outdoor Volumetric Ambient Occlusion
Solution: Take advantage of the top down map used for the
edge fading and “snow shadow” map to compute ambient
occlusion:
- use the height information from the TopDown map as a height
map and do quick ray casting in the GPU to compute the
ambient occlusion
Pros:
✔ More accurate
Cons:
✗ Slow to compute
can be pre-computed for
static objects

39. Outdoors Volumetric Ambient Occlusion
Without ambient occlusion

40. Outdoors Volumetric Ambient Occlusion
With ambient occlusion

41. Snow Blizzard FX Optimization
Problem:
- Uses lots of big particles for the snow blizzard meaning huge
overdraw even using the special fast Weather FX path
[KGC2012 Relic FX talk: http://www.slideshare.net/proyZ/relics-fx-system]
Solution:
- Make the weather FX deferred:
– Render Weather FX in a 1/2 screen resolution texture
– Render that texture over the full screen as a post-process pass
Huge performance improvement, worst case went from a
frame rate drop of 30% to below 4%

42. COH1 Forward Rendering Engine
• Bleeding edge technology Circa
2006/2007:
• Fully destructible environments
• Dynamic Lighting & Shadows
• Spherical harmonics
• Normal mapping
• HDR
• Dynamic Weather
• D3D9 ( SM 1.1, 2.0, 3.0)
• D3D10 (SM 4.0)
• Etc.

43. COH1 Forward Rendering Engine
• Pros:
✔ Support for lots of old hardware by current
standards.
✔ Proven technology.
✔ Optimized for pre-DirectX11 hardware
• Cons:
✗ Limited number of dynamic lights
✗ Hugely complex ubershaders.
✗ Static branching for performance: lots of auto
generated shader permutations.
✗ Even with packed data used almost all available
pixel shader interpolators.
✗ Literally no space in the shaders to add the new
features needed for COH2 (snow, etc.).
Ambient Occlusion
Near Shadow
Far Shadow
Water View
World View
Water View
Post Processing

44. Why Deferred Rendering for COH2
Cons:
✔ Unable to handle transparent objects
✔ Antialiasing is harder to do properly
✔ Not very optimal for older generations of D3D9 Hardware
Pros:
✔ Unlimited number of lights
✔ Less shader permutations
✔ Hugely simplified and faster shader code
✔ Easier to add modern rendering techniques like (SSAO,etc)
✔ Ability to add the needed features for COH2 to represent winter
and still lots of room to spare to add more features in the future.

45. COH2 Rendering
Top Down Map
Sun Near Shadow
Sun Far Shadow
Render Sun Light
Compute Next Light
Shadow Map
Render Next Light
More
Visible
Lights?
Depth and Simplified
Gbuffer Pass
Light Pass
Combiner Pass
Water View
Light Pass
No
World View
Post Processing
Yes
Track Map

46. COH2 Rendering
Sun Near Shadow
Sun Far Shadow
Post-Processing
Depth Prepass
Gbuffer Pass
SSAO / LAO Pass
Light Pass
Render Sun Light
More
Visible
Lights?
Combine/Resolve Pass
Forward Pass
(Fx/Transparent Objects)
Depth and Simplified
Gbuffer Pass
Light Pass
Combiner Pass
Water View
World View
Light Pass
No
Top Down Map
Track Map
Compute Next Light
Shadow Map
Render Next Light
Yes

47. COH2 Rendering

48. Acknowledgements
• Thanks to Remy Saville, Ian Thompson, Bartek
Nowakowski, Stuart Maxwell, Alun Bjorskten and
James Harrison for their contributions to COH2
winter tech.
• Thanks to all the great Artists at Relic that keep
pushing the limit of what the engine can do and
keep us busy with more ideas of how to make it
better.

49. Thank you
• Questions?
• Contact Information:
• daniel.barrero@gmail.com
• http://danielbarrero.com