The document summarizes a presentation about lighting techniques for a spherical planet in the game Project A1. It discusses using deferred cubic irradiance caching for global illumination that varies based on 12 time spans. Reflection probes are relit based on time of day instead of pre-capturing. Directional lighting and shadows change according to longitude. Sky lighting and bent normals are stored in cubemaps.

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Lighting the Planetary World of
Project A1
Hyunwoo Ki
Lead Graphics Programmer

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A1
• New IP of Nexon
– High-End PC / AAA-quality visuals
– MOBA / Space Opera
– UE4 + @@@
– In Development
• Announced our development last month

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A1
• Talk about character rendering at last NDC 2016 talk
• This talk presents techniques for lighting the world of A1
– Used a test scene
– Not the game world

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World of A1
• Spherical planet
• Real-time day and night cycle
• Partially environment destruction
– Trees, buildings, etc.
• Partially terrain modification
– Craters, explosion, etc.

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Challenges
• Spherical coordinates
• Longitudinal time variation
• Time of day lighting changes
• Dynamic
– Moving Sun
– Destruction
– Modification

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Our Approach
• “Fully Dynamic” if possible
– Shadow maps, SSAO, SSR, SSIS Screen Space Inner Shadows, etc.
• Partially Precomputation and Relighting
– Global illumination, sky lighting and reflection environment

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Changes for Planet: 1
• Vector towards the sky
– Vary according to latitude and longitude
– Standard world: just (0, 0, 1)
– Planetary world: normalize(WorldPosition)
• Assuming (0, 0, 0) is the center of the world
• We call this ‘Planet Normal’

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Changes for Planet: 2
• Longitudinal time variation
– Like GMT
float ComputeGMTFromWorldPosition(float3 WorldPosition)
{
float Longitude = atan2(WorldPosition.y, WorldPosition.x) / PI * 0.5f + 0.5f; // [0, 1]
float NormalizedGMT = frac(Frame.NormalizedDayTime – Longitude); // East to West
return abs(NormalizedGMT);
}
* Note: Frame.NormalizedDayTime = GMT+0 = [0. 1)

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Agenda
• Directional Light
• Global Illumination
• Sky Light
• Reflection Environment

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Directional Light

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Directional Light
• Symmetry of two directional lights
– Sun and Moon
• Movable mobility
– For dynamic scenes
– Real-time lighting
• Deferred rendering: opaque materials
• Forward+ rendering: transparent materials

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Two of Directional Lights
• Sun:
– Dominant light
• Moon:
– Night area
– Adding direct specular

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Two of Directional Lights
• Problem:
– Directional lights commonly affect all surfaces on the world
– Incorrect results
• Ex) Moonlight leaks in the daytime
• Slow (2X)
• Solution:
– Cull backside of the planet from the light
– Smoothly attenuate radiance at boundaries

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Time of Day Lighting
• Different time for each pixel
• Overriding light color by using a hand-painted texture in the shader
• Different methods for GI, sky light and reflection environment
– See further slides

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Sun Shadows
• Shadows have an important role to recognize time during game play
• Presented at my NDC 2016 talk
– Use UE4 implementation
• CSM + PCF
– Add improved PCSS
• To control shadow softness by time: only for 0 and 1 cascade splits
• Shadow normal offset: to remove Peter Panning
• Temporal reprojection: to reduce flickering due to slow moving

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Tighter Shadow Bounds
• For both quality and speed
• Setting tighter bounds
– Assuming very far objects on the view do not cast shadows
– Backside of the planet culling + planetary view frustum culling
– More than 1.5X faster rendering

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Planetary View Frustum Culling
• Limit the far plane as distance
between the camera and the center of the planet
– Assume that we can’t see backside of the planet
• Closer the camera, shorter the far plane
– A proportional expression between
the center of the planet and view frustum planes

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Before
Backside culling
Backside culling
+ View frustum culling

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Global Illumination

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Existing Solutions in UE4
• Lightmaps (X)
– Static, and high memory consumption
• LPV (X)
– Slow, and low quality
• DFGI (X)
– Slow, and not supporting skeletal meshes
• Indirect lighting cache
– Be possible!

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UE4 Indirect Lighting Cache
• SH irradiance volume
• Per-primitive caching
– 5x5 volume -> upload to the global volume texture atlas
– For movable components or preview / Update when the component is moved
• “Try to use volume ILC for all types of components in the scene”
– Including static components and terrain

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Lacks of Volume ILC
• Lighting discontinuity (a.k.a. seams)
• Low density at a large geometry
– Need size-dependent cache distribution
• High memory consumption and slow cache update
• High CPU costs
– Per-primitive computation on the render thread
– Need update cache if a primitive is moved or lighting is changed
• Unsuitable for our game 

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Need of a New Method
• Keep using SH irradiance volume
• More efficient data structure
• Seamless
• Faster update (or no cache update)
• Time of day lighting changes

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Related Work
• Far Cry series
• Assassin Creed series
• Quantum Break
• TC: The Division
• …

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Deferred Cubic Irradiance Caching
• ‘Deferred’:
– As post processing: avoiding overdraw
– Faster development iteration: quick recompile shaders
– But use forward rendering for transparency
• ‘Cubic’:
– Exploiting cubemaps: fit to GPUs
– Cache placement on the world: seamless
– Faster addressing: using planet normal = normalize(WorldPosition)

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Time of Day Lighting
• All day light
– Affect all day
• Time of day light
– Limited by time span
– 12 time spans: 0, 2, 4, …, 20, and 22 hour
• Twelve SH irradiance volumes
– Interpolate lighting from nearest 2 volumes
– No cache update

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Overview
• Offline
– Cache placement
– Photon emission
– Irradiance estimation
• Run-time
– Cubemap caching
– SH lighting

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Offline: Cache Placement
• Based on texels of the cubemap
– N x N x 6
– 50 cm space
– Finding Z by using ray casting

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Offline: Cache Placement
• 2.5D cache placement
– Above the surfaces
– No multi-layers or indoor in our game
– Incorrect results for flying characters
• Multiple layered cubemaps?

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Offline: Photon Emission
• UE4 Lightmass: a photon mapping based light builder
• Store ‘time of day light index’ for deposited photons
• No changes for remainders
class FIrradiancePhotonData
{
FVector4 PositionAndDirectContribution;
FVector4 SurfaceNormalAndIrradiance;
int32 TimeOfDayLightIndex;
};

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Offline: Irradiance Estimation
• Twelve SH irradiance volumes
– Sharing world position, bent normal and sky occlusion
– Difference irradiance by time spans
• Per-time span irradiance
– Indirect photon final gathering
• All day lights: always
• Time of day lights: filtering by its index
• Global sky occlusion and bent normal

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Run-time: Cubemap Caching
• Once at loading time
• CPU irradiance cache -> GPU cubemaps
– half4 texture cube array
• 2nd Order SH: encoded on 3 textures (RGB)
• 12 time spans * 3 = 36 elements
• Misc.:
– Bent normal and sky occlusion: 1
– Average color and directional shadowing: 12 – for reflection environment relighting
• Average color is computed by integrating incident radiance from all directions

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Run-time: SH Lighting
• Every frame
• Texture addressing
– Coordinates: planet normal
– Array index: time of day light index + 12 * {0|1|2}
• SH2 diffuse lighting
– Interpolate radiance from nearest 2 time spans
• Total 6 times fetches of a texture cube array

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Run-time: SH Lighting

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Performance
• Light building
– Scene-dependent
– Costly 12 times final gathering but faster than lightmaps
• Rendering
– Scene-independent
• approximately 0.4 ms: GTX970 1080p / ignoring transparency
• Stable visuals
– No seams or flickering
– Consistent looks for characters and environment

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Sky Light

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Transform to Spherical World
• Sky vector
– Not (0, 0, 1)
– Planet normal = normalize(WorldPosition)
• Rotate planet normal to (0, 0, 1) basis
– Heavy ALU
float3 TransformVectorToLandsphereSpace(float3 InVector, float3 WorldPosition)
{
float3 PlanetNormal = normalize(WorldPosition);
float3 SkyUp = float3(0, 0, 1);
float3 RotationAxis = normalize(cross(PlanetNormal, SkyUp));
float RotationAngle = acos(dot(SkyUp, PlanetNormal));
float3x3 Rotator = RotationMatrixAxisAngle(RotationAxis, RotationAngle);
return mul(Rotator, InVector);
}

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Time of Day Lighting
• Multiple sky cubemaps generated by artists
• Interpolated lighting like GI

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Bent Normal and Sky Occlusion
• As large scale ambient occlusion
– RGB: bent normal
– A: sky occlusion
– One R8G8B8A8 cubemap (no time variation)
• Diffuse lighting
– Widen and soft: sqrt
• Specular lighting
– Narrow and sharp: Square
Sky Occlusion + Bent Normal

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Screen Space Inner Shadows
• For GI and sky lighting
• Better looks when a character is on shadowed surfaces
• SSR styled ray marching
– Tracing on scene depth
– Directionality rather than SSAO
– No precomputation or asset building
– See my NDC 2016 presentation

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Reflection Environment

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Capture Probe Placement
• Uniform distribution on the sphere as the base
– Using golden spiral
• Additional placement by artists

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Time of Day Lighting
• Use relighting instead of pre-capturing all day
– Capture the scene without lighting
– Relight probes with tweak
• Relighting
– Geometric properties
• Relighting position = world position + (reflection vector * capture radius * specular occlusion)
• Relighting normal = normalize(relighting position)
– Direct lighting: Sun and SH2 diffuse sky lighting
– Indirect lighting: deferred cubic irradiance caching
• RGB: irradiance = average color of GI
• A: directional shadowing = occlusion of light sources

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Volumetric Lighting
• Average color of GI (irradiance) is
also used for volumetric lighting
• For each ray marching step (for high spec.)
or at the surface (for low spec.)

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Summary
• Spherical world of A1
– Partially dynamic
– Time of day lighting changes
• Different approaches
– Deferred cubic irradiance caching
– Twelve time spans
– Relighting

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Future Work
• Improved GI
– 2nd Order SH -> 3th Order SH
– Layered cubemaps
– Volumetric fog

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Future Work
• Environment destruction and modification
– Multiple versions of irradiance volumes
• Pre-build for destroyed scenes
• Run-time update of cubemaps
• Like Quantum Break did
– Real-time GI
• SSGI?
– Recapture reflection environment

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Thank you
WE ARE HIRING!