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Occlusion Culling in Alan Wake<br />Ari Silvennoinen<br />

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Rendering Challenges in Alan Wake<br />

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Introduction<br /><ul><li>Rendering challenges in Alan Wake

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Large outdoor world

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Dynamic objects

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Local shadow rendering

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Global shadow rendering

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Umbra internals

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Techniques and ideas used to tackle the rendering challenges

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Umbra is the visibility middleware used in Alan Wake

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Supports both hardware and software occlusion </li></li></ul><li>Large Outdoor Worlds<br /><ul><li>Occlusion Culling and LOD

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Efficient data handling

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Only a subset of the world fits in the memory</li></li></ul><li>Requirements for Visibility<br /><ul><li>Scalable culling (view frustum, distance and occlusion)

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Out-of-core objects should be cheap

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Efficient initial construction

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Incremental updates for dynamic objects</li></li></ul><li>Umbra Architecture <br />Client<br />Umbra<br />Insert/Update<br />Spatial database<br />Query begin<br />Query traversal<br />Callbacks<br />Query end<br />

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Spatial Database<br /><ul><li>Axis-aligned BSP-tree with varying SAH split planes

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Premultiplied OBB representation for objects

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4x3 matrix for mapping the unit cube to world space OBB

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Lazy update mechanism based on camera frustum

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Mark nodes as dirty, update when actually traversed

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Automatic amortized updates

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Split plane optimization to improve spatial subdivision

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Dead node cleanup to save memory on out-of-core locations</li></li></ul><li>Occlusion Culling<br /><ul><li>Hierarchical hardware occlusion culling

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Avoiding latency by interleaving work

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Umbra does hierarchy traversal and database update tasks to prevent CPU stalling

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Works well in practice, however 100% utilization impossible due to GPU starvation

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Avoiding latency by delayed reads

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Artifacts!

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Can not use full hierarchy -> not scalable</li></li></ul><li>Occlusion Culling<br /><ul><li>Hybrid solution brings the best of both worlds

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Screen space error bounds for artifacts

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Adaptive query bounds

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Some queries can be guaranteed to be ready with high probability

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Keep the query count low

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Queries are cheap but not free

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Intelligent batching

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Special rules for expensive objects</li></li></ul><li>Occlusion Culling<br />

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Dynamic Objects<br /><ul><li>Dynamic objects live in the same database

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Temporal bounding volumes

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Linear prediction with conservative dynamics model

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Stable and fast

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Temporal bounds are AABBs

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Has even been used with bullets driven by physics 

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Take advantage of lazy and amortized updates

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Keeps hierarchy balanced during movement

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Ensures optimal node placement when object becomes static</li></li></ul><li>Dynamic Objects<br />

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Local Shadows<br /><ul><li>Accurate bounding geometry

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Spheres and frustums

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Aggressive culling

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Distance culling

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Test every frame

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No batching

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Shadow casters use frustum culling</li></li></ul><li>Global Shadows<br /><ul><li>Shadow map costs

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Shadow map generation

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Shadow map sampling + filtering

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Lot of objects + cascaded shadows = high shadow map generation cost

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What do we really need to draw to a shadow map?</li></li></ul><li>Observation I<br />Cull objects in shadow<br />

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Observation II<br />Cull objects in which do not cast a visible shadow<br />

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Shadow Caster Culling<br /><ul><li>Use camera visibility to determine relevant shadow casters

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Test each cascade for visibility

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Check if the cascade contains any visible objects

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Can potentially skip shadow map generation

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Compute light space scissor rectangle for each cascade

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Early geometry culling

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These ideas generalize

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Bittner et. Al, Shadow Caster Culling for Efficient Shadow Mapping, I3D 2011</li></li></ul><li>Cascaded Shadow Maps<br />

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Cascaded Shadow Maps<br />

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Cascaded Shadow Maps<br />

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Cascaded Shadow Maps<br />Small scissor<br />Empty cascade<br />

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Conclusions and Future Work<br /><ul><li>Efficient spatial database in a key role

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Visibility-aware algorithms

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Shadow caster culling

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Streaming?

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Simulation and AI?

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Rethink visibility and spatial connectivity

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See our slides from “Advances in Real-Time Rendering in Games” course </li></li></ul><li>Thanks<br /><ul><li>Remedy Entertainment

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Petri Häkkinen

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Markus Mäki

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Olli Tervo

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TuukkaTaipalvesi

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Colleagues at Umbra Software

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TimoAila

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Ville Miettinen</li></li></ul><li>Questions? <br />Questions?<br />