Terrain Rendering in Frostbite using Procedural Shader Splatting (Siggraph 2007)

Terrain Rendering in Frostbite using Procedural Shader Splatting Johan Andersson Rendering Architect, EA DICE 2.5

Outline Previous games Terrain overview Graph-based shaders Terrain shading & texturing Terrain rendering Undergrowth Conclusions Future

Battlefield 2 terrain ” Traditional terrain rendering” Static geometry Heightfields  optimized meshes Unique low-res color map Tiled detail maps 3-6 detail maps Controlled by unique mask textures Macro detail map Fixed shading & compositing Expensive & difficult to add features Special case for mountains/slopes No destruction  Color map Detail mask map

Our requirements Battlefield: Bad Company Frostbite engine pilot project Xbox 360 & PS3 Low memory usage Scaling up BF2 methods (~45mb) not possible High detail up close and far away Long view distance (16 km) Normal maps, multiple texturing techniques Destruction Affecting geometry, texturing, shading

Terrain overview Multiple high-res heightfield GPU textures Easy destruction Used in both VS and PS 16-bit unsigned integer format (L16) Normals calculated in the shader from heightfield Very high detail lighting in a distance Saves memory

Terrain texturing - general idea Compute instead of store Shading, texturing, material compositing Using procedural techniques in shaders Allows changing & adding materials dynamically Destruction! Splat arbitrary shaders over the terrain Artist created Determines both look and visibility of materials Specialized to requirements of material ” Procedural Shader Splatting”

Terrain material requirements Different shading & texturing requirements depending on Natural complexity Distance from camera Importance in game How well used it is (effort to create) And more Example cases Seafloor material (partially obscured) Parallax mapping only on rocky surfaces

Specialized terrain material shaders Vary features & complexity Texture compositing, side-projection, normalmapping, parallax-mapping, specular Flexible tradeoff of memory, performance and quality for each material Can be complex for artists How we’ve always done shaders for other types of geometry

Graph-based surface shaders Artist-friendly Easy to create, tweak and manage Flexible Programmers & artists can extend & expose features Data-centric Encapsulates resources Can create or transform shaders in automated processes Example surface shader graph Rich high-level shader framework Used by all meshes & systems incl. terrain

Instance shaders Shader graph network that can be instanced as a node in another shader Think C/C++ functions Reduces complexity and allows reuse Hide and encapsulate functionality on multiple levels Choose inputs & outputs to expose

Shader pipeline Big complex offline pre-processing system Used surface shaders & states is gathered per level Generates shading solutions HLSL vertex and pixel shaders States, constants, passes Can trade shader efficiency for amount of shader permutations Example: include fog in all shaders or create seperate shaders with and without fog Lots of optimization opportunities

Procedural techniques There are many interesting procedural texturing techniques For example: Wang tiles But most are Heavy or difficult to run on a GPU Difficult to mipmap correctly Require rendering to offscreen targets Want direct evaluation techniques that can be executed directly inside shaders At least as a base Only computes visible pixels

Procedural parameters Build procedural patterns of basic terrain parameters evaluated in all shaders Uses the GPU heightfield textures Commonly used in offline terrain rendering and texture generation Such as Terragen Normal Height Slope

Normal filtering Simple & fast cross filter Not correct at diagonals, but good enough for us Result is world space normal float3 filterNormal(float2 uv, float texelSize, float texelAspect) { float4 h; h[0] = hmap.Sample(bilSampler, uv + texelSize*float2( 0,-1)).r; h[1] = hmap.Sample(bilSampler, uv + texelSize*float2(-1, 0)).r; h[2] = hmap.Sample(bilSampler, uv + texelSize*float2( 1, 0)).r; h[3] = hmap.Sample(bilSampler, uv + texelSize*float2( 0, 1)).r; float3 n; n.z = (h[0] - h[3]) * texelAspect; n.x = (h[1] - h[2]) * texelAspect; n.y = 2; return normalize(n); }

Material masking Terrain shaders determine material mask I.e. visibility Can use Procedural parameters Typically slope Painted masks Arbitrary texturing or shader computation Or combine them all Multiple material masks of all types Red = slope-based cliff material Pink = painted dirt material

Mountain material example Only grass material With mountain material Slope Mask (slope scaled & biased)

Painted masks Many materials can not be solely distributed on a procedural basis Fields, man-made areas, artist control Support painted per-material masks Memory heavy but flexible 0.5 – 8 pixels/meter Coverage typically low 5-15% coverage of levels Not much overlap Fields with painted masks

Static sparse mask textures (1/2) Store painted masks in sparse quadtree textures Major memory reduction Split painted masks into 32x32 tiles and store in atlas texture Only unique tiles DXT5A compression Can use texture arrays But want more than 64/512 slices Source mask Atlas texture

Static sparse mask textures (2/2) Tile index & level textures cover the terrain Tile index: 16-bit integer index into tile atlas Tile level: 8-bit integer. Size of tile world area Low-res, 16 meters/pixel Lookup with world-space position Calculate atlas texture coordinates Details in course notes 4 masks packed together in RGBA For efficiency and to reduce # of samplers

Destruction mask (1/2) Change material and/or look around craters Render decals into destruction mask texture Covers playable area (2x2 or 4x4 km) Usually 2 pixels/meter Material shaders get access to simple 0-1 value Blends in or replaces textures and colors Crater screenshot Crater mask (point-filtering)

Destruction mask (2/2) Observation: 100% destruction not possible in practice Due to gameplay Can store mask as sparse texture to save memory Indirection texture covers whole area RG88, 128x128 resolution = 16m cells .rg indexes into atlas texture 64x64 L8 tiles Gives virtual 8192x8192 texture with tweakable max coverage 16.7 mb -> ~1.7 mb (10% coverage) Atlas texture Indirection texture

Increasing mask detail All the masking techniques can suffer from bluriness due to low resolution Add detail in the shaders! Many methods for generating detail fBm, noise Detail textures Reusing textures with scale, bias and contrast Colormaps, normalmap.b (”occlusion”) And for compositing/blending in the detail Multiply, add, min, max, overlay, custom Fully programmable since in shaders

Photoshop Overlay blend Perfect for blending in high-frequency details Doesn’t affect areas where base mask is 0.0 or 1.0 Good for dynamic flow control float overlayBlend(float base, float value, float opacity) { float a = base < 0.5 ? 2*base*value : 1 - 2*(1-base)*(1-value); return lerp(base, a, opacity); } Base mask from slope Detail mask Overlay blend result

Shader compositing Multiple overlapping materials on terrain Pre-process gathers all material combinations Of materials used in 16x16 m areas Builds big single pass shaders Links together shader graphs (simple!) Redundant resources & calculations automatically removed Dynamic flow control to avoid texture & ALU instructions for materials with mask = 0 2 materials (r & g) creating 3 combos due to overlap

Sea/river floor material added

End result. Road decals + minor materials

Terrain rendering Quadtree for culling & LOD Subdivided dependent on distance Leaves are 33x33 fixed vertex grids Simple Vertex texture fetch when supported/efficient CPU/SPU-filled semi-static height vertex buffer pool otherwise Fixed grid resolution important for ground destruction

Geometry LOD T-junctions between patches of different LOD Need to be removed, causes rendering artifacts Due to vertex shader heightfield sampling Before Removed

T-junction solution Limit neighboring patches to max 1 level difference Select index buffer depending on which side borders to lower-resolution LOD Only 9 permutations needed

Undergrowth Heightfields with good texturing & shading isn’t enough up close Need detail geometry Undergrowth, small foliage, litter, debris Must be able to change with destruction Manual placement not feasible nor preferred

Undergrowth example No undergrowth With undergrowth

Undergrowth overview (1/2) Instance low-poly meshes around views Alpha-tested / alpha-to-coverage Fillrate and sort-independence Procedural on-demand distribution Using terrain materials & shaders Gigabyte of memory if stored Regenerate areas on destruction GPU-assisted

Undergrowth overview (2/2) Managed through a virtual grid structure 16x16m cells Cells allocated from a fixed pool As view position changes Cells contain Semi-static vertex buffer with 4x3 fp16 instancing transforms List of which instance uses which mesh

Undergrowth generation GPU renders out 4-8 terrain material masks & terrain normal From the area the cell covers 3x 64x64 ARGB8888 MRT CPU/SPU scans through texture and distributes instances Fills instancing transform buffer Good fit for D3D10 Stream Output Generated mask Normalmap

Undergrowth distribution Based on randomly jittered grid pattern Grid size determined by material density Random offsets to grid points of max half cell size Gives uniform but varied distribution No / controlled overlap of instances Looks and performs better than fully random solution Deterministic results Grid cell position as seed Important both locally (revisit) and over network

Undergrowth rendering Simple instanced mesh rendering Uses arbitrary surface shaders Unified per-pixel lighting & shadowing Can use cached terrain normalmap to fit in Overdraw main performance bottleneck Front-to-back cell sorting Undergrowth surface shader Shadows on undergrowth

Conclusions Very scalable & extendable Flexible framework for performance tradeoffs Low memory usage Higher quality but higher cost in general For performance For artists Complex shaders requires technical artists Simple workflow for undergrowth Huge data amplification High bang for the buck

Future / Ideas Very complex surface shaders ALU-based noise Wang-tiles More care for shader antialiasing Vector texture maps as masks Cached procedural texture generation Texture synthesis Displacement mapping Adv. natural undergrowth distribution patterns Fully on GPU or SPU

Questions? Contact: johan.andersson@dice.se

Terrain Rendering in Frostbite using Procedural Shader Splatting (Siggraph 2007)

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