Morphing and Animation GPU Graphics Gary J. Katz University of Pennsylvania CIS 665 Adapted from articles taken from ShaderX 3, 4 and 5 And GPU Gems 1

Morphing  Vertex Tweening – two key meshes are blended varying by time.  Morph Targets – vertex tweening applied only to local displacements. Represent morph targets by relative vectors from the base mesh to the target meshes

Morph Target Animation  Morph Target Animation – one base mesh can morph into multiple targets at the same time. Facial animation Muscle Deformation

Morph Target Animation Linear Interpolation: Relative: Position Output = Position Source + (Position Destination * Factor) Absolute: Position Output = Position Source + (Position Destination – Position Source )*Factor

Relative vs. Absolute 4 3 RelativeAbsolute

Constraints 1. Number of vertices must be the same 2. Faces and attributes must be the same 3. Material must be equal 4. Textures must be the same 5. Shaders, etc must be the same Useful only where skinning fails!

Data Structures for Morphing  DirectX allows for flexible vertex formats  Unsure if OpenGL supports flexible formats  Position 1 holds the relative position for the morph target D3DVERTEXELEMENT9 pStandardMeshDeclaration[] = { { 0, 0, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_POSITION, 0 }, { 0, 12, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_POSITION, 1 }, { 0, 24, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_NORMAL, 0 }, { 0, 32, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_TEXCOORD, 0 }, D3DDECL_END() }

Skeletal Animation  Hierarchical animation 1. Mesh vertex is attached to exactly one bone 2. Transform vertex with the inverse of the bone’s world matrix  Issues Buckling occurs at regions where two bones are connected

Skeletal Subspace Deformation Vertices are attached to multiple bones by weighting 1. Move each vertex into every associated bone space by multiplying the inverse of the initial transformation 2. Apply current world transformation 3. Resulting vertices are blended using morphing

Shader Model 2.0 Approach  Go into Dawn demo here

GPU Animation  Can skip the processing of unused bones or morph targets Need hardware support for:  Dynamic branching  Can separate the modification and the rendering process Need hardware support for:  Four component floating-point texture formats  Multiple render targets Normal Map Position Map Tangent Map

Method 1  Hold the vertex data in texture arrays  Manipulate the data in the pixel shader  Re-output to texture arrays  Pass the output as input to vertex shader

Storage Procedures If: vertex array is one-dimensional frame buffer is two-dimensional index2D.x = index % textureWidth; index2D.y = index / textureWidth; index = index2D.y * textureWidth + index2D.x;

Redefining the View Draw a rectangle of coordinates (0,0), (0,1), (0,1), (1,1) Remap them using the following vertex program float4 VS(float4 index2D: POSITION0, out float4 outIndex2D : TEXCOORD0) : POSITION { outIndex2D = index2D; return float4(2 * index2D.x – 1, -2 * index2D.y + 1, 0, 1); } (-1, 1), (1,1), (-1,-1), (1,-1)

GPU Animation Pixel Shader float2 halfTexel = float2(.5/texWidth,.5/texHeight); float4 PS(float4 index2D : TEXCOORD0, out float4 position : COLOR0, out float4 normal : COLOR1,...) { index2D.xy += halfTexel; float4 vertAttr0 = tex2Dlod(Sampler0, index2D); float4 vertAttr1 = tex2Dlod(Sampler1, index2D);... // perform modifications and assign the final // vertex attributes to the output registers }

Analysis Advantage  Keeps vertex and geometry processing units workload at a minimum Why is this good?  Good for copy operations and vertex tweeningDisadvantage  Per-vertex data has to be accessed through texture lookups  Number of constant registers is less in pixel shader (224) than vertex shader (256)  Can not divide modification process into several pieces because only a single quad is drawn  Therefore, Constant registers must hold all bone matrices and morph target weights for entire object

Method 2  Apply modifications in the vertex shader, do nothing in the pixel shader  Destination pixel is specified explicitly as a vertex shader input  Still writing all vertices to a textureAdvantage Can easily segment the modification groupsDisadvantage Speed issues make this method impractical

Accessing Modified Data  Do NOT want to send the data back to the CPU, except in one case  Solution: Direct-Render-To-VertexBuffer The problem: Direct-Render-To-VertexBuffer doesn’t exist yet (but we can always dream)  Solution 2: Transfer result from render target to vertex buffer object on graphics card Use OpenGL’s ARB_pixel_buffer_object  Solution 3: Use RenderTexture capability and then access the texture in the vertex shader Store the texture lookup in the vertices texture coordinates Problem: Vertex textures are SLOW Can not execute vertex texture lookups and other instructions in parallel

Performance Issues  Preferable to perform modification and rendering in single pass  Accessing vertex attributes using vertex texturing is always slower than performing a fast copy within video memory  Accessing morph in a vertex texture makes the application too slow, must use constants

Usage  To get real speed advantage use a hybrid CPU GPU approach 1. Let the CPU compute the final vertex attributes used during rendering frames n and n+k 2. Let the GPU perform vertex tweening at frames greater than n and smaller than n+k 3. Phase shift the animations between characters so that the processors do not have peak loads  Advantage Vertex tweening is supported on almost all hardware No restrictions on modification algorithms because it is performed on CPU

Massive Character Animation  Can perform simple AI effects  Each pixel of output texture holds one character’s state  Pixel shader computes next state  State is used to determine which animation to use

Simulating Character Behavior  Implement Finite State Machine in Pixel Shader Walk Turn If no Obstacle Run If Chased If Obstacle If Chased If Obstacle

Implementing an FSM on GPU Use dependent texture lookups  Agent-space maps: Contain information about the state of characters (position, state, frame)  World-space image maps: Contain information about the environment to influence the behavior of the character  FSM Maps: Contain information about the behavior for each state and about transitions between states. Rows group transitions within the same state Columns contain conditions to trigger transitions