Procedural Graphics with DirectX 11

James Reid

Meshes

• A mesh is what all the textures are laid on.• Meshes are deformed to give us the nice

terrains seen in games today using height maps or procedural methods.

• More vertices require more GPU power!• Goal: reduce number of vertices in a mesh

without sacrificing quality.

Height Maps• Advantages:

– Easily modeled by an artist.– Easily implemented.– Normal maps can be paired for lighting.– Use far less memory than meshes.– Easier on the CPU/GPU.

• Disadvantages:– Only able to show 256 distinct heights in grayscale.– Can quickly consume hard disk space when having to store

height/normal/tangent/etc maps.• Image: The Elder Scrolls V: Skyrim

What is Procedural?

• From Wikipedia: “content generated algorithmically rather than manually”

• Content is created at run time, not compile time or stored in file.

• Most games use height maps and models.• Image: The Elder Scrolls III: Morrowind

Why Use Procedural?

• Memory usage is far less on both the hard disk and in RAM.

• Size limitations are much larger than those involving height maps.– Allows games to be released on a single CD/DVD.– Makes downloadable games more manageable.

• Image: Minecraft

Basis For Procedural

• Noise algorithms.– Perlin– Voronoi

• Can be implemented in different dimensions.– 1D– 2D (used for terrain/clouds)– 3D (used for clouds the user can fly through)

How Noise Works

• Summing up of static maps generated with pseudorandom numbers.

• Basic Algorithm:

double t = 0;double amplitude = 1;double freq = m_dFrequency;

for(int i = 0; i < m_nOctaves; i++) { t += GetValue(x * freq + m_nSeed, y * freq + m_nSeed) * amplitude; amplitude *= m_dPersistence; freq *= 2;}return t;

Using Perlin Noise

• Start with a mesh.• Generate a height field using Perlin Noise.• Assign the coordinate for each pair in the

mesh.• Result:

Scalability

• Makes very convincing terrain.• Issues:– High number of vertices.– Static meshes can’t adapt to camera’s view.

• Solution:– Use a quadtree structure to store the data.

Quadtree Structure

• Similar to binary trees, but has 4 nodes instead of 2.

Quadtree Mesh

Uses For The Quadtree

• View frustum culling.• Level of detail (LOD).• Collision detection.• Grid location.• Reducing total vertices.

View Frustum

• Field of view the camera has.• Used in culling and LOD.

Frustum Culling

• Removing of primitives that are not seen from the graphics pipeline to increase performance.

• More types:– Backface– Contribution– Occlusion

Level of Detail (LOD)

• The amount of detail shown being based on the camera’s view point.

• Where DirectX 11 makes a difference.– Use of the geometry shader.

Vertex Interpolation Error

• When changing LOD the new vertices may cause popping.

• Several solutions to this, most common is to just take the midpoint of both vertices.

Texture Morphing

• LOD can cause textures to not line up and varying levels of detail.

• Solution: Use texture blending to hide the effect.

Vertex Reduction

• Use minimum number of vertices based on slope of terrain.

• Image: Shamus Young (Twenty Sided)– Top right: no optimization– Bottom left: optimized grid, removed about two-

thirds of total vertices (Huge performance increase!)

Tessellation

• Started in DirectX 10 – can be controlled by user with DirectX 11

• Take a mesh and add more primitives to smooth it out.

Putting It All Together

• Use CPU to determine LOD and any culling to be done based on view frustum.

• Use noise algorithm in real time on the GPU to generate height field.

• Remove any unwanted vertices based on slope of terrain (e.g. flat areas will use two triangles instead of filling up the mesh).

• Morph the textures at the LOD breaks.• Tessellate the mesh in high detail locations.

Examples

• http://www.youtube.com/watch?v=rL8zDgTlXso– Start at 3:20

• http://www.youtube.com/watch?annotation_id=annotation_790483&feature=iv&src_vid=OiGADgezjC8&v=oUSdSjnDB_E– Start at 0:49

• http://www.youtube.com/watch?v=uzXB0m3_MHQ– Start at 0:25

References• Schneider, J., & Westermann, R. (2006). Gpu-friendly high-quality terrain rendering.,

Computer Graphics and Visualization Group, Technische Universität München, Munich, Germany.

• Yusov, E., & Turlapov, V. (2007). Gpu-optimized efficient quad-tree based progressive multiresolution model for interactive large scale terrain rendering., Department of Computational Mathematics and Cybernetics, Nizhny Novgorod State University, Nizhny Novgorod, Russia.

• Perlin, K. (2001). Improving noise., Media Research Laboratory, Dept. of Computer Science, New York University , New York City, NY, .

• Verts, W. T., & Hill, Jr., F. S. (1989). Quadtree meshes, COINS Department, ECE Department, University of Massachusetts, Amherst, MA, .

• Olsen, J. (2004). Realtime procedural terrain generation., Department of Mathematics And Computer Science (IMADA), University of Southern Denmark, .

• Bernhardt, A., Maximo, A., Velho, L., Hnaidi, H., & Cani, M. (2011). Real-time terrain modeling using cpu–gpu coupled computation., INRIA, Grenoble Univ., Univ. Lyon 1, IMPA, France, Brazil.