1. Maths & Technologies for Games DirectX 11 – New Features Tessellation & Displacement Mapping
CO3303
Week 19

2. Today’s Lecture Direct3D 11 – New features
Direct3D 11 – Changes from Direct3D 10
New DX11 Pipeline
Tessellation Overview
Patches
Details: Hull Shader / Tessellation Stage / Domain Shader
Displacement Mapping
Technical Considerations and Issues

3. DirectX 11 – New Features DirectX 11 was introduced with Windows 7
But is also supported on Windows Vista
Major new features introduced in DX11:
Multithreaded rendering
A single device can have several contexts
Different threads that can render using the same resources
Tessellation
Introduced in this lecture
Compute Shaders
General purpose (non-graphics) programming on the GPU
More recently, support has been added for DX10 hardware
Shader Model 5.0 – extra shader language features
High quality texture compression formats

4. Converting from DX10 to DX11
DirectX 11 is a strict superset of DX10.1
Nearly everything in DX10 works with minimal change
Not like the huge changes from DX9 to DX10
Key points when converting:
The device pointer (g_pD3DDevice) has been split in two
A device pointer for overall control & a context pointer for each thread
Use the immediate context pointer for single threaded work
Context pointer used for most rendering: Draw, SetXXShader etc.
The Effects framework (.fx) is not in the provided libraries
Compile it ourselves (add an extra Effects11 project to our solution)
DX maths libraries not in 11, now provided as an extra download
No font support in the D3DX libraries (now an extra download)
Or use Direct2D, DirectWrite, or a 3rd party library
Minor changes to a few DX structures require code tweaks

5. New DX11 Pipeline Stages
In order to support tessellation, DX11 adds three new stages to the rendering pipeline
Two programmable stages:
Hull Shader
Domain Shader
One fixed stage in between
Tessellation stage
No shader
All three must be used together
Only used when tessellating
Disabled otherwise

6. Tessellation - Overview
Input geometry is made of patches and control points
Not strictly vertices and polygons
Each patch has several control points, from 1 to 32
The vertex shader processes each control point
Likely to convert into world space
But probably not into viewport space since they are not ready for rendering
The hull shader also processes each control point
But can access all points for a patch
Used for patch-specific transforms

7. Tessellation - Overview
Hull shader has an associated patch constant function
Called once per patch
Determines the amount of tessellation required for that patch
The tessellation stage tessellates the patch as required
Tessellation occurs in generic 0->1 space, not world space
The domain shader takes the generic tessellation and control points and creates final vertices
Which are sent to the geometry / pixel shaders as normal
Tessellation Stage: Generic tessellations with different tessellation factors
Domain Shader: Control points shape the generic tessellation to create final mesh

8. Patches / Control Points
Model geometry for tessellation uses patches and control points
A patch is a line, triangle or quad, which is bent or shaped by some number of control points
For example, a Bezier spline
DirectX does not specify the available patch types
We can choose any and implement it in hull and domain shaders
This is potentially a huge change for game asset creation
Patches suit artwork creation much better than polygonal modelling
A quad patch and control points
Head made from quad patches

9. Hull Shader
The hull shader gets access to all the control points for a single patch and can process them in any way
It outputs the final control points used to shape the patch
It can output greater or fewer control points if necessary
For many tessellation purposes, we don’t need to change the control points given
Often the vertex shader converts to world space & they’re ready
So many hull shaders just copy input to output
However, they can be used for advanced purposes:
Approximating complex input splines using simpler output splines
Providing per-control-point information to help the patch constant function (see next slide)

10. Patch Constant Function
The patch constant function is called once per patch
It must decide how much to tessellate each patch
It can also output other custom per-patch data
It can access the input control points and the hull shader ouput control points (as arrays) to do its job
Patch tessellation is specified as one Interior Tessellation Factor and three or four Edge Tessellation Factors
That is for a triangle or quad patch. A line only has one factor
These factors specify how much to divide the edges and split up the interior of each patch (see next slides)
A simple patch constant function can just set fixed values
More commonly the factors are increased as models get nearer, or for more complex areas of the geometry

11. Tessellation Stage
The tessellation stage uses the factors specified in the patch constant function
Divides up a unit square, triangle or line based on the factors
It works in a generic 0->1 space
Usually referred to with UV axes
Outputs this generic geometry as vertices for the domain shader next
Several fixed algorithms are available for the tessellation
Specify in the hull shader code
Edge Factors 3,1,1,1 Interior Factors: 1,1 (Horizontal / Vertical)
Edge Factors 1,1,1,1 Interior Factors: 4,4
Edge Factors 4,4,4,4 Interior Factors: 4,4

12. Five Control points shaping a tessellated quad
Domain Shader
The domain shader:
Takes control points output from the hull shader
And the generic vertices output from the tessellation stage
Combine to create final tessellation for the scene
Exactly what this involves depends on the patch type
At least involves transforming generic vertices to world space
Then some manipulation based on the control points
Five Control points shaping a tessellated quad
Number of control points and formula used for shaping decided by programmer, e.g. NURBs Catmull-Clark surfaces, etc.

13. Distance / Density Variation
It is common to vary the amount of tessellation used based on the geometry distance or complexity (density)
Distance variation is simpler:
The patch control function can look at the distance of the control points to the camera
Make tessellation factor decisions based on that
Density variation needs pre-processing:
The patch control function can get an integer ID for each patch
So analyse the complexity of each patch offline
Store complexity in an array (1D texture) indexed by patch ID
Patch control function reads this array to choose tessellation factors

14. “Water-tight” Patch Seams
As soon as we vary tessellation per-patch, there are problems with patch seams
Cracks in the geometry appearing at the edges between patches of different tessellation
That is why we can control the edge tessellation separately
Ensure all edges have the same tessellation factor in the patch on each side (watertight seams)
Additional processing for the patch constant function / hull shader
Adjacent patches, different tessellation – causes crack at seam
Match edge tessellation to create “watertight” seams

15. Tessellation + Height Map
Displacement Mapping
Displacement mapping is adjusting the height of vertices based on heights stored in a texture (a height map)
Effectively, this is parallax mapping done properly
Result has correct silhouettes and no visual problems
Requires finely detailed geometry, so it works well with tessellation
A very effective method to provide fine geometry detail with less expense – and to make most effective use of the tessellation
Can be used on most kinds of patch
Parallax Mapping
Tessellation + Height Map
Displacement Mapping

16. Triangle Tessellation Used on polygonal mesh
Displacement Mapping
Control points and patches seem far from a game’s polygonal models
But, can use displacement mapping and tessellation on polygons too:
Call each triangle a patch
Call the three vertices control points
Use tessellation to create interior vertices and triangles
Hull shader just copies control points
Domain shader transforms generic tessellated triangle to position of original triangle – tangent-space work for this
Displacement mapping on result vertices
Need height map for displacement and a normal map for lighting
Same requirements as parallax mapping
Triangle Tessellation Used on polygonal mesh

17. Technical Issues Tessellation has performance implications:
A very efficient method to draw large number of polygons
However, minimise it: don’t tessellate off-screen, use lower tessellation in the distance, fall back to normal mapping etc.
Displacement mapping brings more seam issues:
Any discontinuity in texture, e.g. where two different textures meet, will cause a discontinuity in displacement (i.e. cracks)
Even with the most careful mapping, this cannot be avoided
Solve by specifying a dominant texture at patch seams
Extra coding in the hull shader / patch constant function
Sharp edges cause cracks (use normal map or averaging at edges)
Models must be designed with displacement in mind
Very low polygon models won’t work well. Start with the basic shape of the height map designed into the geometry (see lab)

18. Future for Tessellation in Games
Will see tessellation / displacement mapping on standard polygonal models in the lab
However, these technologies allow us to start using patch / control point based geometry instead
Artists have long used patch-based modelling for commercial animation
Catmull-Clark subdivision surfaces are quite well suited to GPU tessellation
A low polygon surface that mathematically defines a curved patch
There have been a few experiments with these in games already, expect to see variants of these used more frequently
The polygon is dead (soon…)