Transcript DirectX 11 / Tessellation / Displacement Mapping

Maths & Technologies for Games
DirectX 11 – New Features
Tessellation & Displacement Mapping
CO3303
Week 19
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
7. Displacement Mapping
8. Technical Considerations and Issues
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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
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
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
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
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 Stage: Generic tessellations
with different tessellation factors
– 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
Domain Shader: Control points shape the
generic tessellation to create final mesh
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
A quad patch and control points
– 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
Head made from quad patches
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)
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
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
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.
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:
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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
“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
Adjacent patches, different
tessellation – causes crack at seam
• 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
Match edge tessellation to create
“watertight” seams
Displacement Mapping
• Displacement mapping is adjusting the height of vertices
based on heights stored in a texture (a height map)
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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
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
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)
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…)