Applying Hardware Tessellation - Direct3D Rendering

Graphics Reference

In-Depth Information

Although it is possible to provide multiple tessellation factors (left, right, top, and bottom,

depending on the domain), we are providing a constant tessellation factor across all the

edges by casting TessellationFactor float to a float3 for tri domain or float4

for the quad domain. We use the built-in HLSL functions to round the tessellation factor.

This ensures that the values used for the inside and outside edges are correct for the

specified partitioning method.

ProcessTriTessFactorsMax((float3)TessellationFactor, 1.0,

roundedEdgeTessFactor, roundedInsideTessFactor,

insideTessFactor);

Process2DQuadTessFactorsMax((float4)TessellationFactor, 1.0,

roundedEdgeTessFactor, roundedInsideTessFactor,

insideTessFactor);

The last part of the hull shader constant function returns data to be shared across all the

control points of the patch. The maximum number of outputs supported for a structure

involved in the tessellation pipeline is 32 float4 properties, allowing a total of 1024 bytes

per patch (remember that the structure will be padded to 16 bytes). Generally the amount of

information should be kept to a minimum, and it may be necessary to use buffers or texture

sampling to load additional data.

The domain shader is called for each newly created output control point in each patch by the

fixed function tessellator stage. This shader is responsible for generating appropriate texture

coordinates, and so on, across the newly created geometry. The result is then output to the

pixel shader.

Our two renderers use two different interpolation techniques: barycentric interpolation

(for the tri domain), and bilinear interpolation (for the quad domain with four control points).

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Controlling the tessellation factors for individual outside and inside edges within the hull

shader constant function is especially useful while performing screen adaptive tessellation.

This form of adaptive tessellation determines the tessellation factor for an edge based upon

the length of the edge within screen space.