Tessellation (Direct3D 9)

Tessellator Unit

The tessellator unit has been enhanced. You can now use it to:

  • Perform adaptive tessellation of all higher-order primitives.
  • Look up per-vertex displacement values from a displacement map and pass them on to a vertex shader.
  • Support rectangle-patch tessellation. This is specified through a vertex declaration using D3DDECLMETHOD_PARTIALU or D3DDECLMETHOD_PARTIALV. If a vertex declaration containing these methods is used to draw a triangle patch, IDirect3DDevice9::DrawTriPatch will fail. For more information about vertex declarations, see D3DVERTEXELEMENT9.

In DirectX 8.x, what was called ORDER was really the degree. In Direct3D 9, the degree is now specified by D3DDEGREETYPE.



 // This used to be D3DORDERTYPE and D3DORDER* 
 typedef enum _D3DDEGREETYPE 
 { 
 D3DDEGREE_LINEAR = 1, 
 D3DDEGREE_QUADRATIC = 2, 
 D3DDEGREE_CUBIC = 3, 
 D3DDEGREE_QUINTIC = 5, 
 D3DDEGREE_FORCE_DWORD = 0x7fffffff, 
 } D3DDEGREETYPE; 


The change in degree type affected two other structures.



typedef struct _D3DRECTPATCH_INFO 
 { 
 UINT StartVertexOffsetWidth; 
 UINT StartVertexOffsetHeight; 
 UINT Width; 
 UINT Height; 
 UINT Stride; 
 D3DBASISTYPE Basis; 
 D3DDEGREETYPE Degree; 
 } D3DRECTPATCH_INFO; 




 typedef struct _D3DTRIPATCH_INFO 
 { 
 UINT StartVertexOffset; 
 UINT NumVertices; 
 D3DBASISTYPE Basis; 
 D3DDEGREETYPE Degree; 
 } D3DTRIPATCH_INFO; 


Drivers need to fix compilation errors that will result from this change when they compile with the new headers. No functionality has to be changed.

Adaptive Tessellation

Adaptive tessellation can be applied to high-order primitives including N-patches, rectangle patches, and triangle patches. This feature is enabled by the D3DRS_ENABLEADAPTIVETESSELLATION and adaptively tessellates a patch, based on the depth value of the control vertex in eye space.

The z-coordinates (Zi) of control vertices (Vi), which are transformed into eye space (Zieye) by performing a dot product with a 4-vector, are used as the depth values. The 4D vector (Mdm) is specified by the application using four render states (D3DRS_ADAPTIVETESS_X, D3DRS_ADAPTIVETESS_Y, D3DRS_ADAPTIVETESS_Z, and D3DRS_ADAPTIVETESS_W). This 4-vector could be the third column of the concatenated world and view matrices. It also could be used to apply a scale to Zieye.

The function to compute a tessellation level Ti from Zieye is assumed to be (MaxTessellationLevel/Zieye), which means that the MaxTessellationLevel is the tessellation level at Z = 1 in eye space. The MaxTessellationLevel is equal to a value set by IDirect3DDevice9::SetNPatchMode for N-patches and, for RT-patches, it is equal to pNumSegs. The tessellation level is then clamped to values, defined by the two additional render states D3DRS_MINTESSELLATIONLEVEL and D3DRS_MAXTESSELLATIONLEVEL, which define the minimum and maximum tessellation levels to be clamped to. The Ti's for each vertex along an edge of a patch are averaged to obtain a tessellation level for that edge. The algorithm for computing Ti for rectangle patches, triangle patches, and N-patches differs in what control vertices are used to compute the tessellation level.

For the rectangle patches with a B-spline basis, the four outermost control vertices are used. For example, with D3DORDER_CUBIC order: vertices (1,1) and (1,width-2) are used with pNumSegs[0], vertices (1,width-2) and (height-2,height-2) are used with pNumSegs[1], vertices (height-2,width-2) and (1,width-2) are used with pNumSegs[2], and vertices (2,1) and (1,1) are used with pNumSegs[3].

For the triangle patches, the corner patch vertices are used. With D3DORDER_CUBIC order: vertices (0) and (9) are used with pNumSegs[0], vertices (9) and (6) are used with pNumSegs[1] and vertices (6) and (0) are used with pNumSegs[3].

For N-patches the triangle vertices are used.

For the rectangle and triangle patches with a Bezier basis, the corner-control vertices are used.

Per-vertex tessellation rate control. An application can optionally supply a single positive floating-point value per vertex, which can be used to control the rate of tessellation. This is supplied using the D3DDECLUSAGE_TESSFACTOR, for which usage index must be 0 and input type must be D3DDECLTYPE_FLOAT1. This is multiplied to the per-vertex tessellation level.

Math

The tessellation level (Te) for an edge e, represented by two control vertices (Ve1, Ve2), is computed as shown below :



Vertex Vi: (Xi, Yi, Zi, TFactori (optional)). 
Ze1eye = Ve1 . Mdm 
Ze2eye = Ve2 . Mdm 
Te1 = MaxTessellationLevel * TFactore1 / Ze1eye 
Te2 = MaxTessellationLevel * TFactore2 / Ze2eye 
Te = ( Te1 + Te2 ) / 2; 
if Te > D3DRS_MAXTESSELLATIONLEVEL || Te < 0, then Te = D3DRS_MAXTESSELLATIONLEVEL 
if Te < D3DRS_MINTESSELLATIONLEVEL, then Te = D3DRS_MINTESSELLATIONLEVEL 


When D3DRS_ENABLEADAPTIVETESSELLATION is TRUE, triangle primitives (triangle lists, fans, strips) are drawn as N-patches, IDirect3DDevice9::SetNPatchMode has set value less than 1.0.

API Changes

New render states:



 D3DRS_ENABLEADAPTIVETESSELLATION // BOOL 
 D3DRS_MAXTESSELLATIONLEVEL       // Float 
 D3DRS_MINTESSELLATIONLEVEL       // Float 
 D3DRS_ADAPTIVETESS_X             // Float 
 D3DRS_ADAPTIVETESS_Y             // Float 
 D3DRS_ADAPTIVETESS_Z             // Float 
 D3DRS_ADAPTIVETESS_W             // Float 
 
 // D3DRS_MINTESSELLATIONLEVEL and D3DRS_MAXTESSELLATIONLEVEL 
 // cannot be less than 1 


And their default values:



D3DRS_MAXTESSELLATIONLEVEL = 1.0f 
D3DRS_MINTESSELLATIONLEVEL = 1.0f 
D3DRS_ADAPTIVETESS_X = 0.0f 
D3DRS_ADAPTIVETESS_Y = 0.0f 
D3DRS_ADAPTIVETESS_Z = 1.0f 
D3DRS_ADAPTIVETESS_W = 0.0f 
D3DRS_ENABLEADAPTIVETESSELLATION = FALSE 


New hardware caps:



D3DDEVCAPS2_ADAPTIVETESSRTPATCH // Can adaptively tessellate RT-patches 
D3DDEVCAPS2_ADAPTIVETESSNPATCH  // Can adaptively tessellate N-patches 


 

 

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