The Rendering Pipeline - Tessellation

Hull Shader

Hull Shader

• Provide the tessellator with how small the input geometry will be split.
• Provide the domain shader with processed control patch primitives.
• Two HLSL functions of hull shader
  1. Main function
     • Executed by the number of control points required to generate one output control patch.
     • Output one control patch per execution.
  2. Patch constant function
     • Executed once for each complete control patch.
     • Define tessellation coefficients, which used to subdivide patches.

The structure of hull shader

 

Input of Hull Shader

• Control patch primitives
  • The vertex set as a result of the vertex shader + the primitive stream of input assembler
  • Two parameters of input control patch
    1. Format of data calculated by vertex shader
    2. The number of control points in the input control patch
• SV_OutputControlPointID
  • Index of the control point that caused the current hull shader program to be executed.
  • Unsigned integer, $\le$ the size of the output control patch.
• SV_PrimitiveID
  • Identify the current primitive. (more specifically, the control patch)
  • Unsigned integer

 

Status Description of Hull Shader

• Shader program
  • ID3D11DeviceContext::HSSetShader()
  • The definition of the patch constant function and the list of function level attributes must come before
• Constant buffer
  • ID3D11DeviceContext::HSSetConstantBuffers()
  • The main means of providing data to the hull shader program.
• Shader resource view
  • ID3D11DeviceContext::HSSetShaderResources()
  • A means of providing read-only data to the hull shader program.
• Sampler status object
  • ID3D11DeviceContext::HSSetSamplers()
  • Provide the ability to perform various filitering operation when reading texels from a texture resource.
• Function attribute : individual sentences that change specific settings of the tessellation system.

// Designate the domain of the primitive to be tessellated
[domain("tri")]
// Designate a tessellation method
[partitioning("fractional_even")]
// Designate the type of primitive to be generated through tessellation
[outputtopology("triangle_cw")]
// Designate the number of control points that the hull shader wants to create
[outputcontrolpoints(3)]
// Designate the patch constant function by the name of function
[patchconstantfunc("PassThroughConstantHS")]
// Designate the maximum value of the tessellation coefficient
[maxtessfactor(5)]

 

Process of Hull Shader

• Hull shader program (main function)
  • Receive all control points in the input control patch primitives as one input attribute.
  • Calculate an appropriate number of output control points.
  • Setting up a tesslation algorithm
    1. Possible to change, expand, and reduce the shape of the input control patch.
    2. Replace the tessllation algorithm without affecting the rest of the pipeline.
  • Detailed level of the tessellation algorithm
    1. Objects close to the camera : more sophisticated tessellation algorithm
    2. Objects far away from the camera : less complex tessellation algrorithm
  • General control point calculation : suitable place to calculate values that will be commonly applied to several tessellated points
• Calculate the coefficient of tessellation. (patch constant function)

 

Output of Hull Shader

• Hull shader program
  • Output control points constituting the control patch to be used for tessellation.
  • No system value that must be printed out.
• Patch constant function
  • Calculate the appropriate number of tessellation coefficients.
  • Output tessellation coefficients through SV_TessFactor and SV_InsideTessFactor.
  • Provide all patch constant information through the output attributes.

 

Tessellator

Tessellator

• The only fixed-function stage among pipeline stages that perform tessellation.
• Perform tessellation as much as tessellation coefficients that the patch constant function designate.
• Create a set of coordinates points within the current 'domain'

 

Input of Tessellator

• Input of tessellator : the coefficients of tessellation created by the patch constant fucntion of the hull shader stage through 'calculation'.
• The output is exactly the same if the inputs are the same, regardless of how the tessellation coefficients are calculated.

Status Description of Tessellator

• Designated through the function attributes of the hull shader program, not application program.
• [domain] attributes
  • Determine the points in the domain corresponding to the [domain] attribute.
  • Available value : isoline, tri, quad
  • Must be specified because it is also used to check whether the number of tessellation coefficients is valid.
• [partitioning] attributes
  • Specify a method of partitioning a designated domain.
  • Available value : integer, fractional_even, fractional_odd, pow2
  • Essential attributes that cannot be omitted.
• [outputtopology] attributes
  • Designate the type of primitive to form.
  • Available value : triangle_cw, triangle_ccw, line
  • Essential attributes that cannot be omitted.
• [maxtessfactor] attributes
  • Specify the maximum value of a tessellation coefficient.
  • Used when the driver efficiently preallocates enough memory to contain the results of the tessellation operation.

 

Process of Tessellator

• Process of tessellator
  1. Select a series of points within a given domain.
  2. Use the points to create primitives.
  3. Provide the primitives to the next stages of the pipeline.
• Sample positions
  • Select the tessellation points to create an appropriate number of triangles.
  • More points are selected for the larger portion of the edge and the inner tessellation coefficient.

• Creation of primitives
  • Create the selected sample positions as primitve information necessary for subsequent stages in the pipeline to use as a renderable geometry.
  • The [outputtopology] attribute should not contradict the culling setting used in the rasterizer, in order to ensure that visibility determination occurs properly.

 

Output of Tessellator

• SV_DomainLocation : output texture coordinates
  • For each domain coordinate point, one SV_DaminLocation is transferred to the domain shader stage.
• Primitive topology
  • Delivered directly to the geometry shader without being used by the domain shader.
  • The type of primitive containing adjacent information cannot be transferred to the geometry shader.
  • Calculation requiring adjacent information may be performed by the hull shader or the domain shader.

 

Domain Shader

Domain Shader

• The last stage of the tessellation system.
• The stage of performing practical tessellation operations by applying tessellation algorithm set by hull shader to locations created by the tessellator.

The structure of domain shader

 

Input of Domain Shader

• SV_DomainLocation system value semantic
  • The coordinate points created by the tessellator stage.
  • The domain shader stage is executed once for each coordinate point.
• The completed control patch calculated by the hull shader stage
  • The number of control points must match the number specified in the [outputcontrolpoints] function attribute of the hull shader program.

 

Status Description of Domain Shader

• Shader program : ID3D11DeviceContext::DSSetShader()
• Constant buffer : ID3D11DeviceContext::DSSetConstantBuffers()
• Shader resource view : ID3D11DeviceContext::DSSetShaderResources()
• Sampler status object : ID3D11DeviceContext::DSSetSamplers()

 

Process of Domain Shader

• Create vertices using a set of coordinate points, control patches and patch constants.

• The overall tessellation process is designed by dividing it into several components.
  1. Must create a control patch that defines the surface (curved) we actually want.
  2. Necessary to calculate the tessellation coefficients that minimize the number of required vertices while properly sampling the desired curved surface.
  3. Should implement algorithms to properly sample virtual surfaces from control patch data using a given set of coordinate points.

 

Output of Domain Shader

• The output vertex completed by calculating the required attribute data.
• SV_Position : include the position of the output vertex.
• Other vertex-specific attributes required to determine the final pixel color.