Lesson 12 – Tessellation Shaders
PV227 – GPU Rendering
Jiˇrí Chmelík, Jan ˇCejka
Fakulta informatiky Masarykovy univerzity
15. 12. 2015
PV227 – GPU Rendering (FI MUNI) Lesson 12 – Tessellation Shaders 15. 12. 2015 1 / 11
Tessellation Shaders
new programmable stage (optional), requires OpenGL 4.0
between vertex shader and geometry shader,
use the correct spelling :-)
Figure: Taken from lighthouse3d.com
PV227 – GPU Rendering (FI MUNI) Lesson 12 – Tessellation Shaders 15. 12. 2015 2 / 11
Tessellation Shaders (cont.)
Tessellation Control Shader (TCS)
Hull Shader in HLSL
optional, programmable
Primitive generation
fixed
Tessellation Evaluation Shader (TES)
Domain Shader in HLSL
required, programmable
PV227 – GPU Rendering (FI MUNI) Lesson 12 – Tessellation Shaders 15. 12. 2015 3 / 11
Tessellation Shaders — Primitive Generation
Figure: Triangle Figure: Quad Figure: Lines
Images taken from OpenGL Specification
PV227 – GPU Rendering (FI MUNI) Lesson 12 – Tessellation Shaders 15. 12. 2015 4 / 11
Tessellation Shaders — Shaders
Tessellation Evaluation Shader
computes the data of each generated vertex
similar to vertex shaders
Tessellation Control Shader
computes the level of the tessellation (the density of the mesh)
computes the data of the control points and other patch data
PV227 – GPU Rendering (FI MUNI) Lesson 12 – Tessellation Shaders 15. 12. 2015 5 / 11
Patches
New primitive, only for tessellation shaders
Consist of 1 – 32 vertices
Individual patches, no strips
1 glPatchParameteri (GL_PATCH_VERTICES, 16) ;
2 glDrawArrays (GL_PATCHES, . . . ) ;
OpenGL does not define the mapping between input vertices and
control points, the programmer does!
PV227 – GPU Rendering (FI MUNI) Lesson 12 – Tessellation Shaders 15. 12. 2015 6 / 11
Tessellation Control Shader in GLSL
Consumes one patch, generates one patch, like geometry shader
Unlike geometry shaders, TCS is executed ones per output
vertex.
Number of generated vertices defined by layout(vertices = 4) out;
Use gl_InvocationID to get the index of the vertex for which this TCS is
executed.
Array of per vertex input data from the vertex shader
Example: in vec4 position_vs [];
Every TCS has access to each per vertex input data (read-only)
Array of per vertex output data into the TES
Example: out vec4 position_tcs [];
Every TCS has readonly access to each per vertex output data
Only the one for which TCS is executed can write the data of its
vertex
Use barrier () ; to make sure the data written by TCS are visible to
other TCS.
PV227 – GPU Rendering (FI MUNI) Lesson 12 – Tessellation Shaders 15. 12. 2015 7 / 11
Tessellation Control Shader in GLSL
Per patch data, marked as out patch, passed into TES
Example: out patch int materialIdx;
Tessellation levels defining the density of the mesh
gl_TessLevelInner[2] describes the density inside the patch
gl_TessLevelOuter[4] describes the density at the boundary of the patch
When set to 0, the whole patch is discarded
TCS is optional, when missing:
Per vertex data passes through from vertex shader into TES
The number of patch vertices stays the same
Tessellation levels defined from C++ code using glPatchParameterfv
PV227 – GPU Rendering (FI MUNI) Lesson 12 – Tessellation Shaders 15. 12. 2015 8 / 11
Tessellation Evaluation Shader in GLSL
Defines the patch topology: layout (...) in;
triangles / quads / isolines
fractional_odd_spacing / fractional_even_spacing / equal_spacing
cw / ccw
point_mode / (nothing)
Array of per vertex input data from the TCS
Example: in vec4 position_vs [];
Every TES has access to each per vertex input data (readonly)
Per patch data, marked as in patch, from TES
Example: in patch int materialIdx;
Coordinate of the tessellated vertex in the patch vec3 gl_TessCoord
triangles uses 3 coordinates (xyz)
quads and isolines use 2 coordinates (xy)
PV227 – GPU Rendering (FI MUNI) Lesson 12 – Tessellation Shaders 15. 12. 2015 9 / 11
Example — Patch topology
Try layout (...) in; in TES and tessellation levels in TCS
Figure: Taken from OpenGL Specification
PV227 – GPU Rendering (FI MUNI) Lesson 12 – Tessellation Shaders 15. 12. 2015 10 / 11
Example — Utah teapot
Use quads to smoothly tessellate 32 Bezier patches, each with 16
control points
Add some displacement
PV227 – GPU Rendering (FI MUNI) Lesson 12 – Tessellation Shaders 15. 12. 2015 11 / 11