Lesson 9 – Tessellation shaders
PV227 – GPU Rendering
Jiˇrí Chmelík, Jan ˇCejka
Fakulta informatiky Masarykovy univerzity
12. 11. 2018
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Tessellation Shaders
new programmable stage (optional)
between vertex shader and geometry shader,
use the correct spelling :-)
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Tessellation Shaders
Tessellation Control Shader (TCS)
Hull Shader in HLSL
optional, programmable
computes the parameters of the tessellation (the density of the
mesh)
Primitive generation
fixed
Tessellation Evaluation Shader (TES)
Domain Shader in HLSL
required, programmable
computes the data of each generated vertex, like vertex shaders
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Primitive Generation
Triangle Quad Isolines
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Patches
New primitive, only for tessellation shaders
Consist of 1 – 32 vertices
glPatchParameteri(GL_PATCH_VERTICES, 16);
glDrawArrays(GL_PATCHES, ...);
Individual patches, no strips
OpenGL does not define the mapping between input vertices and
control points, the programmer does!
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Tessellation Control Shader in GLSL
Consumes one patch, generates one patch, like geometry shader
Unlike geometry shaders, TCS is executed once per output vertex.
Computes the following:
parameters of the tessellation
parameters of the whole patch
data of each patch control point.
Number of generated control points (vertices)
layout(vertices = 4) out;
Index of the vertex for which this TCS is executed
gl_InvocationID
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Tessellation Control Shader in GLSL
Parameters of the tessellation:
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
Per patch data, marked as out patch, passed into TES
Example: out patch int materialIdx;
Usually computed only by one thread, e.g. by the thread with
gl_InvocationID = 0;
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Tessellation Control Shader in GLSL
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
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
TCS car write only the data of its own vertex
Use barrier() to make sure the data written by TCS are visible to
other TCS.
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
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Tessellation Evaluation Shader in GLSL
Computes the data of each generated vertex
Defines the patch topology: layout(...) in;
triangles / quads / isolines
fractional_odd_spacing / fractional_even_spacing / equal_spacing
cw / ccw
point_mode / (nothing)
Example: layout(quads, equal_spacing, ccw, point_mode) in;
Array of per vertex input data from TCS: in vec4 position_vs[];
Per patch data, from TCS: 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)
Output: like the output of vertex shader
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Patch topology
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Task: Examine patch topology
Task 0: Examine patch topology and patch parameters
Download, compile, and run project TessViewer from IS
Try different parameters
No code to write :-)
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Task: Tessellate Utah teapot
Use quads to smoothly tessellate 32 Bezier patches, each with 16
control points
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Task: Tessellate Utah teapot
Task 1: Tessellate Utah teapot in a very simple way
Already done: Vertex shader transforms the positions of control
points into world space.
Already done: Tessellation control shader passes the data from
input to output, and sets tessellation factors to a constant value.
Task: In teapot_tess_eval.glsl, compute the position of vertex in
world space, transform it with the view and projection matrices, and
store it into gl_Position. Also, send the untransformed one (in world
space) to fragment shader.
Already done: Fragment shader outputs simple white color.
Use wireframe to see the result.
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Tessellating Bezier patch
1D cubic Bezier curve:
bezier4(V0, V1, V2, V3, t) =
V0(1 − t)3 + 3V1(1 − t)2t + 3V2(1 − t)t2 + V3t3
2D cubic Bezier patch:
bezier4x4(V0 . . . V15, tx , ty ) =
r0 = bezier4(V0, V1, V2, V3, tx )
. . .
r3 = bezier4(V12, V13, V14, V15, tx )
result = bezier4(r0, r1, r2, r3, ty )
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Tessellating Bezier patch
Order of control points in out patches:
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Task: Add texture coordinates
Task 2: Add texture coordinates and texturing
Use the gl_TessCoord as the texture coordinate, send it from TES
to FS.
In teapot_tess_fragment.glsl, use the texture coordinate to sample
the color from color_tex.
We still do not compute the lighting.
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Task: Add texture coordinates
Result
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Task: Add lighting
Task 3: Compute the normal and lighting
Tangent: direction on the surface of tex_coord.x axis
Bitangent: direction on the surface of tex_coord.y axis
Both are precomputed at control points
Both are also already transformed into world space in vertex shader
And are already passed through TCS to TES
Task: In TES, evaluate them the same way as positions. Also
compute the normal as the cross product between them (order is
n = t × b. Pass all three vectors to FS.
In fragment shader, use the normal to compute the lighting.
Optional homework: Compute the tangent and bitangent as
derivation of the position.
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Task: Add lighting
Result
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Adaptive tessellation
One of many places where tessellation shaders can be used
Use more triangles when/where necessary
when the object is close
where there are more geometry details
at the countours
discard the patch when outside of the view
. . .
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Task: Add adaptive tessellation
Task 4: Change TCS, use more triangles when the object is
closer to the viewer
Transform each control point with projection and view matrices,
divide it with w, and multiply it with the window size to get its
position on the screen in pixels. Use barrier to wait for all vertices to
be computed.
In zeroth invocation, compute the approximation of the length of the
four sides of the patch, e.g. sum the length of the three sublines.
Divide these lengths with triangle_size to get the number of
triangles to be tesselated. Use these values as gl_TessLevelOuter.
Average the two opposites outer levels to get gl_TessLevelInner.
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Task: Add adaptive tessellation
Result
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Displacement mapping
Displacement mapping
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Task: Add displacement mapping
Task 5: Displace vertices to add some more geometric details
In TES, sample the height_tex texture (use function textureLod)
Multiply the value with max_displacement and height_scale.
Move the position in the direction of the normal.
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Displacement mapping
Result, notice incorrect lighting
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Normal mapping
Normal mapping
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Normal mapping
Get normal from texture, and transform it from (0, 1) to (-1, 1).
Don’t forget it is in tangent space, i.e. relative to the surface.
nts = texture(tex) · 2 − 1
Transform it into world space:
nws = tangentws
tes · nts
.x + bitangentws
tes · nts
.y + normalws
tes · nts
.z
Use this normal to compute the lighting
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Task: Add normal mapping
Task 6: Implement normal mapping in fragment shader
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Task: Add normal mapping
Result
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