PV227 GPU programming
Marek Vinkler
Department of Computer Graphics and Design
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Geometry shader
new programmable stage (optional),
between vertex shader and the fragment shader,
before the rasterizer,
bit different than the other two.
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Geometry shader
needs input and output format,
recieves the assembled primitives (no strip, fan or loop),
full knowledge of the primitive.
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Input
primitive #vertices
points 1
lines 2
lines_adjacency 4
triangles 3
triangles_adjacency 6
primitive type must match the draw command,
layout ( triangles ) in;
Figure: Taken from lighthouse3d.com
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Input
build-in:
array gl_in of input vertices,
default vertex attributes.
user-defined:
same way as usual,
array, data for each vertex.
1 in gl_PerVertex
2 {
3 vec4 gl_Position ;
4 f l o a t gl_PointSize ;
5 f l o a t gl_ClipDistance [ ] ;
6 } gl_in [ ] ; / / # of ve rt ic e s : gl_in . length ( )
7
8 in i n t g l _ P r i m i t i v e I D I n ;
1 in Data
2 {
3 vec3 normal ;
4 } vertexData [ ] ;
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Output
primitive
points
line_strip
triangle_strip
output type need not match the input type,
GL_MAX_GEOMETRY_OUTPUT_VERTICES (1024),
can output [0, max] primitives,
input primitive is discarded,
layout ( line_strip , max_vertices = 4) out;
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Output
outputs vertices,
attributes passed the same way as in the vertex shader,
vertex definition ended with EmitVertex();
need enough vertices to form primitives,
primitive definition ended with EndPrimitive();
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Examples
culling,
explosion,
tesselation,
normal visualization.
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Culling
render only triangles visible from a point,
do not emit triangles for the others.
Figure: Point view culling
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Explosion
move vertices along the common triangle normal,
color the vertices with R, G, B.
Figure: Explosion in t = 0.5f
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Tessellation
only minor amplification,
create new point in the barycenter.
Figure: Extruded tessellation in t = 0.5f
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Tessellation
one new point and three triangles.
a b
c
d
Figure: Tessellated triangle.
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Tessellation
mind the emit order of primitives,
must follow the winding order of triangle strips.
Figure: Taken from atspace.co.uk
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Normal visualization
draw lines for normals,
visualize both kinds of normals.
Figure: Visualized vertex and face normals.
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