Post on 23-Feb-2016
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GEOMETRY SHADER
Breakdown Basics
Review – graphics pipeline and shaders What can the geometry shader do?
Working with the geometry shader GLSL example
Examples Fur, Fins Particles
Recommended Reading Geometry shader is pretty new concept
Real-Time Rendering mentions in passing Course text has no real discussion
Do some searching online if you are interested in learning more
Basics
Review – Graphics Pipeline
Vertex Shader
Geometry Shader Clipping Screen
Mapping
Triangle Setup
Triangle Traversal Pixel Shader Merger
Review – What are Shaders? Shaders are small programs that run on
the GPU Shaders allow us to implement effects
that we may wish for in our rendered scene Lighting, texturing being the most basic
Three types of shader Vertex Geometry Fragment (or pixel)
What is the Geometry Shader? The geometry shader is the stage that
occurs directly after the vertex shader stage
The geometry shader works with output from the vertex shader in the form of geometry objects Points, lines, triangles
Can also include adjacency data
What Can the Geometry Shader Do?
Geometry shader can manipulate a whole piece of geometry No longer manipulation of a single vertex
Geometry shader can also add geometry No longer single vertex in, single vertex out
Combining these two features allows manipulation of your geometry in a new manner And can create some neat effects
Example
Geometry Shader Input The input into a geometry shader comes
in the form of pieces of geometry Points, lines, triangles, quads
The geometry shader may also take in an adjacency block Vertices adjacent to the piece of geometry
The piece of geometry is basically an array of vertex positions
Geometry Shader Output The geometry shader outputs a strip of the
defined geometry type e.g. triangles in, triangle strip out
The output operates by having a few extra functions in GLSL EmitVertex() – emits a vertex to the next stage EndPrimitive() – ends the vertex creation for
the particular primitive Possible to restrict the number of output
vertices
Stream Output It is also possible to output the data from
the geometry shader stage of the pipeline
Output from the geometry shader can be used for further rendering if required Useful for letting the GPU do point physics
calculations Each vertex has a position, velocity,
acceleration, mass, etc. Output is the new position, velocity
Questions?
Working with the Geometry Shader
GLSL Example Geometry Shader
#version 330 compatibilitylayout(triangles) in;layout(triangle_strip, max_vertices=6) out;uniform mat4 viewMatrix;uniform mat4 projectionMatrix;uniform mat4 modelMatrix;in vec4 normal[];out vec4 vertex_color;
GLSL Examplevoid main(){ mat4 modelViewMatrix = viewMatrix * modelMatrix; mat4 viewProjectionMatrix = projectionMatrix * viewMatrix; int i; //====== First triangle - identical to the input one. for(i=0; i<3; i++) { vec4 view_pos = modelViewMatrix * gl_in[i].gl_Position; gl_Position = projectionMatrix * view_pos; vertex_color = vec4(0.0, 1.0, 0.0, 1.0); EmitVertex(); } EndPrimitive();
GLSL Example //====== Second triangle - translated version for(i=0; i<3; i++) { vec4 N = normal[i]; vec4 world_pos = modelMatrix * (gl_in[i].gl_Position + normalize(N) * 10.0); gl_Position = viewProjectionMatrix * world_pos; vertex_color = vec4(1.0, 1.0, 0.0, 1.0); EmitVertex(); } EndPrimitive();}
Input Values Notice that we can define the input value
types using the layout specification Layout can be used to tell GPU where to send
particular pieces of data The input layout is important for using the
geometry shader correctly Need to know the type of primitive we are
dealing with We can also declare incoming adjacent
vertices
Output Values Notice that we also set the outgoing
data format for the geometry shader Triangle strip in this instance
We can set the output data type, and the number of expected vertices Useful when dealing with potentially large
pieces of geometry
Questions?
Examples
Fur http://www.youtu
be.com/watch?v=tuAC9QGx3vQ
Point Sprite Expansion http://www.youtu
be.com/watch?v=jpXgPkftRcM
Dynamic Particles http://www.youtube.com/watch?v=HwMj
yVB9EB4
Morphing / Shape Manipulation http://
www.youtube.com/watch?v=HTKaq3AIBfQ
Tesselation http://
www.youtube.com/watch?v=9s0onJyTmWE
http://www.youtube.com/watch?v=Bcalc8UoJzo
Questions?
To do this week… Coursework, coursework, coursework As mentioned, I’ll be in the Games Lab
all week Focus on getting the coursework in, then
next week we’ll talk about the exam and module as a whole
Summary Geometry shader allows us to do some
pretty cool effects beyond the capabilities of the vertex and fragment shader Geometric primitives approach
This is such a new area of graphics programming that materials are light on the ground Dig around to see what you can find