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

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 compatibility layout(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 Example void 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  om/watch?v=tuAC9Q Gx3vQ om/watch?v=tuAC9Q Gx3vQ

Point Sprite Expansion  om/watch?v=jpXgPkft RcM om/watch?v=jpXgPkft RcM

Dynamic Particles 

Morphing / Shape Manipulation 

Tesselation  

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