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Geometry Shader

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

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Geometry Shader

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  1. Geometry Shader

  2. Breakdown • Basics • Review – graphics pipeline and shaders • What can the geometry shader do? • Working with the geometry shader • GLSL example • Examples • Fur, Fins • Particles

  3. 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

  4. Basics

  5. Review – Graphics Pipeline

  6. 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)

  7. 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

  8. 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

  9. Example

  10. 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

  11. 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

  12. 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

  13. Questions?

  14. Working with the Geometry Shader

  15. 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;

  16. GLSL Example void main() { mat4 modelViewMatrix = viewMatrix * modelMatrix; mat4 viewProjectionMatrix = projectionMatrix * viewMatrix; inti; //====== 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();

  17. 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(); }

  18. 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

  19. 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

  20. Questions?

  21. Examples

  22. Fur • http://www.youtube.com/watch?v=tuAC9QGx3vQ

  23. Point Sprite Expansion • http://www.youtube.com/watch?v=jpXgPkftRcM

  24. Dynamic Particles • http://www.youtube.com/watch?v=HwMjyVB9EB4

  25. Morphing / Shape Manipulation • http://www.youtube.com/watch?v=HTKaq3AIBfQ

  26. Tesselation • http://www.youtube.com/watch?v=9s0onJyTmWE • http://www.youtube.com/watch?v=Bcalc8UoJzo

  27. Questions?

  28. 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

  29. 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

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