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CS361

Week 3 - Wednesday. CS361. Last time. What did we talk about last time? Project 1 Graphics processing unit Programmable shading. Questions?. Project 1. Assignment 1. XNA Practice. Vertex Shading. Vertex shader. Supported in hardware by all modern GPUs

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CS361

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  1. Week 3 - Wednesday CS361

  2. Last time • What did we talk about last time? • Project 1 • Graphics processing unit • Programmable shading

  3. Questions?

  4. Project 1

  5. Assignment 1

  6. XNA Practice

  7. Vertex Shading

  8. Vertex shader • Supported in hardware by all modern GPUs • For each vertex, it modifies, creates, or ignores: • Color • Normal • Texture coordinates • Position • It must also transform vertices from model space to homogeneous clip space • Vertices cannot be created or destroyed, and results cannot be passed from vertex to vertex • Massive parallelism is possible

  9. Vertex shader effects • Lens effects for distortion • Novel perspective correction • Object definition • Creating a mesh and having the vertex shader form its shape • Object twist, bend, and taper • Procedural deformations • Flags • Cloth • Water

  10. More vertex shading effects • Primitive creation • Degenerate 2D meshes given a third dimension by the shader • Page curls, heat haze, water ripples • Make a mesh of the screen and distort it • Vertex texture fetch • Apply a texture to vertices, making ocean surfaces or terrain in hardware

  11. Geometry Shading

  12. Geometry shader • Newest shader added to the family, and optional • Comes right after the vertex shader • Input is a single primitive • Output is zero or more primitives • The geometry shader can be used to: • Tessellate simple meshes into more complex ones • Make limited copies of primitives

  13. Stream output • The geometry shader is guaranteed to return output in the same order as the input was received • In Shader Model 4.0 and later, the output of the GS can be put into a stream (an ordered array) • This stream can be rasterized or it can be sent back through the pipeline for multi-step effects • For computational purposes, the stream could simply be output non-graphically

  14. Pixel Shading

  15. Pixel shader • Clipping and triangle set up is fixed in function • Everything else in determining the final color of the fragment is done here • Because we aren't actually shading a full pixel, just a particular fragment of a triangle that covers a pixel • So much goes on that we'll have to put it off until later • Various lighting models are a lot of it • The pixel shader is limited in that it cannot look at neighboring pixels • Except that some information about gradient can be given • Multiple render targets means that many different colors for a single fragment can be made and stored in different buffers

  16. Merging Stage

  17. Merging stage • Fragment colors are combined into the frame buffer • This is where stencil and Z-buffer operations happen • It's not fully programmable, but there are a number of settings that can be used • Multiplication • Addition • Subtraction • Min/max

  18. Effects

  19. All this programming gets complicated… • So, people in the industry have tried to collect useful programs for rendering things • A collection of shaders to achieve a particular rendering effect can be stored in an effect file (commonly with extension .fx) • The syntax of the effects language allows your application to set specific arguments

  20. Tools to generate effects • You can download existing .fx files or write your own • There are also tools like NVIDIA's FX Composer 2.5 that allow you to create effects with a GUI • Now, let's examine the book's example effect file for Gooch shading

  21. Standard variables • Camera parameters are supplied automatically • Syntax is type id : semantic • type is a system defined type or a user defined struct • id is whatever identifier the user wants • semantic is a system defined use float4x4 WorldXf : World; float4x4 WorldITXf : WorldInverseTranspose; float4x4 WvpXf : WorldViewProjection;

  22. User variables • Default values are given for these variables • The annotations given inside angle brackets allow the outside program to set them float3 Lamp0Ps : Position < string Object = "PointLight0"; string UIName = "Lamp 0 Position"; string Space = "World"; > = {-0.5f, 2.0f, 1.25f}; float3 WarmColor < string UIName = "Gooch Warm Tone"; string UIWidget = "Color"; > = {1.0f, 0.9f, 0.15f}; float3 CoolColor < string UIName = "Gooch Cool Tone"; string UIWidget = "Color"; > = {0.05f, 0.05f, 0.6f};

  23. User defined structs • Input and output types are usually defined by the user • The TEXCOORD1 and TEXCOORD2 semantics are used for historical reasons structappdata { float3 Position : POSITION; float3 Normal : NORMAL; } structvertexOutput { float4 HPosition : POSITION; float3 LightVec : TEXCOORD1; float3 WorldNormal : TEXCOORD2; };

  24. Vertex shader vertexOutputstd_VS(appdata IN) { vertexOutput OUT; float4 No = float4(IN.Normal,0); OUT.WorldNormal = mul(No,WorldITXf).xyz; float4 Po = float4(IN.Position,1); float4 Pw = mul(Po,WorldXf); OUT.LightVec = (Lamp0Pos – Pw.xyz); OUT.HPosition = mul(Po,WvpXf); return OUT; }

  25. Pixel shader • We linearly interpolate between cool and warm colors based on the dot product float4 gooch_PS(vertexOutput IN) : COLOR { float3 Ln = normalize(IN.LightVec); float3 Nn = normalize(IN.WorldNormal); float ldn = dot(Ln,Nn); float mixer = 0.5 * (ldn + 1.0); float4 result = lerp(CoolColor, WarmColor, mixer); return result; }

  26. Putting it all together • Z-buffer configuration is done here • technique Gooch < string Script = "Pass=p0;"; > • { • pass p0 < string Script = "Draw=geometry;"; > • { • VertexShader = compile vs_2_0 std_VS(); • PixelShader = compile ps_2_a gooch_PS(); • ZEnable = true; • ZWriteEnable = true; • ZFunc = LessEqual; • AlphaBlendEnable = false; • } • }

  27. Shader output • The result of the shader given before applied to a teapot:

  28. All kinds of different effects

  29. Why a teapot? • The Utah teapot was modeled in 1975 by graphics pioneer Martin Newell at the University of Utah • It's actually taller than it looks • They distorted the model so that it would look right on their non-square pixel displays

  30. Original vs. modern • Original • Modern

  31. There's a Stanford Bunny too…

  32. Quiz

  33. Upcoming

  34. Next time… • Linear algebra review • Vectors and matrices

  35. Reminders • Read Appendix A • Finish Assignment 1, due Friday by 11:59 • Keep working on Project 1, due Friday, February 8 by 11:59

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