UCSC CMPS 20 - Introduction to HLSL Shaders in XNA

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Introduction to HLSL Shaders in XNA Game Design Experience Modified from Jim Whitehead’s slides Creative Commons Attribution 3.0 (Except copyrighted images and example Shader) creativecommons.org/licenses/by/3.0What is a Shader? • Recall that all 3D drawing in XNA uses a Shader ► Have been using BasicEffect shader so far • But, more generally, what is a shader? ► Today, gaming computers have both a CPU, and a GPU • CPU is on motherboard, GPU is on graphics card – CPU is an unspecialized computer • GPU is a computer specialized for 3D graphics – Advantage: faster 3D graphics, more effects, larger scenes ► A Shader is a small program that runs on the GPU • Written in a Shader language (HLSL, Cg, GLSL) • XNA supports only the HLSL shader languageShader Languages • Currently 3 major shader languages ► Cg (Nvidia) ► HLSL (Microsoft) • Derived from Cg ► GLSL (OpenGL) • Main influences are ► C language ► pre-existing Shader languages developed in university and industry HLSL (Microsoft, 2002?) GLSL (OpenGL ARB, 2003) ARB Vertex/Fragment (OpenGL ARB, 2002) Source: http://http.developer.nvidia.com/CgTutorial/cg_tutorial_chapter01.html (Modified with information on HLSL and GLSL)Brief history • Initially, computers did not have specialized graphics hardware ► In mid-90’s 3D acceleration hardware appeared • OpenGL typically provided better support ► DirectX 7 (1999) introduced support for hardware T&L • Transform and lighting • Moved vertex transformations and lighting computations from CPU to GPU • Improved game graphics, but at a cost: lighting and display calculations hard-wired into cards • Led to games having similar look ► In 2002, first consumer-level programmable GPUs became available • Led to development of Cg, HLSL, and GLSL shader languages • Benefit: can have game-specific custom graphics programs running on GPU • Games can have very distinctive visualsTypes of Shaders • Shaders (GPU programs) are specialized into 3 different types: ► Vertex shaders • Executed once per vertex in a scene. • Transforms 3D position in space to 2D coordinate on screen • Can manipulate position, color, texture coordinates • Cannot add new vertices ► Geometry shaders • Can add/remove vertices from a mesh • Can procedurally generate geometry, or add detail to shapes ► Pixel shaders (fragment shaders) • Calculates the color of individual pixels • Used for lighting, texturing, bump mapping, etc. • Executed once per pixel per geometric primitiveShader control flow • C#/XNA program sends vertices and textures to the GPU ► These are the input for the vertex and pixel shader • Shader executes vertex shader ► Once per vertex • Shader executes pixel shader ► Once per pixel in each primitive object Vertex Shader Pixel Shader GPU CPU C#/XNA program vertices, textures displayAnatomy of a Shader in HLSL • Shader is a program written in textual form in HLSL • Programs tend to have these parts ► Global variables • Variables used by multiple functions • Way to pass arbitrary data from C#/XNA to Shader ► Data structure definitions • Data structures used within the shader functions ► Vertex and Pixel shaders • Functions written in HLSL ► Techniques • Describe grouping of vertex and pixel shaders • Describe ordering of same Global variables Data structure definitions Vertex shading functions Pixel shading functions Techniques (calls to vertex and pixel shading functions)Common data types in HLSL • HLSL has well known data types ► int, float, bool, string, void • Vectors ► float3, float4 – 3/4 item floating point vector • float4 color = float4(1, 0, 0, 1); • Red, in RGBA (red, green, blue, alpha) color space • Used to represent vertices, colors • Matrices ► floatRxC – creates matrix with R rows, C cols • Float4x4 – a 4x4 matrix • Used to represent transformation matrices • Structures struct structname { variable declarations of members } Example: struct myStruct { float4 position; }Passing Information to/from a Shader • There are two ways information is passed into a Shader ► Directly set global variables • In C#/XNA: • effect.Parameters[“global variable name”].SetValue(value) • Example: • HLSL: float4x4 World;  The global variable • C#/XNA: effect.Parameters[“World”].SetValue(Matrix.Identity); ► Semantics • “Magic” variables • Names and meaning are hard-wired by HLSL language specification • Examples: – POSITION0: a float4 representing the current vertex » When the HLSL program is executing, before each Vertex shader is called, POSITION0 is updated with the next vertex – COLOR0: a float4 representing the current pixel colorExample Shader • Example is Shader from Chapter 13 of Learning XNA 3.0, Aaron Reed, O’Reilly, 2009. float4x4 World; float4x4 View; float4x4 Projection; struct VertexShaderInput { float4 Position : POSITION0; }; struct VertexShaderOutput { float4 Position : POSITION0; }; VertexShaderOutput VertexShaderFunction(VertexShaderInput input) { VertexShaderOutput output; float4 worldPosition = mul(input.Position, World); float4 viewPosition = mul(worldPosition, View); output.Position = mul(viewPosition, Projection); return output; } Global variables Data structures Vertex Shader Computes final output position (x,y,z,w) from input position semanticExample Shader (cont’d) float4 PixelShaderFunction() : COLOR0 { return float4(1, 0, 0, 1); } Technique Technique1 { pass Pass1 { VertexShader = compile vs_1_1 VertexShaderFunction(); PixelShader = compile ps_1_1 PixelShaderFunction(); } } Pixel Shader function Makes every pixel red. Compile Vertex and Pixel shaders using Shader version 1.1 Define a technique combining the vertex and pixel shaders Contains a single pass An output semanticConnecting Shader to C#/XNA Four main steps in using a Shader from XNA 1. Load the Shader via the Content manager ► Creates Effect variable using the loaded shader ► Add shader under Content directory • Move .fx file in file system to Content directory • On Content,


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UCSC CMPS 20 - Introduction to HLSL Shaders in XNA

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