CSE 381 – Advanced Game Programming Shading

What is Shading? Calculating how a 3D object should look, taking lighting into account Depends on: –mesh data vertices, colors, textures, etc. –lighting properties type of light, position of light, etc. –shading algorithm used How is it done? –basically by running a script on your graphics card

What’s a Shader? A good definition from Wikipedia: –a program used to determine the final surface properties of an object or image –this can include arbitrarily complex descriptions of: light absorption light diffusion texture mapping reflection refraction shadowing surface displacement post-processing effects

Simpler Shader Definition A program that can affect: –the position of a vertex –the color of a pixel –or both Create interesting effects by: –manipulating geometry –manipulating color

Shader Languages HLSL, GLSL, Cg (C for Graphics) Compile to Assembly Shader compilers are on the GPU Compiling is done at runtime

Shading Languages Shader programs may be written for different platforms –can operate on GPU Different platforms use different shading languages, e.g. –High-Level Shading Language (HLSL) uses C-like code –OpenGL Shading Language (GLSL) uses C-like code –Nvidia Cg uses assembly-like code –Pixar’s RenderMan the pioneer among shaders

Shader Types Vertex Shader Pixel Shader (Fragment Shader in GLSL) Geometry Shader Compute Shader

Vertex Shader Job is to process vertices and send them to pixel shader Process? –Transformation & lighting Let’s look at an HLSL example & GLSL example

Shading Techniques Flat shading Smooth Shading –Gouraud shading –Phong shading Additional techniques: –bump mapping –cel shading –and many more

Let’s assume we want to render a red ball Ignore textures for now How can we shade the ball to make it realistic?

Flat Shading Assigns a single color to a polygon Simple & fast Color based on: –Material color –the polygon's normal –the position and intensity of a light source.

Gouraud Shading invented by Henri Gouraud in 1971 simulates smoothly shaded surfaces How? –computes vertex colors using normals & lighting –interpolates vertex colors across a polygon's surface done linearly fast and resource-conscious

Phong Shading invented by Bui Tuong Phong a smooth shading technique approximates curved-surface lighting How? –interpolates the vertex normals of a polygon across the surface Most realistic but expensive

Bump Mapping invented by Jim Blinn an extension of Phong shading simulates bumpy or wrinkled surfaces How? –at each pixel along a surface, slight changes are made to the normal –these changes are usually stored in a texture map

Cel Shading also called “toon shading” technique of making computer generated graphics appear to look hand drawn How? –objects have black outlines (as though done with pencil) done via inversing back-face culling with black vertices & slight translation –only a few shades of each color used for objects

Example of Cel Shading From The Legend of Zelda: The Wind Talker

Different Effects using Different Shaders

Texture Shading Textures can be lit as well –we’ve already seen this How? –make polygon white & light normally –when laying texture, calculate texel brightness using polygon colors before texture We can exert further control over how textures appear –ex: texture blending

Rendering Pipeline Ordered operations involved in transforming a scene to a screen –transformations, hiding, lighting, shading, projection Rendering Options: –Fixed-function pipeline actually emulated these days –Programmable pipeline

What’s wrong with fixed-function? Certain graphics algorithms are pre-programmed into graphics cards (ex: Gouraud shading) So what? –What if we want to use different algorithms? Why is the programmable pipeline necessary? –can perform non-standard lighting/shading –can achieve higher quality lighting/shading –do these things on the hardware level

Typical Graphics Pipeline In the traditional Programmable pipeline, which 2 stages may we control? –vertex processing –pixel processing

Types of Shaders Vertex Shaders –allows programmer to control transformation & lighting operations (T&L) Pixel Shading –the output of a vertex shader provides input to a pixel shader –allows programmer to ultimately decide final pixel colors before rasterization Shader Models –standardized support for shader programs –ex: Microsoft’s Shader Model 3.0

Possible uses of Vertex Shaders Procedural Geometry (Isidoro/Gosslin) –cloth simulation, soap bubbles Advanced Vertex Blending for Skinning and Vertex Morphing (Gosselin) –tweening Texture Generation (Riddle/Zecha) Advanced Keyframe Interpolation –complex facial expression and speech Particle System Rendering Real-Time Modifications of the Perspective View –lens effects, underwater effect Advanced Lighting Models –in cooperation with the pixel shader Displacement Mapping (Calver) And whatever will appear at next year’s SIGGRAPH Conference

Possible Uses of Pixel Shaders Single pass, per-pixel lighting True phong shading (Beaudoin/Guardado) Anisotropic lighting (Isidoro/Brennan) Non-Photorealistic-Rendering (Card/Mitchell) –Ex: cartoon shading, hatching, gooch lighting, image space techniques Per-pixel fresnel term (Brennan) Volumetric effects (Kraus, Hart) Advanced bump mapping (self-shadowing bump maps) –also known as Horizon Mapping Procedural textures (Zecha) Texture perturbation (Isidoro/Riguer) Bidirectional reflectance distribution functions (Moravánsky) And whatever will appear at next year’s SIGGRAPH Conference

Shader Tools Most modeling tools provide shader support –WYSIWYG use of algorithms –generation of shader code –shader work encapsulated in effects Ex: –Maya –3D Studio Max, –NVIDIA’s Effects Browser, Shader City, & Shader Studio

Next Time Shader Examples GLSL & HLSL

Resources GameDev.net – – HowStuffWorks – MSDN – us/directx9_c/directx/graphics/programmingguide/gettingstarted/architecture. asphttp://msdn.microsoft.com/library/default.asp?url=/library/en- us/directx9_c/directx/graphics/programmingguide/gettingstarted/architecture. asp Wikipedia – –