Admin Grad student TAs may have had their

Admin • Grad student TAs may have had their accounts disabled – Please check and email the lab if there is a problem • If you plan on graduating with any degree in the coming year, you should see Lorene and collect a questionnaire – List compiled from questionnaires will be provided to employers • Sooner, or later, Lith. Tech install CDs will be available to borrow overnight – Arrangements yet to be finalized CS 638, Fall 2001

NTSC vs. PAL • Two major differences: – Vertical resolution: 625 vs. 525 (not all useable) – Frame rate: 50 Hz vs. 60 Hz (approx) • Issues: – Artwork appearance, particularly for menus and other 2 D art – Animation timing: Detach animation clock from frame rate clock, which is good practice anyway CS 638, Fall 2001

Graphics Review • Recall the standard graphics pipeline (Open. GL in this case): CS 638, Fall 2001

Normal Vectors • The surface normal vector describes the orientation of the surface at a point – Mathematically: Vector that is perpendicular to the tangent plane of the surface • What’s the problem with this definition? – Just “the normal vector” or “the normal” – Will use N to denote • Many lighting calculations are parameterized by the normal vector – Later, see how to exploit this CS 638, Fall 2001

Local Shading Models • Local shading models provide a way to determine the intensity and color of a point on a surface – The models are local because they don’t consider other objects at all – We use them because they are fast and simple to compute – They do not require knowledge of the entire scene, only the current piece of surface • Works well for pipelined architectures, because pipeline only knows about one piece of geometry at a time CS 638, Fall 2001

“Traditional” Shading Model • What it captures: – Direct illumination from light sources – Diffuse and Specular components – (Very) Approximate effects of global lighting • What it doesn’t do: – – Shadows Mirrors Refraction Lots of other stuff … CS 638, Fall 2001

“Standard” Lighting Model • Consists of three terms linearly combined: – Diffuse component for the amount of incoming light reflected equally in all directions – Specular component for the amount of light reflected in a mirror-like fashion – Ambient term to approximate light arriving via other surfaces • It doesn’t do shadows, mirrors, refraction, lots of other stuff … CS 638, Fall 2001

Diffuse Illumination • Incoming light, Ii, from direction L, is reflected equally in all directions – No dependence on viewing direction • Amount of light reflected depends on: – Angle of surface with respect to light source • Actually, determines how much light is collected by the surface, to then be reflected – Diffuse reflectance coefficient of the surface, kd • Don’t want to illuminate back side. Use CS 638, Fall 2001

Diffuse Example Where is the light? CS 638, Fall 2001

Specular Reflection (Phong Model) L H • Incoming light is reflected primarily in the mirror direction, R. (H is half vector, N is normal) – Perceived intensity depends on the relationship between the viewing direction, V, and the mirror direction – Bright spot is called a specularity • Intensity controlled by: – The specular reflectance coefficient, ks – The parameter, n, controls the apparent size of the specularity • Higher n, smaller highlight CS 638, Fall 2001 V R

Specular Example CS 638, Fall 2001

Putting It Together • Global ambient intensity, Ia: – Gross approximation to light bouncing around of all other surfaces – Modulated by ambient reflectance ka • Just sum all the terms • If there are multiple lights, sum contributions from each light • Several variations, and approximations CS 638, Fall 2001

Flat shading • Compute shading at a representative point and apply to whole polygon – Open. GL uses one of the vertices • Advantages: – Fast - one shading value per polygon • Disadvantages: – Inaccurate – Discontinuities at polygon boundaries CS 638, Fall 2001

Gourand Shading • Shade each vertex with it’s own location and normal • Linearly interpolate across the face • Advantages: – Fast - incremental calculations when rasterizing – Much smoother - use one normal per shared vertex to get continuity between faces • Disadvantages: – Specularities get lost CS 638, Fall 2001

Phong Interpolation • Interpolate normals across faces • Shade each pixel • Advantages: – High quality, narrow specularities • Disadvantages: – Expensive – Still an approximation for most surfaces • Not to be confused with Phong’s specularity model CS 638, Fall 2001

CS 638, Fall 2001

Texture Mapping • The problem: Colors, normals, etc. are only specified at vertices. How do we add detail between vertices? • Solution: Specify the details in an image (the texture) and specify how to apply the image to the geometry (the map) • Works for shading parameters other than color, as we shall see – The basic underlying idea is the mapping CS 638, Fall 2001

Basic Mapping • The texture lives in a 2 D space – Parameterize points in the texture with 2 coordinates: (s, t) – These are just what we would call (x, y) if we were talking about an image, but we wish to avoid confusion with the world (x, y, z) • Define the mapping from (x, y, z) in world space to (s, t) in texture space • With polygons: – Specify (s, t) coordinates at vertices – Interpolate (s, t) for other points based on given vertices CS 638, Fall 2001

Basic Mapping CS 638, Fall 2001

I assume you recall… • Texture sampling (aliasing) is a big problem – Mipmaps and other filtering techniques are the solution • The texture value for points that map outside the texture image can be generated in various ways – Repeat, Clamp, … • Texture coordinates are specified at vertices and interpolated across triangles • Width and height of texture images is constrained (powers of two, sometimes must be square) CS 638, Fall 2001

Textures in Games • The game engine provides some amount of texture support • Artists are supplied with tools to exploit this support – They design the texture images – They specify how to apply the image to the object • Commonly, textures are supplied at varying resolutions to support different hardware performance – Note that the texture mapping code does not need to be changed just load different sized maps at run time • Textures are, without doubt, the most important part of a game’s look CS 638, Fall 2001

Example Texture Tool CS 638, Fall 2001

Packing textures • Problem: The limits on texture width/height make it inefficient to store many textures – For example: long, thin objects • Solution: Artists pack the textures for many objects into one image – The texture coordinates for a given object may only index into a small part of the image – Care must be taken at the boundary between sub-images to achieve correct blending – Mipmapping is restricted – Best for objects that will be at known resolution (weapons, for instance) CS 638, Fall 2001

Combining Textures CS 638, Fall 2001

Texture Matrix • Normally, the texture coordinates given at vertices are interpolated and directly used to index the texture • The texture matrix applies a homogeneous transform to the texture coordinates before indexing the texture • What use is this? – Two examples in this lecture: Animating textures and projective texturing CS 638, Fall 2001

Animating Texture (method 1) • The texture matrix can be used to translate or rotate the texture • If the texture matrix is changed from frame to frame, the texture will appear to move on the object • This is particularly useful for things like flame, or swirling vortices, or pulsing entrances, … CS 638, Fall 2001

Demo CS 638, Fall 2001

Projective Texturing • The texture should appear to be projected onto the scene, as if from a slide projector • Solution: – Equate texture coordinates with world coordinates – Think about it from the projector’s point of view: wherever a world point appears in the projector’s view, it should pick up the texture – Use a texture matrix equivalent to the projection matrix for the projector – maps world points into texture image points • Details available in many places • Problems? What else could you do with it? CS 638, Fall 2001

Multitexturing • Some effects are easier to implement if multiple textures can be applied – Future lectures: Light maps, bump maps, shadows, … • Multitexturing hardware provides a pipeline of texture units, each of which applies a standard texture map operation – Fragments are passed through the pipeline with each step working on the result of the previous stage – Texture parameters are specified independently for each unit, further improving functionality – For example, the first stage applies a color map, the next modifies the illumination to simulate bumps, the third modifies opacity – Not the same as multi-pass rendering - all applied in one pass CS 638, Fall 2001

What’s in a Texture? • The graphics hardware doesn’t know what is in a texture – It applies a set of operations using values it finds in the texture, the existing value of the fragment (pixel), and maybe another color – The programmer gets to decide what the operations are, within some set of choices provided by the hardware – Examples: • the texture may contain scalar “luminance” information, which simply multiplies the fragment color. What use is this? • the texture might contain “alpha” data that multiplies the fragment’s alpha channel but leaves the fragment color alone. What use is this? – Future lectures will look at creative interpretations of textures CS 638, Fall 2001