CS 638, Fall 2001

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, LithTech 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

2D 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 (OpenGL 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

)( NL id Ik

)0,max( NL id Ik

CS 638, Fall 2001

Diffuse Example

Where is the light?

CS 638, Fall 2001

Specular Reflection (Phong Model)

• 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

nisIk )( NH

LR

VH

CS 638, Fall 2001

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

nsdiaa kkIIkI )()( NHNL

CS 638, Fall 2001

Flat shading

• Compute shading at a representative point and apply to whole polygon– OpenGL 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 2D 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