CS361Week 8 - Monday

Last time

What did we talk about last time? Workday Before that:

Image texturing▪ Magnification▪ Minification

Mipmapping Summed area tables

Questions?

Project 2

Mipmap Review

Mipmapping in action

Typically a chain of mipmaps is created, each half the size of the previous

That's why cards like square power of 2 textures Often the filtered version is made with a box

filter, but better filters exist The trick is figuring out which mipmap level to

use The level d can be computed based on the

change in u relative to a change in x

Trilinear filtering

One way to improve quality is to interpolate between u and v texels from the nearest two d levels

Picking d can be affected by a level of detail bias term which may vary for the kind of texture being used

Summed-area table

Sometimes we are magnifying in one axis of the texture and minifying in the other

Summed area tables are another method to reduce the resulting overblurring

It sums up the relevant pixels values in the texture It works by precomputing all possible rectangles

Anisotropic filtering

Summed area tables work poorly for non-rectangular projections into texture space

Modern hardware uses unconstrained anisotropic filtering The shorter side of the projected area determines d, the

mipmap index The longer side of the projected area is a line of anisotropy Multiple samples are taken along this line Memory requirements are no greater than regular

mipmapping

Other Texturing Issues

Volume textures

Image textures are the most common, but 3D volume textures can be used

These textures store data in a (u, v, w) coordinate space

Even volume textures can be mipmapped Quadrilinear interpolation!

In practice, volume textures are usually used for fog, smoke, or explosions 3D effects that are inconsistent over the

volume

Cube maps

A cube map is a kind of texture map with 6 faces

Cube maps are used to texture surfaces based on direction

They are commonly used in environment mapping

A ray is made from the center of the cube out to the surface The component with the largest

magnitude selects which of the 6 faces

The other components are used for (u,v) coordinates

Cube maps can cause awkward seams when jumping between faces

Texture caching

You will never need to worry about this in this class, but texture memory space is a huge problem

There are many different caching strategies, similar ones used for RAM: Least Recently Used (LRU): Swap out the least

recently used texture, very commonly used Most Recently Used (MRU): Swap out the most

recently used texture, use only during thrashing Prefetching can be useful to maintain

consistent frame rates

Texture compression

JPEG and PNG are common compression techniques for regular images

In graphics hardware, these are too complicated to be decoded on the fly That's why the finished SharpDX projects have

pre-processed.tkb files Most DirectX texture compression divides

textures into 4 x 4 tiles Two 16-bit RGB values are recorded for each tile Each texel uses 2 bits to select one of the two

colors or two interpolated values between them

More texture compression Ericsson texture compression (ETC) is used in OpenGL

It breaks texels into 2 x 4 blocks with a single color It uses per-pixel luminance information to add detail to the blocks

Normal maps (normals stored as textures) allow for interesting compression approaches Only x and y components are needed since the z component can

be calculated The x and y can then be stored using the BC5 format for two

channels of color data

Procedural texturing

A procedural texture is made by computing a function of u and v instead of looking up a texel in an image

Noise functions are often used to give an appearance of randomness

Volume textures can be generated on the fly

Values can be returned based on distance to certain feature points (redder colors near heat, for example)

Texture animation

Textures don't have to be static The application can alter them over time Alternatively, u and v values can be

remapped to make the texture appear to move

Matrix transformations can be used for zoom, rotation, shearing, etc.

Video textures can be used to play back a movie in a texture

Blending between textures can allow an object to transform like a chameleon

Material Mapping

The lighting we have discussed is based on material properties Diffuse color Specular color Smoothness coefficient m

A texture can be used to modify these values on a per-pixel basis

A normal image texture can be considered a diffuse color map

One that affects specular colors is a specular color map (usually grayscale)

One that affects m is a gloss map

Alpha Mapping

Alpha values allow for interesting effects Decaling is when you apply a texture

that is mostly transparent to a (usually already textured) surface

Cutouts can be used to give the impression of a much more complex underlying polygon 1-bit alpha doesn't require sorting Cutouts are not always convincing from

every angle

Student Lecture: Bump Mapping

Bump Mapping

Bump mapping

Bump mapping refers to a wide range of techniques designed to increase small scale detail

Most bump mapping is implemented per-pixel in the pixel shader

3D effects of bump mapping are greater than textures alone, but less than full geometry

Macro, meso, and micro

Macro-geometry is made up of vertices and triangles Limbs and head of a body

Micro-geometry are characteristics shaded in the pixel shader, often with texture maps Smoothness (specular color and m parameter) based on

microscopic smoothness of a material Meso-geometry is the stuff in between that is too

complex for macro-geometry but large enough to change over several pixels Wrinkles Folds Seams

Bump mapping techniques are primarily concerned with mesoscale effects

Blinn's methods

James Blinn proposed the offset vector bump map or offset map Stores bu and bv values at each texel,

giving the amount that the normal should be changed at that point

Another method is a heightfield, a grayscale image that gives the varying heights of a surface Normal changes can be computed from

the heightfield

Normal maps

The results are the same, but these kinds of deformations are usually stored in normal maps Normal maps give the full 3-

component normal change Normal maps can be in world

space (uncommon) Only usable if the object never

moves Or object space

Requires the object only to undergo rigid body transforms

Or tangent space Relative to the surface, can assume

positive z Lighting and the surface have to

be in the same space to do shading

Filtering normal maps is tricky

Parallax mapping

Bump mapping doesn't change what can be seen, just the normal

High enough bumps should block each other Parallax mapping approximates the part of

the image you should see by moving from the height back to the view vector and taking the value at that point

The final point used is:z

xy

v

h vpp

adj

Parallax mapping continued At shallow viewing angles, the previous

approximation can look bad A small change results in a big texture change

To improve the situation, the offset is limited (by not scaling by the z component)

It flattens the bumpiness at shallow angles, but it doesn't look crazy

New equation: xyh vpp adj

Relief mapping

The weakness of parallax mapping is that it can't tell where it first intersects the heightfield

Samples are made along the view vector into the heightfield

Three different research groups proposed the idea at the same time, all with slightly different techniques for doing the sampling

There is still active research here Polygon boundaries are still flat in most models

Heightfield texturing

Yet another possibility is to change vertex position based on texture values Called displacement mapping

With the geometry shader, new vertices can be created on the fly

Occlusion, self-shadowing, and realistic outlines are possible and fast

Unfortunately, collision detection becomes more difficult

Upcoming

Next time…

Radiometry Photometry Colorimetry BRDFs

Reminders

Start reading Chapter 7Finish Project 2

Due on Friday