cs361
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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
vh 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
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