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Unformatted text preview: CS 445 / 645
Introduction to Computer Graphics
Lecture 20
Antialiasing Environment Mapping
Used to model a object that reflects surrounding textures
Used
to the eye
to
• Polished sphere reflects walls and ceiling textures
• Cyborg in Terminator 2 reflects flaming destruction Texture is distorted fisheye view of environment
Spherical texture mapping
Spherical
creates texture coordinates
that correctly index into
this texture map
this Materials from NVidia
http://developer.nvidia.com/object/Cube_Mapping_Paper.html Sphere Mapping Blinn/Newell Lattitude Mapping Cube Mapping Multitexturing
Pipelining of multiple texture applications to one
Pipelining
polygon
polygon
The results of each texture unit application is
The
passed to the next texture unit, which adds its
effects
effects
More bookkeeping is required to pull this off Antialiasing What is a pixel?
A pixel is not…
• A box
• A disk
• A teeny tiny little light
A pixel is a point
• It has no dimension
• It occupies no area
• It cannot be seen
• It can have a coordinate A pixel is more than a point, it is a sample Samples
Most things in the real world are continuous
Most
continuous
Everything in a computer is discrete
Everything
discrete
The process of mapping a continuous function to a
The
discrete one is called sampling
sampling
The process of mapping a continuous variable to a
The
discrete one is called quantization
quantization
Rendering an image requires sampling and
Rendering
quantization
quantization Samples Samples Line Segments
We tried to sample a line segment so it would map
We
to a 2D raster display
to
We quantized the pixel values to 0 or 1
We saw stair steps, or jaggies Line Segments
Instead, quantize to many shades
But what sampling algorithm is used? Area Sampling
Shade pixels according to the area covered by thickened
Shade
line
line
This is unweighted area sampling
This unweighted A rough approximation formulated by dividing each pixel
rough
into a finer grid of pixels
into Unweighted Area Sampling
Primitive cannot affect intensity of pixel if it does
Primitive
not intersect the pixel
not
Equal areas cause equal intensity, regardless of
Equal
distance from pixel center to area
distance Weighted Area Sampling
Unweighted sampling colors two pixels identically
Unweighted
when the primitive cuts the same area through
the two pixels
the
Intuitively, pixel cut through the center should be
Intuitively,
more heavily weighted than one cut along
corner
corner Weighted Area Sampling
Weighting function, W(x,y)
• specifies the contribution of primitive passing through
specifies
the point (x, y) from pixel center
the
Intensity W(x,y) x Images
An image is a 2D function I(x, y) that specifies
An
(x,
intensity for each point (x, y)
intensity Sampling and Image
Our goal is to convert the continuous image to a
Our
discrete set of samples
discrete
The graphics system’s display hardware will
The
attempt to reconvert the samples into a
continuous image: reconstruction
reconstruction Point Sampling an Image
Simplest sampling is on a grid
Sample depends
solely on value
at grid points Point Sampling
Multiply sample grid by image intensity to obtain a
Multiply
discrete set of points, or samples.
discrete Sampling Geometry Sampling Errors
Some objects missed entirely, others poorly
sampled
sampled Fixing Sampling Errors
Supersampling
• Take more than one sample for each pixel and combine
Take
them
them
– How many
How
samples is
enough?
enough?
– How do we
How
know no
features are
lost?
lost? 150x15 to 100x10
200x20 to 100x10
300x30 to 100x10
400x40 to 100x10 Unweighted Area Sampling
Average supersampled points
All points are weighted equally Weighted Area Sampling
Points in pixel are weighted differently
• Flickering occurs as object moves
across display Overlapping regions eliminates flicker Signal Theory Intensity Convert spatial signal to frequency domain Pixel position across scanline
Example from Foley, van Dam, Feiner, and Hughes Signal Theory
Represent spatial
Represent
signal as sum of
sine waves
(varying frequency
and phase shift)
and
Very commonly
used to represent
sound “spectrum” Fourier Analysis
Convert spatial domain to frequency domain • Let f(x) indicate the intensity at a location in space, x (pixel value)
Let f(x)
(pixel
• u is a complex number representing frequency and phase shift
– i = sqrt (1) … frequently not plotted
• F(u) is the amplitude of a particular frequency in a signal
– In this case the signal is f(x)
In
f(x) Fourier Transform
Examples of spatial
Examples
and frequency
domains
domains Nyquist Sampling Theorem
The ideal samples of a continuous function
The
contain all the information in the original function
if and only if the continuous function is sampled
at a frequency greater than twice the highest
frequency in the function
frequency Nyquist Rate
The lower bound on the sampling rate equals
The
twice the highest frequency component in the
image’s spectrum
image’s
This lower bound is the Nyquist Rate Bandlimited Signals
If you know a function contains no components of
If
frequencies higher than x
• Bandlimited implies original function will not require any
Bandlimited
ideal functions with frequencies greater than x
ideal
• This facilitates reconstruction Flaws with Nyquist Rate
Samples may not align with peaks Flaws with Nyquist Rate
When sampling below Nyquist Rate, resulting
When
signal looks like a lowerfrequency one
signal
• With no knowledge of bandlimits, samples could have
With
been derived from signal of higher frequency
been Lowpass Filtering
• We know we are limited in the resolution of our screen
• We want the screen (sampling grid) to have twice the resolution
We
of the signal (image) we want to display
of
• How can we reduce the highfrequencies of the image?
How
– Lowpass filter
– Bandlimits the image Lowpass Filtering
In frequency domain
• If signal is F(u)
• Just chop off parts of F(u) in high frequencies using a second
Just
function, G(u)
function,
– G(u) ==
G(u) 1 when –k <= u <= k
when
0 elsewhere – This is called the pulse function Lowpass Filtering
In spatial domain
• Multiplying two Fourier transforms in the spatial domain
Multiplying
corresponds exactly to performing an operation called
convolution in the spatial domain
convolution
• f(x) * g(x) = h(x) the convolution of f with g…
f(x)
– The value of h(x) at x is the integral of the product of f(x)
The
with the filter g(x) such that g(x) is centered at x
with
• The pulse (frequency) == sinc (spatial) Lowpass Filtering
In spatial domain
Sinc: sinc(x) = sin (π x)/π x
• Note this isn’t perfect way to eliminate high frequencies
– “ringing” occurs Sinc Filter
Slide filter along
Slide
spatial domain and
compute new pixel
value that results
from convolution
from Bilinear Filter
Sometimes called a tent filter
Easy to compute
• just linearly interpolate between samples Finite extent and no negative values
Still has artifacts Sampling Pipeline How is this done today?
Full Screen Antialiasing
Nvidia GeForce2
• OpenGL: render image 400% larger and supersample
• Direct3D: render image 400%  1600% larger Nvidia GeForce3
• Multisampling but with fancy overlaps
– Don’t render at higher resolution
– Use one image, but combine values of neighboring pixels
– Beware of recognizable combination artifacts Human perception of patterns is too good GeForce3
Multisampling
• After each pixel is rendered, write pixel value to two
After
different places in frame buffer
different GeForce3  Multisampling
After rendering two copies of entire frame
• Shift pixels of Sample #2 left and up by ½ pixel
• Imagine laying Sample #2 (red) over Sample #1 (black) GeForce3  Multisampling
Resolve the two samples into one image by
Resolve
computing average between each pixel from
Sample 1 (black) and the four pixels from
Sample 2 (red) that
are 1/ sqrt(2) pixels
away
away GeForce3  Multisampling No AA
No AA Multisampling
Multisampling GeForce3  Multisampling • 4x Supersample Multisampling ATI Smoothvision
ATI SmoothVision
• Programmer selects
Programmer
samping pattern
samping
• Some supersampling
thrown in ATI NVidia comparison
ATI Radeon 8500
ATI
AA Off 2x AA NVidia GF3 Ti 500
Quincunx AA http://www.anandtech.com/video/showdoc.html?i=1562&p=4 3dfx
3dfx Multisampling
• 2 or 4frame shift and average Tradeoffs? ...
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 Spring '08
 BLOOMFIELD,A
 Computer Graphics

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