Week 8 - Rasterisation and Anti-Aliasing Flashcards
Name 2 examples of vector displays.
Pen plotters
Tektronix cathode ray tubes
Name 5 examples of raster displays.
Monitors
Televisions
Laser, ink-jet, dot matrix printers
Liquid crystal displays
Arrays of LEDs
What is rasterisation?
The conversion of ideal, mathematical graphics primitives onto raster displays
What is scan conversion?
Rasterisation line by line of the raster display
How does scanline interpolation work?
Use Z-buffer algorithm to determine screen locations of vertices
Intensities or normals interpolated from vertices
Interpolate along scanline from edges
What is the parity rule for filling polygons?
A point is inside a polygon if a line to a distant point intersects boundary an odd number of times
What is the non-zero winding number rule for filling polygons?
A point is inside a polygon if the polygon winds clockwise around the point a non-zero number of times
What is scan-line coherence?
An interior pixel is adjacent to another interior pixel unless a boundary is encountered
How does scan-line coherence work?
For each scan line:
- Find intersections of scan line with all polygon edges
- Sort intersections into increasing order of x coordinate
- Fill alternate segments of scan line (spans) according to parity rule
What are the two ways to choose which pixels to illuminate for a line?
Every pixel the line intersects
The vertically closest pixel
What algorithm is used to illuminate the pixel vertically closest to a line?
Naïve midpoint algorithm
What does DDA stand for?
Digital Differential Analyser
What is the Bresenham circle algorithm?
Choose pixel with centre closest to circle
Work on a single octant and draw others by symmetry
Can be written in integer arithmetic
What causes aliasing?
Undersampling (not taking enough discrete samples to capture a fast rate of change)
What does ADC stand for?
Analog-to-Digital Converter
What does DAC stand for?
Digital-to-Analog Converter
Why will there always be aliasing, no matter how densely we sample?
Repetitive patterns can have arbitrarily high frequencies due to perspective effects
Object edges create sharp changes -> high frequencies
How does anti-aliasing work in computer graphics?
The scene is rendered
No access to a continuous image
Low-pass filtering of continuous image has to be simulated/approximated
What are the three filtering approaches?
No filtering
Pre-filtering (simulation)
Post-filtering (approximation)
What does SSAA stand for?
Super-Sampling Anti-Aliasing
What is SSAA?
Take many point samples per pixel, and assign grey level as the (weighted) average value
What is a pro and a con of SSAA?
Pro: smoother, nicer looking results
Con: ridiculously expensive and can lead to low performance and framerate
What is multisample anti-aliasing?
Faster processing by processing fragment shaders only once, and only supersampling depth and stencil buffers
What does FXAA stand for?
Fast Approximate Anti-Aliasing
What is FXAA?
Pixel-level fast algorithm (implemented as fragment shader) identifies depth edges in the image, and smooths edges at pixel level
What are two issues to be aware of with anti-aliasing and texture maps, and how are they fixed?
Compression (several texels map to a single pixel) - texture image should be sampled many times to calculate pixels, but this is very expensive
Magnification (single pixel lies deep within texel) - higher resolution texture map required
What are the three types of filtering in MIP mapping?
Bilinear filtering, trilinear filtering, anisotropic filtering
What is bilinear filtering?
Approximate pixel value from pixels around it
What is trilinear filtering?
Also approximate between levels of MIP mapping
What is anisotropic filtering?
Sample texture with different horizontal and vertical scale ratios
What is anisotropic filtering?
Sample texture with different horizontal and vertical scale ratios
