| Full Scene Anti-Aliasing (FSAA) |
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| May 2011 | |
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How does anti-aliasing fool
the eye?
The clever system that anti-aliasing uses to fool your eyes into thinking that a line is not jagged is to use subtle changes in colour around the curved or diagonal area. These slight changes in colour make the image blend around curves and giving the impression that the line in true. The colour changes are on such a small scale that you eyes cannot detect them under normal circumstances.   Going back to the first example with the letters above, have a closer look at the second letter and you can see in this zoomed in image the changes in greys around the curve.   What is FSAA? FSAA stands for Full Scene Anti Aliasing and is used as the most common term for Anti-Aliasing in the gaming world. FSAA refers to a technique of Anti-Aliasing that affects the entire screen not just a certain image. When playing a game with FSAA turned on the graphics card will run an Anti-Aliasing algorithm on every frame from top to bottom removing jagged edges from the entire scene. This can have a large hit on performance in games and has only been overcome recently by having much more powerful graphics cards on the market. Types of Anti Aliasing There are two main types of Anti-Aliasing techniques, Multisampling and Supersampling. There are varieties of each as graphics card manufacturers find improved ways of getting the same result. Finding ways to limit the performance hit is the main priority. Here we will look at the overall picture of the two techniques. First we will concentrate on Supersampling. Supersampling Supersampling is a method of Anti-Aliasing by taking the corners of each pixel and creating what would be the average colour. This is then the displayed pixel on the screen. By doing this you are effectively smudging the image and averaging out the colour along a curve. 
This very basic graphic shows you what would happen when Supersampling is used in the four squares in the centre of this image. 3 of the 4 squares are in both red and white area's and so will be displayed as a shade of orange. One of the squares is fully in the white zone and so that pixel will be displayed as a pure white pixel. If one of the samples was taken from a area that was full in the red zone then that pixel would be displayed as fully red. Obviously samples are much more complex than this, having many more colours to average out, however the principles remain the same. A mathematical calculation is done to determine the average colour for each pixel. Effectively Supersampling renders the scene 4 times larger than the true scene and is then scaled down once the calculations are complete. This system has a massive performance hit but does give the best results. Multisampling Multisampling is a more efficient but slightly less pretty form of Anti-Aliasing. Multisampling takes multiple samples for each pixel. In a fairly standard example the quincunx system takes 4 samples in the corners and 1 sample in the middle. Each of these samples is given a weight the corners are given a weight of 1/8 each and the centre sample is given a weight of 1/2. the colour of the pixel is then determined by similar calculations to Super sampling. 
With multisampling each cell on has two samples in it. Looking at the diagram above you can see that the other 3 samples are taken from neighbouring cells. So multisampling takes into account the colours around the pixel in question. This is how you get a blend of colour to achieve the desired result. It is important to note that Multisampling only happens when a cell is covered my more than one colour, otherwise a single colour is chosen and does not need to be calculated. There are pro's and cons for using anti-aliasing in both games and applications. We have been through them but here is a quick summery to help you make up your mind if using AA or FSAA is right for you and your PC Pro's
  FSAA on FSAA off Return to Advanced Features page |