Week 11 Tutorial
Question 1
In this question you are to work out the colour of a pixel using different aliasing schemes. The pixel is covered by parts of two polygons, Polygon A (coloured red (1,0,0)) and Polygon B (coloured green (0,1,0)). The polygons share a common edge that runs horizontally exactly across the middle of the pixel. The rule we use when filling polygons is that if the centre of the pixel is exactly on the edge of the polygon, it is filled if it is a top edge of the polygon, and not filled if it is a bottom edge. In this case, Polygon B is rendered first and then Polygon A (ie A is on top.)
a) What colour does the pixel end up if there is no anti-aliasing?
b) What colour would it be if we could take an infinite number of samples?
c) What colour does it end up if we use pre-filtering? Assume that the background colour is black.
d) What colour does it end up if use 3×3 post-filtering with the samples as indicated in the first picture (ie no jitter)?
e) What colour does it end up if we use 3×3 jittered samples as shown in the second picture?
f) Which of c,d and e is closest to the correct answer?
Question 2
When texture polygons used for floors and walls, mipmaps have a problem — the scaling of textures is not proportional. For example, a square texel might get mapped to a rectangle 1 pixel wide and a quarter pixel high. The best we can do when mipmapping is to choose the one quarter sized image, but this blurs the result much more than necessary.
Anisotopic filtering is an improvement on mipmapping that allows for the possibility of non-proportional scaling of textures. One technique uses a ripmap (rip = rectim im parvo), where non proportional scaled versions are stored as well. For a mipmap of a 256 x 256 texture we store 128 x 128, 64 x 64, 32 x 32, etc versions, while in a ripmap we also store 256×128, 128×256, 256×64, 128×64, 64×256, 128×256, etc.
Calculate how much extra space is need for a mipmap texture and for a ripmap texture.