Index Making Pictures

Sampling Regular Textures

Thanks to Dan Su for preparing the pictures.

The above example shows sampling defects, which arise from performing a point sampling of the scene. Firstly, edges between grey and white are stepped. These steps are visible throughout the image except where the edges happen to align exactly with the scan of the display. Secondly, in the distance the rectangles gradually break up. Look at the edges of the picture, where this problem is most apparent. Finally, look close to the horizon and you can see that the rectangles get larger. This is because successive samples hit the same colour, grey say. There should be white in between but the samples don't hit the white part just there. So what should be a high spatial frequency (rapidly changing between grey and white) appears as a lower frequency. We say that the high frequency appears under the alias of the low frequency. Correcting this is generically called anti-aliasing.

The second image has been mip-mapped. The aliasing on edges between grey and white has gone (compare the edges of the foreground rectangles in both pictures) and the distance effects are greatly reduced. Although not entirely eliminated, they are not as visible and so not so objectionable. The details blur together in the distance, as you would expect in reality, becoming an average grey at the horizon.