I understand what you mean. It only appears on monitor screen, you won't have this strange visual effect when print.
Was wondering if someone here could explain to me why fill patterns of parallel lines seems to form crazy hatch patterns when zooming in or out. Does the same thing happen in the real world or only with monitors? If my written explanation is too vague I'll find some pics.
It's due to the way pixels are arranged in a grid, it's why on tv they make sure not to wear clothes that will cause that problem (checkered patterns etc.), you have the same thing in CRT screens. What I think happens is that the pixels get a read pattern of colour/ no colour because the colours fall in between where they display on the screen... only I'm sure there is some way to express that in a way that makes sense...
Falconius describes part of the problem. The real problem is how the image is sampled. A quick internet search on "image resampling" and "moire patterns" and "aliasing" will turn up a lot of discussions on the subject. The (overly) short version is that if you make an image smaller by just picking a pixel at a multiple of the image size (e.g. shrinking an image to 1/3 its size by picking every third pixel), then any pattern that happened to be in the original image at every third pixel will be made more apparent. A grid, for example, might drop out lines and a pattern that is composed of every other sample will show as a larger checkerboard pattern. Very broadly speaking, shrinking an image involves using all pixels in the source image to produce the destination image (typically, a blur as big as your shrink ratio is a good starting point); enlarging an image involves trying to create detail at the right scale to avoid artifacts like blockiness and ringing. http://www.cartographersguild.com/tu...explained.html has some good discussions on the subject.