There are two kinds of images: rasters and vectors
Rasters are your typical image formats (bmp, png, JPEG) They aren't very good when scaled up or down.
Vectors, however, are pictures stores as a series of mathematical algorithms and so can be scaled up and down without any loss in image quality and are usually a smaller file size.
Why is the latter so uncommon?
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The pure insanity of defining those equations perfectly. Think about it this way; if it were that easy, photo-realistic cgi would be child's play.