Polygon compression for Integer coordinates

Question

I have polygon in raster with "signed int" 2D coordinates. Polygon has its vertices ordered in clockwise direction.

I want to store this polygon in some more "space friendly" way, that just store the sequence of int x, y coordinates. If I compress this file with rar, I got sizes about 3.5x smaller than uncompressed. Is there any better representation than simply storing x,y in a sequence?

The compression should be lossless.

Answer 1

If your polygon is (x1, y1), (x2, y2) ... (xn, yn), you can write it as,

x1, x2-x1, x3-x2, xn-xn-1, y1, y2-y1 ..., yn-yn-1

Typically the differences between adjacent x,y values will be smaller than the absolute values, so you can store the deltas as variable length ints. By doing this, each x,y pair can typically be stored in 2 or 4 bytes rather than 8.