How to link four points to a convex polygon

How to link four points to a convex 开发者_StackOverflowpolygon? I mean how to identify the order of these four points.

Thanks.

Zhong

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Take the center point (i.e. average of x and y coords), then calculate x/y values for y<centery, then for y>=centery. would be fastest I guess.

(that is, if I understood the question in the first place...)

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Sort them vertically, connect 2 top most to each other and two lowest to each other.
Sort horizontally and then connect 2 leftmost to each other and two rightmost to each other.

EDIT: anyways, SO's cool related section on the right suggests an answered duplicate: Sort Four Points in Clockwise Order

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The atan2() method is handy for this, and is found in most languages.

atan2(y,x) and converts rectangular coordinates (x,y) to the angle theta from the polar coordinates (r,theta).

Given 4 points, find their average. Then calculate the four (x,y) vectors obtained by subtracting the average from each of the four points.

For each of these (x,y) vectors, calculate the angle θ = atan2(y,x). θ will be between -π/2 and π/2.

Sort the θ's. This will give you the order of the points, in clockwise order.

This only works for convex quadrilaterals.