Sort points in clockwise order

algorithmcomputational-geometrygeometryluamath

Given an array of x,y points, how do I sort the points of this array in clockwise order (around their overall average center point)? My goal is to pass the points to a line-creation function to end up with something looking rather "solid", as convex as possible with no lines intersecting.

For what it's worth, I'm using Lua, but any pseudocode would be appreciated.

Update: For reference, this is the Lua code based on Ciamej's excellent answer (ignore my "app" prefix):

function appSortPointsClockwise(points)
    local centerPoint = appGetCenterPointOfPoints(points)
    app.pointsCenterPoint = centerPoint
    table.sort(points, appGetIsLess)
    return points
end

function appGetIsLess(a, b)
    local center = app.pointsCenterPoint

    if a.x >= 0 and b.x < 0 then return true
    elseif a.x == 0 and b.x == 0 then return a.y > b.y
    end

    local det = (a.x - center.x) * (b.y - center.y) - (b.x - center.x) * (a.y - center.y)
    if det < 0 then return true
    elseif det > 0 then return false
    end

    local d1 = (a.x - center.x) * (a.x - center.x) + (a.y - center.y) * (a.y - center.y)
    local d2 = (b.x - center.x) * (b.x - center.x) + (b.y - center.y) * (b.y - center.y)
    return d1 > d2
end

function appGetCenterPointOfPoints(points)
    local pointsSum = {x = 0, y = 0}
    for i = 1, #points do pointsSum.x = pointsSum.x + points[i].x; pointsSum.y = pointsSum.y + points[i].y end
    return {x = pointsSum.x / #points, y = pointsSum.y / #points}
end

Best Answer

First, compute the center point. Then sort the points using whatever sorting algorithm you like, but use special comparison routine to determine whether one point is less than the other.

You can check whether one point (a) is to the left or to the right of the other (b) in relation to the center by this simple calculation:

det = (a.x - center.x) * (b.y - center.y) - (b.x - center.x) * (a.y - center.y)

if the result is zero, then they are on the same line from the center, if it's positive or negative, then it is on one side or the other, so one point will precede the other. Using it you can construct a less-than relation to compare points and determine the order in which they should appear in the sorted array. But you have to define where is the beginning of that order, I mean what angle will be the starting one (e.g. the positive half of x-axis).

The code for the comparison function can look like this:

bool less(point a, point b)
{
    if (a.x - center.x >= 0 && b.x - center.x < 0)
        return true;
    if (a.x - center.x < 0 && b.x - center.x >= 0)
        return false;
    if (a.x - center.x == 0 && b.x - center.x == 0) {
        if (a.y - center.y >= 0 || b.y - center.y >= 0)
            return a.y > b.y;
        return b.y > a.y;
    }

    // compute the cross product of vectors (center -> a) x (center -> b)
    int det = (a.x - center.x) * (b.y - center.y) - (b.x - center.x) * (a.y - center.y);
    if (det < 0)
        return true;
    if (det > 0)
        return false;

    // points a and b are on the same line from the center
    // check which point is closer to the center
    int d1 = (a.x - center.x) * (a.x - center.x) + (a.y - center.y) * (a.y - center.y);
    int d2 = (b.x - center.x) * (b.x - center.x) + (b.y - center.y) * (b.y - center.y);
    return d1 > d2;
}

This will order the points clockwise starting from the 12 o'clock. Points on the same "hour" will be ordered starting from the ones that are further from the center.

If using integer types (which are not really present in Lua) you'd have to assure that det, d1 and d2 variables are of a type that will be able to hold the result of performed calculations.

If you want to achieve something looking solid, as convex as possible, then I guess you're looking for a Convex Hull. You can compute it using the Graham Scan. In this algorithm, you also have to sort the points clockwise (or counter-clockwise) starting from a special pivot point. Then you repeat simple loop steps each time checking if you turn left or right adding new points to the convex hull, this check is based on a cross product just like in the above comparison function.

Edit:

Added one more if statement if (a.y - center.y >= 0 || b.y - center.y >=0) to make sure that points that have x=0 and negative y are sorted starting from the ones that are further from the center. If you don't care about the order of points on the same 'hour' you can omit this if statement and always return a.y > b.y.

Corrected the first if statements with adding -center.x and -center.y.

Added the second if statement (a.x - center.x < 0 && b.x - center.x >= 0). It was an obvious oversight that it was missing. The if statements could be reorganized now because some checks are redundant. For example, if the first condition in the first if statement is false, then the first condition of the second if must be true. I decided, however, to leave the code as it is for the sake of simplicity. It's quite possible that the compiler will optimize the code and produce the same result anyway.

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