Sort points in clockwise order?

Question

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.

What you're asking for is a system known as polar coordinates. Conversion from Cartesian to polar coordinates is easily done in any language. The formulas can be found in this section.

After converting to polar coordinates, just sort by the angle, theta.

An interesting alternative approach to your problem would be to find the approximate minimum to the Traveling Salesman Problem (TSP), ie. the shortest route linking all your points. If your points form a convex shape, it should be the right solution, otherwise, it should still look good (a "solid" shape can be defined as one that has a low perimeter/area ratio, which is what we are optimizing here).

You can use any implementation of an optimizer for the TSP, of which I am pretty sure you can find a ton in your language of choice.

Another version (return true if a comes before b in counterclockwise direction):

    bool lessCcw(const Vector2D &center, const Vector2D &a, const Vector2D &b) const
    {
        // Computes the quadrant for a and b (0-3):
        //     ^
        //   1 | 0
        //  ---+-->
        //   2 | 3

        const int dax = ((a.x() - center.x()) > 0) ? 1 : 0;
        const int day = ((a.y() - center.y()) > 0) ? 1 : 0;
        const int qa = (1 - dax) + (1 - day) + ((dax & (1 - day)) << 1);

        /* The previous computes the following:

           const int qa =
           (  (a.x() > center.x())
            ? ((a.y() > center.y())
                ? 0 : 3)
            : ((a.y() > center.y())
                ? 1 : 2)); */

        const int dbx = ((b.x() - center.x()) > 0) ? 1 : 0;
        const int dby = ((b.y() - center.y()) > 0) ? 1 : 0;
        const int qb = (1 - dbx) + (1 - dby) + ((dbx & (1 - dby)) << 1);

        if (qa == qb) {
            return (b.x() - center.x()) * (a.y() - center.y()) < (b.y() - center.y()) * (a.x() - center.x());
        } else {
            return qa < qb;
       } 
    }

This is faster, because the compiler (tested on Visual C++ 2015) doesn't generate jump to compute dax, day, dbx, dby. Here the output assembly from the compiler:

; 28   :    const int dax = ((a.x() - center.x()) > 0) ? 1 : 0;

    vmovss  xmm2, DWORD PTR [ecx]
    vmovss  xmm0, DWORD PTR [edx]

; 29   :    const int day = ((a.y() - center.y()) > 0) ? 1 : 0;

    vmovss  xmm1, DWORD PTR [ecx+4]
    vsubss  xmm4, xmm0, xmm2
    vmovss  xmm0, DWORD PTR [edx+4]
    push    ebx
    xor ebx, ebx
    vxorps  xmm3, xmm3, xmm3
    vcomiss xmm4, xmm3
    vsubss  xmm5, xmm0, xmm1
    seta    bl
    xor ecx, ecx
    vcomiss xmm5, xmm3
    push    esi
    seta    cl

; 30   :    const int qa = (1 - dax) + (1 - day) + ((dax & (1 - day)) << 1);

    mov esi, 2
    push    edi
    mov edi, esi

; 31   : 
; 32   :    /* The previous computes the following:
; 33   : 
; 34   :    const int qa =
; 35   :        (   (a.x() > center.x())
; 36   :         ? ((a.y() > center.y()) ? 0 : 3)
; 37   :         : ((a.y() > center.y()) ? 1 : 2));
; 38   :    */
; 39   : 
; 40   :    const int dbx = ((b.x() - center.x()) > 0) ? 1 : 0;

    xor edx, edx
    lea eax, DWORD PTR [ecx+ecx]
    sub edi, eax
    lea eax, DWORD PTR [ebx+ebx]
    and edi, eax
    mov eax, DWORD PTR _b$[esp+8]
    sub edi, ecx
    sub edi, ebx
    add edi, esi
    vmovss  xmm0, DWORD PTR [eax]
    vsubss  xmm2, xmm0, xmm2

; 41   :    const int dby = ((b.y() - center.y()) > 0) ? 1 : 0;

    vmovss  xmm0, DWORD PTR [eax+4]
    vcomiss xmm2, xmm3
    vsubss  xmm0, xmm0, xmm1
    seta    dl
    xor ecx, ecx
    vcomiss xmm0, xmm3
    seta    cl

; 42   :    const int qb = (1 - dbx) + (1 - dby) + ((dbx & (1 - dby)) << 1);

    lea eax, DWORD PTR [ecx+ecx]
    sub esi, eax
    lea eax, DWORD PTR [edx+edx]
    and esi, eax
    sub esi, ecx
    sub esi, edx
    add esi, 2

; 43   : 
; 44   :    if (qa == qb) {

    cmp edi, esi
    jne SHORT $LN37@lessCcw

; 45   :        return (b.x() - center.x()) * (a.y() - center.y()) < (b.y() - center.y()) * (a.x() - center.x());

    vmulss  xmm1, xmm2, xmm5
    vmulss  xmm0, xmm0, xmm4
    xor eax, eax
    pop edi
    vcomiss xmm0, xmm1
    pop esi
    seta    al
    pop ebx

; 46   :    } else {
; 47   :        return qa < qb;
; 48   :    }
; 49   : }

    ret 0
$LN37@lessCcw:
    pop edi
    pop esi
    setl    al
    pop ebx
    ret 0
?lessCcw@@YA_NABVVector2D@@00@Z ENDP            ; lessCcw

Enjoy.