Signed area calculation is incorrect ... about mapbox-vector-tile-cs HOT 3 CLOSED

Hi, I've added a testcase for the signed area 100197f

The signed area function assumes the polygon is closed (first point is the same as last point), maybe I should add a check on that.

In your simple triangle case it results in area == -1 (when polygon closed), its negative because polygon is defined clockwise. So I think the area calculation is correct?

I'll take an extra check on the CW/CCW issue you mention later this week.

from mapbox-vector-tile-cs.

Hi. In the definition seen in the 4.3.4.4 above, it is a positive area that is clockwise, not a negative area. This is because the Y axis is inverted. Your code claims a negative area is clockwise, but it is the opposite that is true.

    public bool IsCW()
    {
        return SignedArea() < 0;
    }

Here is your calculation for the signed area:
for (var i = 0; i < points.Count-1; i++)
{
sum = sum + (points[i].X * points[i + 1].Y - (points[i].Y * points[i + 1].X));
}

It is missing out the term for point[Count-1].X. This X value has to loop back to the point[0].Y. That is why it's called the shoelace formula, the X and Y values cross over, and the last X and Y values go back to the beginning.

Your code misses out the XnY1 and -X1Yn terms, where n is point.Count - 1. This corrects the sign of the area and also the area itself.

from mapbox-vector-tile-cs.

can you create a pr for the code changes involved?

from mapbox-vector-tile-cs.