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fixed circle center finding

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This commit is contained in:
Mikhail Allen 2021-06-01 15:10:29 -04:00
commit ab9fb3c2f1
1 changed files with 22 additions and 18 deletions
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40
src/geo/base.js
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@ -316,29 +316,31 @@
return (d2y * d1x) - (d2x * d1y);
}
/**
* find circle center given 3 points in XY plane
*/
function circleCenter(A,B,C)
{
let yDelta_a = B.y - A.y;
let xDelta_a = B.x - A.x;
let yDelta_b = C.y - B.y;
let xDelta_b = C.x - B.x;
let center = {z: A.z};
let cheat = 0;
if (!xDelta_a) { xDelta_a = 0.00000000001; cheat++ } // cheat!
if (!xDelta_b) { xDelta_b = 0.00000000001; cheat++ } // cheat!
if (!yDelta_a) { yDelta_a = 0.00000000001; cheat++ } // cheat!
if (!yDelta_b) { yDelta_b = 0.00000000001; cheat++ } // cheat!
let aSlope = yDelta_a / xDelta_a;
let bSlope = yDelta_b / xDelta_b;
center.x = (aSlope * bSlope * (A.y - C.y) + bSlope * (A.x + B.x) - aSlope*(B.x+C.x)) / (2 * (bSlope-aSlope));
center.y = (-1 * (center.x - (A.x + B.x) / 2)) / aSlope + ((A.y+B.y) / 2);
if (cheat > 1 || [center.x,center.y].hasNaN()) {
// console.log({xDelta_a, xDelta_b, aSlope, bSlope, cheat});
return undefined;
}
let xmat = [[A.x*A.x + A.y*A.y, A.y, 1],[B.x*B.x + B.y*B.y, B.y, 1],[C.x*C.x + C.y*C.y, C.y, 1]];
let ymat = [[A.y, A.x*A.x + A.y*A.y, 1],[B.y, B.x*B.x + B.y*B.y, 1],[C.y, C.x*C.x + C.y*C.y, 1]];
let center = {x:determinant33(xmat), y:determinant33(ymat), z:A.z};
return center;
}
/**
* find the determinant of a 3x3 matrix array organized [row, col]
*/
function determinant33(mat33)
{
let cofactor00 = mat33[0][0] * (mat33[1][1] * mat33[2][2] - mat33[1][2] * mat33[2][1]);
let cofactor01 = -mat33[0][1] * (mat33[1][0] * mat33[2][2] - mat33[1][2] * mat33[2][0]);
let cofactor02 = mat33[0][2] * (mat33[1][0] * mat33[2][1] - mat33[1][1] * mat33[2][0]);
return cofactor00 + cofactor01 + cofactor02;
}
/**
* return circle center given three points
* from https://stackoverflow.com/questions/4103405/what-is-the-algorithm-for-finding-the-center-of-a-circle-from-three-points
@ -353,6 +355,8 @@
return center;
}
/**
* return two possible circle centers given two points and a radius
* https://stackoverflow.com/questions/36211171/finding-center-of-a-circle-given-two-points-and-radius