通過三個不共線的平面點繪製圓形/MATLAB實現
1: 三個不共線的點求解圓心和半徑
假設三個不共線的點的座標分別爲P1(x1,y1)、P2(x2,y2)和P3(x3,y3),設過三個不共線點的圓的圓心和半徑分別爲(x,y)和R,根據圓的公式可以得到:⎩⎪⎨⎪⎧(x−x1)2+(y−y1)2=R2(x−x2)2+(y−y2)2=R2(x−x3)2+(y−y3)2=R2將上述公式展開移項後得到:⎩⎪⎨⎪⎧x2+y2+x12+y12=2x1x+2y1y+R2x2+y2+x22+y22=2x2x+2y2y+R2x2+y2+x32+y32=2x3x+2y3y+R2通過對上述公式合併整理可以得到:
⎩⎪⎨⎪⎧(x2−x1)x+(y2−y1)y=0.5(x22+y22−x12−y12)(x3−x1)x+(y3−y1)y=0.5(x32+y32−x12−y12)(x3−x2)x+(y3−y2)y=0.5(x32+y32−x22−y22)將上述方程構成矩陣形式,既有:Aθ=B其中A=⎣⎡x2−x1x3−x1x3−x2y2−y1y3−y1y3−y2⎦⎤,B=0.5⎣⎡x22+y22−x12−y12x32+y32−x12−y12x32+y32−x22−y22⎦⎤
由於上述方程爲線性方程,也不存在誤差,利用最小二乘求得圓心的座標爲:θ=(ATA)−1ATB
那麼圓心的座標有:x=θ(1),y=θ(2)
利用得到的圓心可以直接求得圓的半徑爲:R=(x−x1)2+(y−y1)2
2: 得到圓的邊的座標
當得到過三點圓的圓心和半徑後,可以利用圓的參數方程得到圓的邊的座標,既有:{x=x0+Rcosαy=y0+Rsinα,其中α∈[0,2π]其中,x爲圓邊上某一點的橫座標,y爲圓邊上某一點的縱座標,α爲圓邊上某一點對應的角度,x0和y0爲該圓的圓心,R爲該圓的半徑。
3: 代碼實現-MATLAB
function Result = ThreePoint2Circle(P1, P2, P3)
%% 求圓心和半徑
x1 = P1(1); x2 = P2(1); x3 = P3(1);
y1 = P1(2); y2 = P2(2); y3 = P3(2);
z1 = x2^2 + y2^2 - x1^2 - y1^2;
z2 = x3^2 + y3^2 - x1^2 - y1^2;
z3 = x3^2 + y3^2 - x2^2 - y2^2;
A = [(x2-x1), (y2-y1); (x3-x1), (y3-y1); (x3-x2), (y3-y2)];
B = 0.5*[z1; z2; z3];
P0 = (A'*A)\A'*B;
R1 = sqrt( (P0(1) - P1(1))^2 + (P0(2) - P1(2))^2 );
R2 = sqrt( (P0(1) - P2(1))^2 + (P0(2) - P2(2))^2 );
R3 = sqrt( (P0(1) - P3(1))^2 + (P0(2) - P3(2))^2 );
R = (R1 + R2 + R3)/3;
%% 繪製圓
theta = (0:pi/360:2*pi)';
Result = zeros(size(theta,1),4);
for i = 1: size(theta,1)
Result(i,1) = i;
Result(i,2) = theta(i);
Result(i,3) = P0(1) + R*cos(theta(i));
Result(i,4) = P0(2) + R*sin(theta(i));
end
figure();plot(Result(:,3),Result(:,4),'b-');hold on;
grid on; xlabel('x');ylabel('y'); axis equal;
end
輸出的結果爲0.5°間隔,其中第1列爲點號,第2列爲角度,第3列爲橫座標,第4列爲縱座標。
4: 代碼測試
(1)過(1,1)、(3,2)、(5,1)畫圓
(2)過(1.2,1.2)、(8,0.5)、(13.8,1.2)畫圓
結束!