# How to fix number of points of a cone using Matlab

조회 수: 2(최근 30일)
M.S. Khan 2020년 8월 18일
댓글: M.S. Khan 2020년 8월 21일
The code below is great which generates a cone of infinite points. Could anyone help me how to fix the number of points. For example, if i want only 10, 000 points or 20,000 point on the cone. How can i fix on the cone using code below. Thanks for support and help. Regards.
r=r1:-0.5:0;
h=input('enter ht');
% --------------------
xvec = floor(-r1):ceil(r1);
yvec = xvec;
hvec = 0:ceil(h);
[X, Y, H] = ndgrid(xvec, yvec, 0:floor(h));
r_at_H = r1 * (1 - H/h);
is_in_cone = abs(X) <= r1 & abs(Y) <= r1 & H <= h & sqrt(X.^2+Y.^2) <= r_at_H;
Xc = X(is_in_cone);
Yc = Y(is_in_cone);
Hc = H(is_in_cone);
pointsize = 20;
figure(2)
scatter3(Xc, Yc, Hc, pointsize, 'filled')

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### 채택된 답변

Bruno Luong 2020년 8월 19일
편집: Bruno Luong 2020년 8월 20일
Grid pattern
The volume of the cone of base radius R and height h
Vcone = pi*R^2*h/3
Assuming you dsicretize by small volume (dx x dx x dx), you 'll get
N ~ Vcone/dx^3. So if you want an approximative of N with grid pattern:
% Cone parameters
R = 10;
h = 10;
N = 10000;
dx = (pi*R^2*h/3/N)^(1/3);
rvec = linspace(-R,R,ceil(2*R/dx));
hvec = linspace(0,h,ceil(h/dx));
[X,Y,Z] = ndgrid(rvec,rvec,hvec);
is_in_cone = (X.^2+Y.^2) <= (R/h*(h-Z)).^2; % Edit bug fix
Xc = X(is_in_cone);
Yc = Y(is_in_cone);
Zc = Z(is_in_cone);
figure(3)
pointsize = 3,
scatter3(Xc, Yc, Zc, pointsize, 'filled')
axis equal
numel(Xc) % should be close to N
##### 댓글 수: 5표시 이전 댓글 수: 4숨기기 이전 댓글 수: 4
M.S. Khan 2020년 8월 21일
Thanks for late reply. Out of station. You are very helpful as usual.
Regards,

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### 추가 답변(1개)

KSSV 2020년 8월 18일
m = input('enter number of points you want along radius') ;
r = linspace(r1,0,m);
h = input('enter ht');
m = input('enter number of points you want along height') ;
% --------------------
xvec = floor(-r1):ceil(r1);
yvec = xvec;
hvec = 0:ceil(h);
[X, Y, H] = ndgrid(xvec, yvec, linspace(0,floor(h),m));
r_at_H = r1 * (1 - H/h);
is_in_cone = abs(X) <= r1 & abs(Y) <= r1 & H <= h & sqrt(X.^2+Y.^2) <= r_at_H;
Xc = X(is_in_cone);
Yc = Y(is_in_cone);
Hc = H(is_in_cone);
pointsize = 20;
figure(2)
scatter3(Xc, Yc, Hc, pointsize, 'filled')
##### 댓글 수: 4표시 이전 댓글 수: 3숨기기 이전 댓글 수: 3
KSSV 2020년 8월 19일
You have used in the following lines:
xvec = floor(-r1):ceil(r1);
hvec = 0:ceil(h);
[X, Y, H] = ndgrid(xvec, yvec, 0:floor(h));
Replace them with linspace by specifying the number of points you want.

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