# How to draw a vector/line in a mesh

조회 수: 30 (최근 30일)
댓글: Lama Hamadeh 2021년 3월 19일
Hi all,
I have the follwoing mesh:
%Defining space variables
L = 4; %boundary legth
ns = 25; %number of points on S axis
np = 25; %number of points on P axis
s = linspace(0,L,ns); %boundary variable
p = linspace(-1,1,np); % direction/angle variable
[S,P] = meshgrid(s,p); %construct coordinates meshgrid
% mesh needs X,Y and Z so create z
Z = zeros(size(S));
%Visualise the grid
figure;
mesh(S,P,Z,'Marker','o','MarkerFaceColor','k','EdgeColor',"k")
axis equal tight
view(2)
xlabel('\$S\$','Interpreter','latex')
ylabel('\$P\$','Interpreter','latex')
set(gca,'TickLabelInterpreter','latex')
set(gca,'FontSize',16)
And I want to draw a line/vector starting from the bottom of the mesh upwards in a way that it's inclined to the right with a certain angle. Could you please help on how to draw such a line in the mesh and how to calculate its angle with the norm (not with the x axis).
Thanks.
Lama
##### 댓글 수: 2없음 표시없음 숨기기
darova 2021년 3월 17일
Can you make a simple drawing or something? Can you show how you want to draw a vector?
For simplicity, let's suppose we have a unit length square with an initial vector v0 that makes a angle θ0 with the norm as shown in this illustrative picture:
the code for constructing the square is as follwoing:
%the main coordinates of the square
x1 = 0; y1 = 0;
x2 = 1; y2 = 0;
x3 = 1; y3 = 1;
x4 = 0; y4 = 1;
x5 = 0; y5 = 0;
%the x and y variables
x = [x1 x2 x3 x4 x5];
y = [y1 y2 y3 y4 y5];
%plotting the square
plot(x,y,'b','LineWidth',2)
axis([-0.2 1.2 -0.2 1.2])
set(gca,'TickLabelInterpreter','latex')
set(gca,'FontSize',16)
But how can I create the line/vector, v0, based on its position and the angle that it makes with the norm. I suppose that based on its position and the angle, I can know the equation of the line.
Thanks.

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### 채택된 답변

darova 2021년 3월 18일
Thanks for the drawing, i understand
Here is a way:
[x,y] = meshgrid([0 1]); % coordinates for square
t0 = 45;
[u0,v0] = pol2cart(t0,1); % angle and radius
surf(x,y,x*0,'facecolor','none')
quiver(0.5,0,u0,v0) % start position and components of vector
axis([-1 2 -1 2])
##### 댓글 수: 2없음 표시없음 숨기기
A question, though, I tried to code the Polar -> Cartesian conversion as follwoing:
%boundary length
L = 1;
%number of points on S axis
ns = 25;
%boundary variable
s = linspace(0,L,ns);
%construct coordinates meshgrid
[X,Y] = meshgrid(s,s);
% mesh needs X,Y and Z so create z
Z = zeros(size(X));
%Visualise the grid
figure;
mesh(X,Y,Z,'Marker','o','EdgeColor',"k") %or surf
axis equal tight
view(2)
set(gca,'ytick',[])
xlabel('\$S\$','Interpreter','latex')
set(gca,'TickLabelInterpreter','latex')
set(gca,'FontSize',16)
hold on
%---------
%From Polar to Cartesian
theta_deg = 45; %angle from the x-axis in radians
theta_rad = theta_deg*pi/180; %angle in degress
r = 0.7071; %the length of the vector
x = r*cos(theta_rad); %the x projection of the vector
y = r*sin(theta_rad); %the y projection of the vector
quiver(0,0,x,y,'b','LineWidth',2) %show the vector on the mesh
%---------
Although the above code should result a vector that has x=0.5 and y=0.5, but as shown below, they are x=0.45 and y=0.45. That's puzzling!
I just found out that it relates to scaling factor which is by default embedded in quiver MAtlab funciton. When scale is 'off' or 0, such as quiver(X,Y,U,V,'off'), then automatic scaling is disabled and the arrow reaches the specified x and y correctly.

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