MATLAB Surface Plotting - matlab

I want to plot the region (of integration) in 3d-space given by the conditions -1<=x<=1, 0<=y<=1, 0<=z<=1-x^2 with translucent single-coloured faces in essentially 'wireframe'.
My solution basically draws the four bounding surfaces of the region separately. I used surf() for the bottom face that lies on the x-y plane and the top face; however surf() doesn't seem to work with the two vertical faces. Hence I guess that there has to be an easier way to draw this...
Cludgiest code ever:
figure
hold on
x = linspace(-1,1,30);
y = linspace(0,1,30);
[X,Y] = meshgrid(x,y);
Z = 1-X.^2;
h = surf(X,Y,Z);
set(h,'edgecolor','none','FaceColor','r','FaceAlpha',0.5);
Z = zeros(30);
h = surf(X,Y,Z);
set(h,'edgecolor','none','FaceColor','r','FaceAlpha',0.5);
Y = ones(30);
z = 1-x.^2;
y = x.*0;
h = fill3(x,y,z,'r');
alpha(h,0.5);
y = y+1;
h = fill3(x,y,z,'r');
alpha(h,0.5);
y = linspace(0,1,30);
x = y.*0+1;
z = y.*0
plot3(x,y,z,'k')

Related

Adding mesh to enclosed semicircle MATLAB

I am currently trying to produce a 2D heat transfer analysis on an alloy which has a shape of a semicircle, (and then introduce a heat source on this shape). I found myself unable to produce a mesh for that specific shape, the code below shows what I have done up until now, if anyone could possibly point me in the right direction it would be greatly appreciated!
initially I set up the grid parameters as follows:
%-------------------GRID SETTINGS----------------------%
Sx = 10; %physical width
Sy = 10; %physical height
Nx = 100; %number of points x-axis
Ny = round(Nx*Sy/Sx); %number of points y-axis
Nt = 500;
%-------------------GRID RESOLUTION---------------------%
dx = Sx/(Nx);
dy = Sy/(Ny);
and worked on creating the shape of an enclosed semicircle as below:
%---------------------SHAPE CREATION--------------------%
r1 = 0.07; %radius 1, m
r2 = 5; %radius 2, m
phi = 0:0.01:pi;
x = r2*cos(phi); x = [x,x(1)]; %x coordinates
y = r1*sin(phi); y = [y,y(1)]; %y coordinates
[X, Y] = meshgrid(x,y);
I'd use polar coordinates for your mesh and then visualize the results in cartesian coordinates. Check out the example below, hope this gives you a starting point (not really sure why you applied two different radius (r1, r2) to your x,y coordinates - so I assumed you have a minimal and maximal radius).
Nx = 100;
r1 = 1; %radius 1, m
r2 = 5; %radius 2, m
phi = 0:0.01:pi;
% generate mesh grid in polar coordinates
radius = linspace(r1, r2, Nx/2);
[Radius, Phi] = meshgrid(radius, phi);
heat = Radius.^2; % some "fake" heat data
% plot results (in cartesian coordinates)
mesh(Radius.*cos(Phi), Radius.*sin(Phi), heat)
axis equal
% top view
view([-0.2 90.0])

Calculate volume of hollow tilted cylinder inside cube, without a grid

I have a hollow cylinder with inner radius r1 and outer radius r2. This cylinder is tilted from the Z axis towards the X axis, then rotated around the Z axis and then translated along X and along Y. I also have a cube between a range of X and Y values. I then want to calculate how large a volume of the hollow cylinder is inside the cube.
%set variables
r1 = 0.9; r2 = 1.0;
z1 =-1.0; z2 = 1.0;
beta = pi/4; gamma = pi/8;
%create inner surface
[X,Y,Z] = cylinder(r1, 100);
%outer surface
[Xx,Yy,Zz] = cylinder(r2, 100);
%fix coordinates
X = [X,Xx]; Y = [Y,Yy]; Z = [Z,Zz];
Z(1,:) = z1; Z(2,:) = z2;
%elongate in first rotation direction
X = X/cos(beta);
%shift top and bottom accordingly
X(1,:) = X(1,:) + z2/tan(beta);
X(2,:) = X(2,:) + z1/tan(beta);
%also perform another rotation in XY plane
Xnu = X*cos(gamma) + -Y*sin(gamma);
Y = X*sin(gamma) + Y*cos(gamma);
X = Xnu;
%translation not included for this example
surf(X,Y,Z); axis equal
%try to make a cube from its nodes/corners
ix=-1;iy=-1;dx=0.5;dy=dx;
[X,Y,Z] = ndgrid( ix*dx+[-dx/2, dx/2] , iy*dy+[-dy/2, dy/2] , [z1, z2] );
X = reshape(X, 2, []);Y = reshape(Y, 2, []);Z = reshape(Z, 2, []);
%failed
surf(X,Y,Z); axis equal
The hollow cylinder seems OK. Just no 'cap' on the top of the edge.
The cube looks nothing like a cube, but at least the volume between the corners should be correct.
My question is, how do I get the intersection of these two shapes and then calculate the volume?
I actually need to iterate over many of these tubes and cubes, but once I have this single case fixed then it should be easy. Each cube represents a measured pixel from an experiment and each hollow cylinder a physical object that was in the experiment. When simulating this on a meshgrid itself I encounter memory issues very fast.

Matlab - Subtracting two 3D figures with surf and min function

I'm trying to make a surf plot that looks like:
So far I have:
x = [-1:1/100:1];
y = [-1:1/100:1];
[X,Y] = meshgrid(x,y);
Triangle1 = -abs(X) + 1.5;
Triangle2 = -abs(Y) + 1.5;
Z = min(Triangle1, Triangle2);
surf(X,Y,Z);
shading flat
colormap winter;
hold on;
[X,Y,Z] = sphere();
Sphere = surf(X, Y, Z + 1.5 );% sphere with radius 1 centred at (0,0,1.5)
hold off;
This code produces a graph that looks like :
A pyramid with square base ([-1,1]x[-1,1]) and vertex at height c = 1.5 above the origin (0,0) is erected.
The top of the pyramid is hollowed out by removing the portion of it that falls within a sphere of radius r=1 centered at the vertex.
So I need to keep the part of the surface of the sphere that is inside the pyramid and delete the rest. Note that the y axis in each plot is different, that's why the second plot looks condensed a bit. Yes there is a pyramid going into the sphere which is hard to see from that angle.
I will use viewing angles of 70 (azimuth) and 35 (elevation). And make sure the axes are properly scaled (as shown). I will use the AXIS TIGHT option to get the proper dimensions after the removal of the appropriate surface of the sphere.
Here is my humble suggestion:
N = 400; % resolution
x = linspace(-1,1,N);
y = linspace(-1,1,N);
[X,Y] = meshgrid(x,y);
Triangle1 = -abs(X)+1.5 ;
Triangle2 = -abs(Y)+1.5 ;
Z = min(Triangle1, Triangle2);
Trig = alphaShape(X(:),Y(:),Z(:),2);
[Xs,Ys,Zs] = sphere(N-1);
Sphere = alphaShape(Xs(:),Ys(:),Zs(:)+2,2);
% get all the points from the pyramid that are within the sphere:
inSphere = inShape(Sphere,X(:),Y(:),Z(:));
Zt = Z;
Zt(inSphere) = nan; % remove the points in the sphere
surf(X,Y,Zt)
shading interp
view(70,35)
axis tight
I use alphaShape object to remove all unwanted points from the pyramid and then plot it without them:
I know, it's not perfect, as you don't see the bottom of the circle within the pyramid, but all my tries to achieve this have failed. My basic idea was plotting them together like this:
hold on;
Zc = Zs;
inTrig = inShape(Trig,Xs(:),Ys(:),Zs(:)+1.5);
Zc(~inTrig) = nan;
surf(Xs,Ys,Zc+1.5)
hold off
But the result is not so good, as you can't really see the circle within the pyramid.
Anyway, I post this here as it might give you a direction to work on.
An alternative to EBH's method.
A general algorithm from subtracting two shapes in 3d is difficult in MATLAB. If instead you remember that the equation for a sphere with radius r centered at (x0,y0,z0) is
r^2 = (x-x0)^2 + (y-y0)^2 + (z-z0)^2
Then solving for z gives z = z0 +/- sqrt(r^2-(x-x0)^2-(y-y0)^2) where using + in front of the square root gives the top of the sphere and - gives the bottom. In this case we are only interested in the bottom of the sphere. To get the final surface we simply take the minimum z between the pyramid and the half-sphere.
Note that the domain of the half-sphere is defined by the filled circle r^2-(x-x0)^2-(y-y0)^2 >= 0. We define any terms outside the domain as infinity so that they are ignored when the minimum is taken.
N = 400; % resolution
z0 = 1.5; % sphere z offset
r = 1; % sphere radius
x = linspace(-1,1,N);
y = linspace(-1,1,N);
[X,Y] = meshgrid(x,y);
% pyramid
Triangle1 = -abs(X)+1.5 ;
Triangle2 = -abs(Y)+1.5 ;
Pyramid = min(Triangle1, Triangle2);
% half-sphere (hemisphere)
sqrt_term = r^2 - X.^2 - Y.^2;
HalfSphere = -sqrt(sqrt_term) + z0;
HalfSphere(sqrt_term < 0) = inf;
Z = min(HalfSphere, Pyramid);
surf(X,Y,Z)
shading interp
view(70,35)
axis tight

How to plot with pcolor one 2D function with two x axis ? MATLAB

I would like to plot one function with two x axis. One on the bottom one on the top. The top is just different vector. How to add the second x axis on the top ?
X = rand(100,100);
x = linspace(1,100,100); %bottom x axis
x2 = linspace(0.1,1,100); %top x axis
y = linspace(100,200,100);
pcolor(x,y,X);
shading interp
This might help,
X = rand(100,100);
x = linspace(1,100,100);
x2 = linspace(0.1,1,100);
y = linspace(100,200,100);
h2 = axes('XAxisLocation','top','XTick',linspace(0.1,1,10),'YTick',[]);
h2_pos = get(h2,'Position');
h1 = axes('XTick',linspace(1,100,10),'YTick',[],...
'Position',h2_pos);
pcolor(x,y,X,'Parent',h1);
shading interp
which gives this,

Interpolate surface of 3D cylinder in Matlab

I have a dataset that describes a point cloud of a 3D cylinder (xx,yy,zz,C):
and I would like to make a surface plot from this dataset, similar to this
In order to do this I thought I could interpolate my scattered data using TriScatteredInterp onto a regular grid and then plot it using surf:
F = TriScatteredInterp(xx,yy,zz);
max_x = max(xx); min_x = min(xx);
max_y = max(yy); min_y = min(yy);
max_z = max(zz); min_z = min(zz);
xi = min_x:abs(stepSize):max_x;
yi = min_y:abs(stepSize):max_y;
zi = min_z:abs(stepSize):max_z;
[qx,qy] = meshgrid(xi,yi);
qz = F(qx,qy);
F = TriScatteredInterp(xx,yy,C);
qc = F(qx,qy);
figure
surf(qx,qy,qz,qc);
axis image
This works really well for convex and concave objects but ends in this for the cylinder:
Can anybody help me as to how to achieve a nicer plot?
Have you tried Delaunay triangulation?
http://www.mathworks.com/help/matlab/ref/delaunay.html
load seamount
tri = delaunay(x,y);
trisurf(tri,x,y,z);
There is also TriScatteredInterp
http://www.mathworks.com/help/matlab/ref/triscatteredinterp.html
ti = -2:.25:2;
[qx,qy] = meshgrid(ti,ti);
qz = F(qx,qy);
mesh(qx,qy,qz);
hold on;
plot3(x,y,z,'o');
I think what you are loking for is the Convex hull function. See its documentation.
K = convhull(X,Y,Z) returns the 3-D convex hull of the points (X,Y,Z),
where X, Y, and Z are column vectors. K is a triangulation
representing the boundary of the convex hull. K is of size mtri-by-3,
where mtri is the number of triangular facets. That is, each row of K
is a triangle defined in terms of the point indices.
Example in 2D
xx = -1:.05:1; yy = abs(sqrt(xx));
[x,y] = pol2cart(xx,yy);
k = convhull(x,y);
plot(x(k),y(k),'r-',x,y,'b+')
Use plot to plot the output of convhull in 2-D. Use trisurf or trimesh to plot the output of convhull in 3-D.
A cylinder is the collection of all points equidistant to a line. So you know that your xx, yy and zz data have one thing in common, and that is that they all should lie at an equal distance to the line of symmetry. You can use that to generate a new cylinder (line of symmetry taken to be z-axis in this example):
% best-fitting radius
% NOTE: only works if z-axis is cylinder's line of symmetry
R = mean( sqrt(xx.^2+yy.^2) );
% generate some cylinder
[x y z] = cylinder(ones(numel(xx),1));
% adjust z-range and set best-fitting radius
z = z * (max(zz(:))-min(zz(:))) + min(zz(:));
x=x*R;
y=y*R;
% plot cylinder
surf(x,y,z)
TriScatteredInterp is good for fitting 2D surfaces of the form z = f(x,y), where f is a single-valued function. It won't work to fit a point cloud like you have.
Since you're dealing with a cylinder, which is, in essence, a 2D surface, you can still use TriScatterdInterp if you convert to polar coordinates, and, say, fit radius as a function of angle and height--something like:
% convert to polar coordinates:
theta = atan2(yy,xx);
h = zz;
r = sqrt(xx.^2+yy.^2);
% fit radius as a function of theta and h
RFit = TriScatteredInterp(theta(:),h(:),r(:));
% define interpolation points
stepSize = 0.1;
ti = min(theta):abs(stepSize):max(theta);
hi = min(h):abs(stepSize):max(h);
[qx,qy] = meshgrid(ti,hi);
% find r values at points:
rfit = reshape(RFit(qx(:),qy(:)),size(qx));
% plot
surf(rfit.*cos(qx),rfit.*sin(qx),qy)