I would like to draw an arbitrary curve on Matlab with my mouse, say a heart or pikachu. How exactly can I do that? Also, I would like to obtain data points from that curve such if I use scatter, the data points look like they form the curve I drew. How can I do that?
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TL;DR - How do I only analyse the surface data points for surface estimation?
I have a 3D object, and I would like to estimate the surface shape.
The problem is: MATLAB curve fitting toolbox takes into account all the data points of the object. See the example of a cylinder and it's approximated polynomial. MATLAB is taking into account all the data points for surface estimation, what can I do to over come this?
Assuming that the outer and inner surface have identical shape, you can first do an imfill to make the object solid and then use morphological skeleton, bwmorph(BW,'skel',Inf) to turn it into a single line, which you then can approximate the shape from.
I have a 3D scatter plot and I want to visually show COVARIANCE on it. One can show COVARIANCE, for example with an ISO LINE. With this method, one generally gets an ellipse aligned with the shape of the scatter plot. Do you know how I can do this with MATLAB or any other method.
Thanks
I don't understand how would you like to display covariance on a 3D plot. I think what you are looking for is pca , it would give you the three vectors corresponding to maximum variance in your 3D scatter plot. You can then determine the variance along each of those vectors and plot an ellipsoid which represents sort of a confidence region. The final figure would something like this:
There is a little bit of Linear algebra and rotation matrices knowledge involved with this approach.
Suppose I have a dataset which consists of three vectors which represent a trajectory in 3D. This temporal data can be plotted in Matlab with the following command:
plot3(Data(:,1),Data(:,2),Data(:,3),'.r');
The output is a "cloud" of points:
I would like to visualize the trajectory, so my question is: How do I modify the plot so that the color of the points represent the index (time) of the temporal data?
Just to make my point a bit clearer, imagine a trajectory of points that change color "smoothly" from red to blue in a way that will enable me to visualize the trajectory.
I can think of two answers:
use the surface function on a 3D line like this:
color=1:length(Data(:,1));
surface([Data(:,1);Data(:,1)],[Data(:,2);Data(:,2)][Data(:,3);Data(:,3)],[color ;color],...
'facecol','no','edgecol','interp');
this is a very nice trick, but it plots a line.
If you want to plot points, you can define an RGB color and plot single points with hold on like this:
hold on
for i=1:length(Data(:,1))
plot3(Data(i,1),Data(i,2),Data(i,3),'Color',[(i/100*255)/255 0/255 (255-(i/100*255))/255],'LineWidth',2)
end
shg
I have a 3D data set of a surface that is not a function graph. The data is just a bunch of points in 3D, and the only thing I could think of was to try scatter3 in Matlab. Surf will not work since the surface is not a function graph.
Using scatter3 gave a not so ideal result since there is no perspective/shading of any sort.
Any thoughts? It does not have to be Matlab, but that is my go-to source for plotting.
To get an idea of the type of surface I have, consider the four images:
The first is a 3D contour plot, the second is a slice in a plane {z = 1.8} of the contour. My goal is to pick up all the red areas. I have a method to do this for each slice {z = k}. This is the 3rd plot, and I like what I see here a lot.
Iterating this over z give will give a surface, which is the 4th plot, which is a bit noisy (though I have ideas to reduce the noise...). If I plot just the black surface using scatter3 without the contour all I get is a black indistinguishable blob, but for every slice I get a smooth curve, and I have noticed that the curves vary pretty smoothly when I adjust z.
Some fine-tuning will give a much better 4th plot, but still, even if I get the 4th plot to have no noise at all, the result using scatter3 will be a black incomprehensible blob when plotted alone and not on top of the 3D contour. I would like to get a nice picture of the full surface that is not plotted on top of the 3D contour plot
In fact, just to compare and show how bad scatter3 is for surfaces, even if you had exact points on a sphere and used scatter3 the result would be a black blob, and wouldn't even look like a sphere
Can POV-Ray handle this? I've never used it...
If you have a triangulation of your points, you could consider using the trisurf function. I have used that before to generate closed surfaces that have no boundary (such as polyhedra and spheres). The downside is that you have to generate a triangulation of your points. This may not be ideal to your needs but it definitely an option.
EDIT: As #High Performance Mark suggests, you could try using delaunay to generate a triangulation in Matlab
just wanted to follow up on this question. A quick nice way to do this in Matlab is the following:
Consider the function d(x, y, z) defined as the minimum distance from (x, y, z) to your data set. Make sure d(x, y, z) is defined on some grid that contains the data set you're trying to plot.
Then use isosurface to plot a (some) countour(s) of d(x, y, z). For me plotting the contour 0.1 of d(x, y ,z) was enough: Matlab will plot a nice looking surface of all points within a distance 0.1 of the data set with good lighting and all.
In povray, a blob object could be used to display a very dense collection of points, if you make them centers of spheres.
http://www.povray.org/documentation/view/3.6.1/71/
If you want to be able to make slices of "space" and have them colored as per your data, then maybe the object pattern (based on a #declared blob object) might do the trick.
Povray also has a way to work with df3 files, which I've never worked with, but this user appears to have done something similar to your visualization.
http://paulbourke.net/miscellaneous/df3/
I have a formula that depends on theta and phi (spherical coordinates 0<=theta<=2*pi and 0<=phi<=pi). By inserting each engle, I obtained a quantity. Now I have a set of data for different angles and I need to plot the surface. My data is a 180*360 matrix, so I am not sure if I can use SURF or MESH or PLOT3. The figure should be a surface that include all data and the axes should be in terms of the quantity, not the quantity versus the angles. How can I plot such a surface?
I see no reason why you cannot use mesh or surf to plot such data. Another option I tend to use is that of density plots. You basically display the dependent variable (quantity) as an image and include the independent variables (angles) along the axis, much like you would with the aforementioned 3D plotting functions. This can be done with imagesc.
Typically you would want your axes to be the dependent variables. Could you elaborate more on this point?
If I understand you correctly you have calculated a function f(theta,phi) and now you want to plot the surface containing all the points with the polar coordinated (r,theta,phi) where r=f(theta,phi).
If this is what you want to do, the 2D version of such a plot is included in MATLAB under the name polar. Unfortunately, as you pointed out, polar3 on MatlabCentral is not the generalization you are looking for.
I have been able to plot a sphere with the following code, using constant r=1. You can give it a try with your function:
phi1=0:1/(3*pi):pi; %# this would be your 180 points
theta1=-pi:1/(3*pi):pi; % your 360 points
r=ones(numel(theta1),numel(phi1));
[phi,theta]=meshgrid(phi1,theta1);
x=r.*sin(theta).*cos(phi);
y=r.*sin(theta).*sin(phi);
z=r.*cos(theta);
tri=delaunay(x(:),y(:),z(:));
trisurf(tri,x,y,z);
From my tests it seems that delaunay also includes a lot of triangles which go through the volume of my sphere, so it seems this is not optimal. So maybe you can have a look at fill3 and construct the triangles it draws itself: as a first approximation, you could have the points [x(n,m) x(n+1,m) x(n,m+1)] combined into one triangle, and [x(n+1,m) x(n+1,m+1) x(n+1,m+1)] into another...?