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Slice plot of a 3D-data set with color code

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Arne
Arne on 19 Nov 2014
Commented: Arne on 24 Nov 2014
Hello,
I have a rather simple question - tried to reread other posts but it simply doesnt work out so far..
I have several thousands of points in a 3D field - each represented by X,Y and Z value. Each point is described by a fourth number. Coloring should be according to the value of the fourth variable.
I have done a 3D-scatter plot but chances are I miss out interesting things in the middle that are blocked out of my view.
Hence, I would have to do a compilation of slices to visualize the inside. Slice planes should go from the bottom of the 3D-figure to the top.
I know of the slice-function in matlab. But my input arguments produce errors I cannot interprete.
Can someone provide a sample code? Sadly, I could not find one before.
I know that it should be something like -> slice (X,Y,Z,V, PlanevectorX, PlanevectorY, PlanevectorZ)
what would be V in my case?

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Arne
Arne on 20 Nov 2014
I have thought about this question a bit. Maybe it might be the easiest thing to separate the matrix into several smaller matrices according to their Z-value. For example Z ranges from -428 to -152. Then I divide the giant matrix into smaller ones; first one with Z-values from -425 to -400, next one with Z-values from -400 to -375, and so on.. And then actually plot the scatter3 plots of those - I guess I can separate the matrix with histc but I only did this for vectors.. any idea how to histc a matrix according to a column and then de-index them to get the small matrices?
Best

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Answers (2)

Adam
Adam on 20 Nov 2014
Edited: Adam on 21 Nov 2014
I'm not familiar with the slice function, although it does look very useful for the work I do also, but I'm pretty sure that V should be your colour data, of the same size as the X, Y and Z data which should be provided using meshgrid e.g.
[X,Y,Z] = meshgrid(-2:.2:2,-2:.25:2,-2:0.1:2);
The parameterisation of meshgrid should match your sampling to give you X, Y, and Z matrices whose dimensions match your colour values.

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Arne
Arne on 21 Nov 2014
Thank you. But it is not totally clear to me. I have four vectors from my data. X-coordinates, Y-coordinates, Z-coordinates and the property vector (colour vector). The x,y,z you mentioned have nothing to do with my X,Y,Z I guess? It only establishes a grid of a defined dimension in meshgrid??
Yes, I also think V should be the colouring property. Yet, I cant use it in slice as a vector. Matlab tells me V should be a 3d-array. I have no idea how to transform my vector into a reasonable 3d-array.
If your x,y,z has nothing to do with my coordinate-vectors, I also wonder where the function will receive the necessary spatial information from.
Adam
Adam on 21 Nov 2014
The x, y, z in my comment above are now the same as your x, y, z. I changed naming convention from my original answer just to match the way the help names things.
X, Y and Z in my comment answer are the meshgrid outputs that I named x, y, z originally.
(I have now edited my original answer to use consistent variable naming)
Arne
Arne on 24 Nov 2014
Thank you! I understand a little bit more now - I tried to create the meshgrid like you suggested:
[X,Y,Z] = meshgrid(my_x,my_y,my_z);
Unfortunately, I get an error message called: Error using repmat Out of memory. Type HELP MEMORY for your options.
Looks like the vecotors I have are too large for this operation (31147 lines).

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matt dash
matt dash on 21 Nov 2014
Answer to your updated comment: Use the second output (ind) of histc, which is the index of which bin the rows are in. Then use this to plot subsets of the data.
Example: see how data is binned according to the 4th column (which corresponds to color in the scatter plot)
data = rand(10000,4)
[counts,ind]=histc(data(:,4),[0 0.25 0.5 0.75 1]);
for i = 1:max(ind)
figure;
axes;
idx = ind==i;
scatter3(data(idx,1),data(idx,2),data(idx,3),16,data(idx,4),'filled')
set(gca,'clim',[0 1])
end

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