2D Plotting a 3D point cloud as set of lines

Question

I have a three dimensional array of points. I need to plot them on a 2D [x,z] grid, where every line is based on the value of Y inside a range. es. the first line is made by points with 0 < y < 1, the second with 1 < y < 2, etc..

I can plot points just fine using this script(to have a better looking graph i'm also changing the color for every set of point by moving inside an rgb triplet). set is the range of the line (in my case Y is time and i need to plot every 0.1 seconds). i is the index for my array of points.

w= 0;
yend = 100;
set = 0.1;
numberOfColors = 1/2*(Yinterval)*(1/set);
incrementOfColor = 1/numberOfColors;
red = 0;
green = 0;
blue = 1;
color=[red green blue];

while w < yend

while y(i)>w&&y(i)<w+set

    figure(1)
    hold on        
    plot(x(i),z(i),'.','Color',color);
    hold on
    i=i+1;
end
    w=w+set;

   if red < 1-incrementOfColor && blue > 0
       red = red + incrementOfColor;
       blue = blue - incrementOfColor;

   end
  if red > incrementOfColor && blue < incrementOfColor
      red = red-incrementOfColor;
      green = green + incrementOfColor;
      blue = 0;
  end
   color = [red green blue];


end

And this is the result: https://i.stack.imgur.com/1R4IN.png When there's too much data the plot becomes cumbersome to read, and with too little data isolated points don't really tell much. I've tried converting the points inside a "set" interval into an array but it really didn't work out:

while w < yend

while y(i)>w&&y(i)<w+set
    vX(a,:)=x(i);
    vZ(a,:)=z(i);
    i=i+1;
 a=a+1;
end    


    figure(2)
    hold on        
    plot(vX,vZ,'-','Color',color);

Gave this result: imgur.com/S7OasUn.jpg

Is my conversion wrong or arrays are just not the proper way to handle this? If so, what should I use?

I'm rather new to matlab so if this is really obvious or i'm asking something in the wrong way, please excuse me.

edit: This is the result i want to achieve:
imgur.com/jPZTO8E.png and it was obtained with Origin's contour profile tool. I guess you mean this with functional example:

myPoints[i]: 
i=0, x = 1, y= 0.03, z = 1
i=1, x = 2, y= 0.06, z = 3
i=2, x = 2.5, y = 0.09, z = 4
i=3, x = 1.2, y = 1.01, z = 3.1
i=4, x = 1.3, y = 1.04, z = 1.1
i=5, x = 1.2, y = 1.06, z = 2.5
i=6, x = 2, y = 1.09, z = 3.1
i=7, x = 1.2, y = 2.02, z = 3.1

etc..

i want the points i 0,1,2 plotted as one line, i 3,4,5 as another line and i 7 etc.. plotted as a third line. In my data i have a lot of points.

this is one example of the data i'm using. https://drive.google.com/file/d/0B9GHAkIYepQcRXdBdy03dFJtT1k/view?usp=sharing where:

x = myPoints(:,1);
y = myPoints(:,2);
z = myPoints(:,3);

In this specific case, i only care about the points with y value between 85.85 and 90.85, and i want to "place an edge" (?) every 0.1

x = myPoints(:,1);
y = myPoints(:,2);
z = myPoints(:,3);
ymin = 85.85;
ymax = 90.85;
set = 0.1;
nbins = (ymax-ymin)/set;
binedge = linspace(ymin, ymax, (nbins + 1));

Answer 1

I think I'm understanding what you're attempting to accomplish and I believe you can accomplish your task with a vectorized use of plot rather than having to loop through each of your points.

Given your example data we can give this method a shot:

load('mypoints.mat');
x = myPoints(:,1);
y = myPoints(:,2);
z = myPoints(:,3);

% Bin your data by y values
ymin = 85.85;
ymax = 90.85;
set = 0.1;
nbins = (ymax-ymin)/set;
binedge = linspace(ymin, ymax, (nbins + 1));
[~, ~, binidx] = histcounts(y, binedge);
binloop = unique(binidx);

% Fix for error with unique. If you try to use a for loop with a column
% vector it passes the whole vector as your iterator, so you need to
% transpose it to a row vector.
temp = size(binloop);
if temp(1) ~= 1
    binloop = binloop';
end

% Plot
figure
hold on
for ii = binloop
    if ii == 0
        % These fall outside of our bins, don't plot them
        continue
    else
        dataidx = find(binidx == ii);
        % Plot has its own color cycle, or you can add your color changing logic to this loop
        plot(x(dataidx), z(dataidx), '-', 'LineWidth', 0.25);
    end
end
hold off

What I've done here is to utilize the third output of histcounts to bin your y data and return a bin index for each value in your y vector.

---

To break the code down a bit:

Define your bin boundaries:

ymin = 85.85;
ymax = 90.85;
set = 0.1;
nbins = (ymax-ymin)/set;
binedge = linspace(ymin, ymax, (nbins + 1));

The linspace call returns the edges of each data bin. Given your example data we get binedge = [86.85, 86.95, 86.05, ..., 90.85]. We then plug in these edges and your y vector to histcounts to obtain our bin indices:

[~, ~, binidx] = histcounts(y, binedge);

Where the bins are defined per the documentation:

The value X(i) is in the kth bin if edges(k) ≤ X(i) < edges(k+1). The last bin also includes the right bin edge, so that it contains X(i) if edges(end-1) ≤ X(i) ≤ edges(end).

We can use unique to loop through all of the bins that are present. We can utilize find with MATLAB's logical indexing to plot all data from a single bin in each loop iteration. I ignore where ii == 0 as it indicates where data does not fall under one of the bins (and 0 index is meaningless in MATLAB).

I also played around a bit with the line sizes to try and make the plot a bit easier to read. There's a lot of data to visualize here but it helps a bit. You'll need to do a bit of pruning of your data since your x values wrap back, but that should be trivial.