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3D point clouds with variable marker transparency, color and size

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Tim
Tim on 23 Dec 2019
Edited: Tim on 14 Feb 2020
I would like to make 3D point clouds in which the markers have variable color, transparency and/or marker size. I would like to do this in one function call, because it seems like Matlab can have some strange occlusion issues if multiple point-clouds are combined via a loop and the viewpoint is changed.
I have tried using the hidden MarkerHandle settings as described here and it works brilliantly with the exception that transparency is only binary for markers of the '.' type (i.e. the only alpha value that affects the graph is 0, in which case the point is un-rendered), unless the marker size is > 15, in which it seems that a different openGL primitive may be used. To get small points with variable transparency one can use e.g. 'o', and set EdgeColorType/Binding/Data and FaceColorType/Binding/Data appropriately.
In both cases, however, if the number of points >25,000 then rotating, zooming or scaling the figure resets the marker handles on-the-fly which makes it impossible to interact with the point cloud while maintaining the custom markerhandle settings.
Variable markersize could function as an alternative to transparency but I don't see how to do that with a single function call, and in any case the marker handle resetting still causes problems for interactive data exploration for large point clouds. Any pointers on how to accomplish this?
  5 Comments
Tim
Tim on 24 Dec 2019
That's not at all to be expected. If I have two random 3D point clouds normally distributed around the origin, and each distribution has it's own color, I shouldn't see one distribution layered on top of the other. The second version, using MarkerHandles, is how the graph should appear:LoopDemo.PNG

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Answers (3)

Yair Altman
Yair Altman on 3 Jan 2020
As an alternative to scatter3, you could keep using plot3 and attach a callback function to the axes that gets invoked whenever the axes is repainted. In this callback function you could reattach the transparency to the ploted points (don't replot - just update the plotted handles' color property/ies). You can attach such a callback in several maners:
  1. The zoom, pan, and rotate3d functions enable setting a callback function that gets invoked whenever they are used. This is fully-documented functionality, but requires you to set the callback seperately in several places.
  2. You can attach the callback to the axes' MarkedClean event, as explained on http://undocumentedmatlab.com/articles/undocumented-hg2-graphics-events
I'm not claiming that this is a better alternative than scatter3, but in cases where you wish to avoid scatter3 (for example, if performance is problematic), then it might be worth checking.
  1 Comment
Tim
Tim on 14 Feb 2020
Thank you for the input, Yair. I didn't realize, when I originally marked this question as answered, that scatter3 doesn't allow the transparency values of points to be independently set on a per-point basis, though it has this functionality with color. As a result I still have to use the MarkerHandle property, (as outlined on your amazing blog) to independently adjust these values for each point. Interestingly, unlike plot3, if scatter3 is used then zoom/pan/rotate can be performed without the transparencies being reset, even if more than 25,000 points are being rendered.
Scatter3 still doesn't solve the problem of correctly rendering multiple point clouds generated from different function calls as demonstrated above (occlusions aren't correct), but it does take care of the difficulty with interacting with larger point clouds for which transparencies of points have been independently defined using MarkerHandle. This seems to me to be a nice under-the-hood increase of functionality in scatter3 vs plot3.

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Adam Danz
Adam Danz on 24 Dec 2019
"I would like to make 3D point clouds in which the markers have variable color, transparency and/or marker size. "
I suggest using scatter3() where marker size and marker color can be set for each individual point. Here's a demo that creates a 3D volume of 30k points with varying colors and marker sizes that are defined by the depth of each point along the z-axis. The transparency of all points are set to 0.5.
% Create data (30k 3D points)
n = 30000;
x = rand(1,n)*100;
y = rand(1,n)*100;
z = rand(1,n)*100;
% Define color of each point based on z value
colors = jet(numel(z));
[~, ~, depthIdx] = unique(abs(z));
colorData = colors(depthIdx,:);
% Define size gradient based on z value
dotSize = linspace(10,100,numel(x));
sizeData = dotSize(depthIdx);
% Plot 3D points
scatter3(x,y,z,sizeData,colorData,'filled','o','MarkerEdgeColor','k', ...
'MarkerFaceAlpha',0.5);
grid on
box on
xlabel('x')
ylabel('y')
zlabel('z')
% Choose your viewing angle:
view(0,180) % (x,z) view
view(2) % (x,y) view
view(3) % (x,y,z) view
191224 060821-Figure 1.png
  2 Comments
Adam Danz
Adam Danz on 24 Dec 2019
You can use '.' markers as shown below but you'll want to increase the range of the marker sizes.
% Define size gradient based on z value
dotSize = linspace(10,1000,numel(x));
sizeData = dotSize(depthIdx);
% Plot 3D points
scatter3(x,y,z,sizeData,colorData,'.','MarkerFaceAlpha',0.5);
And yes, it does take some time to render 1-million points with varying colors and sizes but it also solves your problem.

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Tim
Tim on 24 Dec 2019
Well, it’s probably the official MATLAB function that comes closest... the scatter3 examples posted above pretty clearly don’t render transparency or layer markers correctly, at least for dense clusters of points. I’ll confess what I’m trying to do is find a MATLAB alternative to PPTK, a fairly buggy OpenGL point cloud renderer with a Python interface that makes fast renderings of large point clouds (>20 million) using primitives that handle transparency. It renders beautifully but has many other problems, and doesn’t interface natively with MATLAB. I’ll mark this answered but I think Scatter3’s rendering needs some improvement.
  4 Comments
Tim
Tim on 28 Dec 2019
  • My problem with the rendering is not the axis scaling or point density, it is the problematic rendering of transparency that you can see in, for example, the bottom right figure of the set above (which is absent in all the others). You should not, for example, be seeing any dark blue points near the center of the cloud - they should be occluded by the red layer on top.
  • This problematic rendering seems to occur for different systems above different numbers of points
  • Both renderers (pptk and Matlab) are openGL (Matlab defaults to OpenGL / Hardware supported if it can, and at least in my case it is using openGL with full hardware support)
I thought the problem might be that depthpeeltransparency was flipping off for points above a threshold but for n=1,000,000 rendererinfo() shows that it is indeed still on, even though the rendering is being corrupted, so it is still a mystery to me.

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