Question about speeding up line plotting

Hello, thanks for reading this,

What I do currently in some MATLAB code is plot3 a series of lines, defined by a beginning and end point. I have 3d coordinate info stored in a nx3 pt matrix, and the connectivity information stored in a mx2 matrix. The data can look something like:

    ptMx = [-0.0004    0.0003    1.9039
            -0.0004    0.0003    1.1424
            -0.1502    0.0001    0.6856
             0.1397    0.0004    0.6853];

    faceMx = [1     2
              2     3
              2     4]

So point 1 is connected to point 2, 2 to 3, and 2 to 4. I plot these with the following code:

    for i=1:size(faceMx,1)
        hold on
        plot3([ptMx(faceMx(i,1),1) ptMx(faceMx(i,2),1)], ...
              [ptMx(faceMx(i,1),2) ptMx(faceMx(i,2),2)], ...
              [ptMx(faceMx(i,1),3) ptMx(faceMx(i,2),3)])
    end

So as you can see, I go through every element and hold on a plot3 line. My problem is this can take a very large amount of time, and it makes rotating/translating the plot very slow. Is there a better/faster way to draw a series of lines this way?

Thanks!

EDIT: I know I can switch plot3 to line to speed this up a little bit, I was hoping if there was a better way available. Sometimes, I can be plotting up to 50k lines, and during these times patches of surface meshes with triangle patches with a similar number is many times faster.

Answer 1

[Mike Garrity]

I did a post on the MATLAB Graphics blog a while back about this.

Basically you want to batch your graphics up into bigger chunks. The graphics card is really best when it's getting hundreds or thousands of lines per object. You really don't need to change the type of graphics object in this case though. Just use nans to combine disjoint strips together.

Your example would look something like this:

nlines = size(faceMx,1);
x = zeros(3,nlines);
y = zeros(3,nlines);
z = zeros(3,nlines);
for i=1:nlines
  x(:,i) = [ptMx(faceMx(i,1),1); ptMx(faceMx(i,2),1); nan];
  y(:,i) = [ptMx(faceMx(i,1),2); ptMx(faceMx(i,2),2); nan];
  z(:,i) = [ptMx(faceMx(i,1),3); ptMx(faceMx(i,2),3); nan];
end
plot3(x(:),y(:),z(:))

I'm assuming here that nlines is actually much larger than 3. As you can see from the performance charts in that blog post, you're not really going to see benefits from this approach until the number of line segments is quite large.

Also, be sure to check out the comments at the end of that blog post. Several readers made very good suggestions on how to improve your graphics performance.

[Brian]

Thank you for the answer! I will check this when I step in to lab today. Could I replace plot3 with line? Plot3 is a bit chunkier of a command, and in my experience line is usually a bit faster as a primitive.

[Mike Garrity]

Sure, but I don't expect that it'll make an awfully big difference here.

[Brian]

Gotcha, thanks, I was able to use this and it speeded up the visualization considerably! My only last question comes to attributing linewidth in each segment.

If I do the visualization that way, would it be possible to have a linewidth for each segment? My understanding is it has to be a scalar, which would not be applicable here.

[Mike Garrity]

That's the rub. If you combine the segments into a single object, you gain a lot in performance, but you lose control over the properties of the individual segments.

[Brian]

Fair enough, one last question for you if you're willing:

If I make these into cylinder objects, and visualized them with a patch command, would that be simpler/faster than plotting each one with a line command?

I've visualized surface meshes via patches with considerable complexity and size that are much more responsive than this.

[Mike Garrity]

I'm not sure that anyone thinks that patch is simpler than line. It's interface is a bit complicated. On the other hand, all of those features do give you more control. For example, if I combine multiple segments into one line object, then they're all going to get the same color. But if I do the same thing with a patch object, then I can control the colors of the individual segments. Unfortunately, linewidth per segment is not one of the features that patch gives you. As you said, you could use patch to turn each segment into a tube, and that would give you that control. But that's going to be a bit complicated, and I'm note sure I would want to guess whether the performance would really be better once you got the tubes looking just the way you liked.

There's one trick we do in some of the charting objects which you might be able to use here. If you're only using a couple of different line widths, you could create one line object for each, and then keep adding the next segment to the right object. There's a bit of bookkeeping involved, but if it could get you down from thousands of objects to half a dozen, it might be worth it.

Answer 2

[Walter Roberson]

You should be using patch() with ptMx as all your vertices, and your first row of your Faces list would be [1 2 3 4] . That would produce a closed face; if you want an open face then add a vertex at nan as the list

    ptMx = [nan        nan       nan
            -0.0004    0.0003    1.9039
            -0.0004    0.0003    1.1424
            -0.1502    0.0001    0.6856
             0.1397    0.0004    0.6853];

faces [2 3 4 5 1; .... ]

[Brian]

I'm a little bit confused how to make this work programmatically. Can you give me an example I can follow? I will test it when I get in to lab.

I've gotten patch to work in the past by defining points and faces, this was with 3-point faces to make triangles. I got that to work in the past via:

    a = patch('XData', ..., 'YData', ..., 'ZData', ..., 'Faces', ...);

I haven't tried it, but would it work this way for lines with 2 point faces?