MATLAB: Question about speeding up line plotting

drawhold onplot3

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.

Best Answer

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.
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