Why does the PRINT command take so long with a scatter plot in MATLAB 7.5 (R2007b)?

The PRINT command with the -djpeg flag takes a very long time to run. I have created a scatter plot with a large amount of data. I would like to export it to a jpeg but the print command takes a long time or freezes MATLAB completely.

Reproduction steps:

NumPoints=2000

x=[0:1:NumPoints];

sinx=0.25.*x.*sin(x);

cosx=0.25.*x.*cos(x);

scatter(sinx,cosx,'.');

tic

print -djpeg testspiral.jpg;

toc

Gives:

 Elapsed time is 33.067874 seconds.

Compare that result with this one:

%

phmin=deg2rad(70);

phmax=deg2rad(50);

thmin=0;

thmax=2*pi;

NumPoints=200000

ph = phmin+(phmax-phmin).*rand(NumPoints,1);

th = thmin+(thmax-thmin).*rand(NumPoints,1);

[x y]=pol2cart(th,ph);

scatter(x,y,'.');

tic

print -djpeg myrand_cut.jpg

toc

which gives

 Elapsed time is 13.166636 seconds.

Notice that the second example runs in less time with a data set that is 100 times larger.

For some reason the PRINT command has a particularly hard time with this type of cyclical data.

Answer 1

[MathWorks Support Team]

Before the image is printed to file all of the elements that will appear need to be sorted. Depending on the data set this sort can take a long time.

To work around this issue try randomizing the order of the vectors that are passed to the SCATTER function.

For instance, if the data is contained in three vectors x, y and z instead of using

scatter3(x,y,z);

use

%Get the length of the input vectors.

len=length(x);

%Create a vector of random integers from 1 to len

rand_ind=randperm(len);

%Randomize the input vectors.

randx=x(rand_ind);

randy=y(rand_ind);

randz=z(rand_ind);

%Plot

scatter3(randx,randy,randz);

An alternative workaround can be to print in EPS format which uses vector graphics and hence is much faster. The EPS file can then be converted to JPEG using third party converters.

print -depsc2 -painters testspiral