Computationally Fast Way to Save Matlab Figures as Image (eg PNG) - matlab

Question

I have a program that creates a lot of plots which I want to save as fig001, fig002, .... At the moment, the only way I can do this is to either save them one by one as I go along using print() or so save them into a graphics object FIG.
Unfortunately, I have to create the plots sequentially (can't parallelise it). If I save as I go, then this takes quite a long time; if I store in FIG then do a parfor-loop over all the figures, then this is faster. However, it's still pretty slow, and it's a major bottleneck in my code.
Matlab is automatically writing a 1200x900 pixel image. In reality, it's just a graph with some (horizontal) straight lines on it. I really do not need a high image quality whatsoever; maybe reducing this would help speed things up? I can't find how to do this either.
I've had a look online, in particular at other SE questions, but I haven't been able to come up with a solution. There's various stuff about "playing around with the hardcore function". I'm a mathematician who wants a code to get some intuition for the problem; I'm not a proper programmer! As such, "play around with a function" (that might cause Matlab to crash) is rather difficult for me!
I've sorted this by using the Matlab movie function. Thank you Cecilia for your assistance!

Answer 1

Since your intention is to use the images in a slideshow. I would suggest making a movie and using that instead. I've run a few tests, and saving a movie is much faster than saving individual pngs. I see about a 5x speed up in my simple test.
numImages = 25;
%Saving one figure at a time
tic;
for i= 1:numImages
x = 0:0.1:2*pi;
y = i*sin(x);
plot(x, y);
fig = gcf;
print('img.png', '-dpng', '-r50'); %Downsample resolution
end
toc;
tic;
%Saving a movie
v = VideoWriter('mov.avi');
open(v);
for i = 1:numImages
x = 0:0.1:2*pi;
y = i*sin(x);
plot(x, y);
drawnow; %Force the figure to render
frame = getframe; %Convert the figure to a movie frame
writeVideo(v, frame); %Write the frame to the movie file
end
close(v);
toc;
Using writeVideo also has the advantage of never storing the whole movie in memory. Instead, as each frame is captured, it is immediately written to the file. The one downside is that because the frames need to be written in the correct sequence, you can not use a parfor
You could also consider making a movie matrix. Each element would be a frame. So to initialize the size of the matrix, you would need something like
numImages = 25;
mov(numImages) = struct('cdata',[],'colormap',[]);
Then using the parfor, you can assign all the images to your mov matrix in the correct order. After your loop is finished, you can write the movie to a file using VideoWriter. However, while generating your figures, you will need to keep your whole movie in memory, and there is an overhead associated with parfor, so it may not end up being more efficient.

Answer 2

If you made Matlab output a file like this, called "lines.txt" with each line describing the x1,y1 and x2,y2 of a line on your output image:
10 100 600 100
50 400 800 400
100 820 350 820
Then you could use ImageMagick to draw the lines like this:
#!/bin/bash
# Create the output image, 1200x900
convert -size 1200x900 xc:gray80 result.png
# Loop, reading one line at a time from the file "lines.txt"
while read x1 y1 x2 y2; do
echo Read line $x1,$y1 $x2,$y2
# Tell ImageMagick to draw the line on the image
convert result.png -stroke blue -strokewidth 5 -draw "line $x1,$y1 $x2,$y2" result.png
done < lines.txt
Obviously, you can draw non-horizontal lines and you could output a colour after the x,y coordinates and/or a thickness...
ImageMagick is installed on most Linux distros and is available for OSX and Windows. There are more efficient ways of plotting the lines, but this may be plenty to get you started.
Another option may be to use Gnuplot, see my other answer here.

Answer 3

On top of Cecilia's answer which suggests using getframe to generate a movie, I'd like to add that getframe can also be used to improve speed for saving static images. Combine it with frame2im to convert the figure into a rasterized image, and then use imwrite to save the rasterized image to a file.
frame = getframe(fig); % fig is the figure handle to save
[raster, raster_map] = frame2im(frame); % raster is the rasterized image, raster_map is the colormap
if isempty(raster_map)
imwrite(raster, fig_file);
else
imwrite(raster, raster_map, fig_file); % fig_file is the path to the image
end
In my particular case the time used to save the figures is reduced from ~60s for 19 files (using saveas) down to 7~9s using this method.
Do note that this method
Does not change the background color to white as saveas does;
Does not seem to support resolution and dpi other than that of the current figure;
Does not support saving as a vector image (obviously).