Performance: scatter vs. line

Wednesday, October 14th, 2009

Following my previous article on the undocumented behavior of the scatter function, one of my readers, Benoit Charles, reported a discovery that in many circumstances the line function generates identical plots much faster than scatter.

Unlike scatter, line does not enable specific data-point marker customization, although the colors could be modified. On the other hand, line only uses a single handle object, saving memory and system resources compared to scatter keeping a separate handle for each data point. So, if you just need to quickly plot a bunch of scattered points then line could be a better choice than scatter.

Here is a simple code snippet, which generates identical plots and shows the performance difference:

>> x=rand(1000,1); y=rand(1000,1);
 
>> tic, for idx=1:100, cla; h=scatter(x,y); end; toc
Elapsed time is 2.521322 seconds.
 
>> props = {'LineStyle','none','Marker','o','MarkerEdge','b','MarkerSize',6};
>> tic, for idx=1:100, cla; h=line([x,x],[y,y],props{:}); end; toc
Elapsed time is 0.333369 seconds.

In the past, I have posted about other undocumented performance aspects, comparing the documented ismember function with the undocumented ismembc and about cellfun‘s undocumented options. If you are aware of other similar functions having identical outputs and a significant performance difference, please let me know.

Related posts:

  1. Undocumented scatter plot behavior The scatter plot function has an undocumented behavior when plotting more than 100 points: it returns a single unified patch object handle, rather than a patch handle for each specific point as it returns with 100 or less points....
  2. Undocumented scatter plot jitter Matlab's scatter plot can automatically jitter data to enable better visualization of distribution density. ...
  3. Multi-line tooltips Multi-line tooltips are very easy to set up, once you know your way around a few undocumented hiccups....
  4. Multi-line uitable column headers Matlab uitables can present long column headers in multiple lines, for improved readability. ...
  5. Array resizing performance Several alternatives are explored for dynamic array growth performance in Matlab loops. ...
  6. Plot performance Undocumented inner plot mechanisms can be used to significantly improved plotting performance...

Categories: Handle graphics, Low risk of breaking in future versions, Memory, Stock Matlab function

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4 Responses to Performance: scatter vs. line

  1. Naor says:
    October 14, 2009 at 1:05 pm

    Now this is really interesting. Now I’m curious about generating 1000 line objects with one data point/marker each, and saving into a handle vector? Is this what scatter is doing under the hood?

    Reply
  2. Benoit says:
    October 15, 2009 at 2:54 am

    In this case, lines are faster than scatter only when colors are used and different for each point (???). Observe this script:

    %% Scatter creation
T = 0;
cla
for i=1:10
	x = rand(1,1000);
	y = rand(1,1000);
	cla
	t = cputime;
	h = scatter( x,y );
	drawnow
	T = T + (cputime-t);
end
fprintf( 'Scatter creations: %.2f\n', T )
 
%% Lines creation
T = 0;
cla
for i=1:10
	x = rand(1,1000);
	y = rand(1,1000);
	cla
	t = cputime;
	h = line( [x;x], [y;y], props{:} );
	drawnow
	T = T + (cputime-t);
end
fprintf( 'Line creations: %.2f\n', T )
 
%% Scatter modification
T = 0;
cla
x = rand(1,1000);
y = rand(1,1000);
h = scatter(x,y);
for i=1:10
	y = rand(1,1000);
	t = cputime;
	set( h, 'YData', y );
	drawnow
	T = T + (cputime-t);
end
fprintf( '\nScatter modifications: %.2f\n', T )
 
%% Lines modification
T = 0;
cla
x = rand(1,1000);
y = rand(1,1000);
h = line( [x;x], [y;y], props{:} );
for i=1:10
	y = rand(1,1000);
	t = cputime;
	set( h, {'YData'}, num2cell([y;y]',2) );
	drawnow
	T = T + (cputime-t);
end
fprintf( 'Line modifications: %.2f\n', T )
 
%% Scatter color creation
T = 0;
cla
for i=1:10
	x = rand(1,1000);
	y = rand(1,1000);
	c = rand(1000,3);
	cla
	t = cputime;
	h = scatter( x,y,50,c );
	drawnow
	T = T + (cputime-t);
end
fprintf( '\nScatter color creation: %.2f\n', T )
 
%% Lines color creation
T = 0;
cla
for i=1:10
	x = rand(1,1000);
	y = rand(1,1000);
	c = rand(1000,3);
	cla
	t = cputime;
	h = line( [x;x], [y;y], props{:} );
	set( h, {'MarkerEdgeColor'}, num2cell(c,2) );
	drawnow
	T = T + (cputime-t);
end
fprintf( 'Line color creation: %.2f\n', T )
 
%% Scatter color modification
T = 0;
cla
x = rand(1,1000);
y = rand(1,1000);
c = rand(1000,3);
h = scatter(x,y,50,c);
for i=1:10
	y = rand(1,1000);
	t = cputime;
	set( h, 'YData', y );
	drawnow
	T = T + (cputime-t);
end
fprintf( '\nScatter color modification: %.2f\n', T )
 
%% Lines modification
T = 0;
cla
x = rand(1,1000);
y = rand(1,1000);
c = rand(1000,3);
h = line( [x;x], [y;y], props{:} );
set( h, {'MarkerEdgeColor'}, num2cell(c,2) );
for i=1:10
	y = rand(1,1000);
	t = cputime;
	set( h, {'YData'}, num2cell([y;y]',2) );
	drawnow
	T = T + (cputime-t);
end
fprintf( 'Line color modification: %.2f\n', T )

    When you execute this script, you obtain:

    Scatter creations: 0.76
Line creations: 9.53
 
Scatter modifications: 0.61
Line modifications: 1.51
 
Scatter color creation: 4.02
Line color creation: 17.07
 
Scatter color modification: 4.34
Line color modification: 1.53

    Magic, isn’t it?
    Sometimes, I wonder if Matlab was not developed at Hogwarts…

    Reply
  3. Naor says:
    October 15, 2009 at 3:41 pm

    very strange indeed.

    Reply
  4. damayi says:
    May 22, 2012 at 7:59 pm

    In my application, I will use line as much as possible.
    What’s more, line is more simple and easy to control.

    Reply

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<pre lang="matlab">
a = magic(3);
sum(a)
</pre>