scrolling plot speed and axis jitter

11 views (last 30 days)
Jim Hokanson
Jim Hokanson on 4 Nov 2017
Commented: Jim Hokanson on 6 May 2019
I'd like to create auto-scrolling plots in which data scrolls by, automatically, as it gets collected (from a DAQ). Before worrying about adding data I wanted to first check how plots behave with a pre-defined data vector where I adjust the x-limits to simulate scrolling. Just to clarify, I don't intend to have any manual scrolling, just calls to adjust the axis limits.
This is the baseline code for plotting:
t = 0:1/1000:20;
s = rand(1,length(t)).*sin(0.67*2*pi*t);
ax(1) = subplot(3,1,1);
h1 = plot(t,s);
toc
ax(2) = subplot(3,1,2);
h2 = plot(t,s);
ax(3) = subplot(3,1,3);
h3 = plot(t,s);
Now I'd like to do something like the following:
update_tick = 0;
set(ax,'ylim',[-1 1])
tic
for t2 = 0:0.01:18
set(ax,'xlim',[t2 t2+2]) %Move the data view
update_tick = update_tick + 1;
%At some interval rerender the ticks
if mod(update_tick,5) == 0
xtick = t2:0.1:t2+2;
set(ax,'XTick',xtick)
end
drawnow
%pause(0.005)
end
toc
The code above changes to the x-limits to simulate scrolling/new data and occasionally updates the displayed ticks.
This code however is horribly slow. This takes about 50 seconds to run on my computer. It gets faster if the drawnow is replaced with pause (about 12s) but I have other things to do in the DAQ collection loop and I'd like this number to be closer to 1-2 seconds. For reference, Matlab 2017b documentation states that using the pause function is equivalent to the drawnow command, but I find I like this explanation better (basically a drawnow if possible within allotted time): https://www.mathworks.com/matlabcentral/answers/7309-is-pause-a-superset-of-drawnow#answer_192103
Additionally, as the plotting is occurring the axes are shaking just slightly which is really annoying.
Any suggestions on speeding up the code?
  3 Comments
Jim Hokanson
Jim Hokanson on 9 Nov 2017
Update: Answers to faster plotting in Matlab generally focus on reducing object creation and manipulating the underlying data directly. Eventually I would like to add data dynamically, but this is focused on updating the scene.
I have tried a few things with surprisingly little luck.
  • set(ax,'DrawMode','fast') (obsolete, alias now for SortMethod?)
  • set(ax,'SortMethod','childorder') - not critical for single lines
  • set(ax,'NextPlot','replacechildren')
  • set(ax,'Clipping','off')
  • set(gcf,'GraphicsSmoothing','off')
  • set(gcf,'Renderer','opengl')
  • set(gcf,'Renderer','painters')
  • opengl hardwarebasic
  • set(ax,'Visible','off')
The only thing that made any difference was turning off the axes visibility, which isn't ideal.

Sign in to comment.

Answers (1)

Joshua  Robinson
Joshua Robinson on 29 Apr 2019
Your main problem is that you're forcing matlab to recalculate and update the position and names of all the ticks every couple of iterations. So even if t2 changed by a little bit, every single tick is recalculated.
Instead it would make more sense to only update the tick values that actually change as t2 moves along (aka the first and last ticks).
The quickest way to achieve this is to round t2 to the nearest tick value and add a tick offset array to the rounded t2 value ala...
dtick = 0.1;
tickbase = (0:dtick:2);
% main loop
for t2=tmin:dt:tmax;
% BLAH BLAH BLAH
% calculate new ticks
nearest_tick = dtick*round(t2/dtick);
xtick = nearest_tick + tick_offset;
set(ax,'XTick',xtick)
% BLAH BLAH BLAH
end
So the final code should be something along the lines of...
clear all
close all
t = 0:1/1000:20;
s = rand(1,length(t)).*sin(0.67*2*pi*t);
ax(1) = subplot(3,1,1);
h1 = plot(t,s);
toc
ax(2) = subplot(3,1,2);
h2 = plot(t,s);
ax(3) = subplot(3,1,3);
h3 = plot(t,s);
% my edit
ax(1).XGrid = 'on';
ax(1).YGrid = 'on';
ax(2).XGrid = 'on';
ax(2).YGrid = 'on';
ax(3).XGrid = 'on';
ax(3).YGrid = 'on';
dtick = 0.1;
tick_offset = (0:dtick:2);
update_tick = 0;
set(ax,'ylim',[-1 1])
tic
for t2 = 0:dtick:18
set(ax,'xlim',[t2 t2+2]) %Move the data view
update_tick = update_tick + 1;
% %At some interval rerender the ticks
if mod(update_tick,5) == 0
nearest_tick = dtick*round(t2/dtick); % Code from above
xtick = nearest_tick + tick_offset;
set(ax,'XTick',xtick)
end
drawnow;
end
toc
  3 Comments
Jim Hokanson
Jim Hokanson on 6 May 2019
Thanks for the insight. The goal of this example was not to exactly replicate the code that I'm running, but rather to work out a strategy to minimize the time spent rendering the ticks. Thanks though.

Sign in to comment.

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!