MATLAB Answers

Knut
0

How to overlay a log-plot on top of an image

Asked by Knut
on 26 Mar 2013
Latest activity Commented on by Scott Webster on 19 Feb 2015
Here is the use-case: You have access to an image of a MATLAB semilogx() plot from a paper, a website or something similar. The original data are not easily available. You want to manually approximate the original vector. If one could overlay a local MATLAB semilogx() plot on top of the figure, that would make the tedious job slightly less tedious.

  1 Comment

If I'm understanding what you are trying to do correctly, you might want to considering using alternative tools to get at the data, several of which exist, e.g.

Sign in to comment.

1 Answer

Answer by Knut
on 26 Mar 2013
Edited by Knut
on 26 Mar 2013
 Accepted Answer

Here is my suggested solution. Nothing ground-breaking, but a bit fiddly for my skills using axes.
%%generate some trivial data
[b1,a1] = butter(2,1200./48e3);
[b2,a2] = butter(2,1200./48e3,'high');
H1 = freqz(b1,a1,512,48e3);
H2 = freqz(b2,a2,512,48e3);
im = imread('ngc6543a.jpg');
%%do the actual plotting
figure
%define plot extent so that image can be aligned with plot
xrng = [20 30e3]./1000;
yrng = [-60 10];
image(xrng,yrng,im);
ax1 = gca;
%get rid of pixel ticklabels
set(ax1,'YTickLabel', [], 'XTickLabel', [])
%color=none to make the image visible
ax2 = axes('Position',get(ax1,'Position'),'Color','none','XScale', 'log');
ylim(yrng)
xlim(xrng)
xlabel('Frequency - kHz')
ylabel('power - dB')
title('Frequency response')
line(W, 20*log10(abs([H1 H2])),'Parent',ax2);
legend('lopass', 'hipass')
Inspired by:

  2 Comments

I've actually just resorted to taking screenshots and overlaying it manually like this:
@Stanley Kubrick — You should probably post that also as a Comment to your post Semilog plot with background image.

Sign in to comment.