Plot points over image in Octave/Matlab

Question

A project I'm working on has an 'old-school' subject matter expert. He has provided me with important plots on X-Y axes. The plots are hand drawn, but very carefully made. I need a way to get the decimal values of each point into a file. Once I have this, I'll use an interpolation function from matlab or octave.

My first option is to just eyeball the plots and guess at the values of each point and type them into Excel or something. I really don't want to do this.

In Matlab and Octave, I understand there is a function called ginput with which the user can click on points in the plane to plot them. This is getting closer to what I want to do.

To ensure the best accuracy, I would like to have the original plot from the SME underneath ginput, like as a background image. I would make the ginput axes as close to the hand drawn ones as possible and then just click on the points in the original image to plot them.

Is there a way to do this with Octave/Matlab? Maybe another tool?

It is certainly possible to place an image on the figure, then use ginput to grab those co-ordinates. However, if you want the actual co-ordinates of your plot (not pixel co-ordinates), this may be difficult because if you want to nail the exact co-ordinates with respect to pixel/screen co-ordinates, you'll need to know the DPI of your scanner that will scan in that plot for you. For example, if you know that you have say 150 ppi, this means that for each inch on your paper, this corresponds to 150 pixels. Once you know this, you can scale your co-ordinates to get the true XY. — rayryeng
Personally, the tool I use for this is plotdigitizer (plotdigitizer.sourceforge.net ). — nkjt

Andy Andy · Accepted Answer · 2014-10-06T15:27:45

I've done this before with octave when I had to digitize spectral response from sensors from scanned datasheets.

img = imread ('datasheet.png');
imshow(img)
[X, Y, BUTTONS] = ginput ();

I then clicked the 4 corners to calculate the scale and the position of ther origo and then as much datapoints as I want. When I'm finished I press RETURN and have the values in X, Y which I calculate back to the desired units.

Plot points over image in Octave/Matlab

2 Answers