How may one digitize a curve from an image

The attached jpeg contains battery discharge curves at various discharge currents (0.68 to 10 A).

I'd like to create arrays (stored in a .mat file) which contains these discharge curves (voltage vs capacity).

Ideally I just need the data for the 0.68 A curve.

How would one "digitize" the 0.68A curve into X and Y coordinates/values given a minimum step size?

Thank you

Answer 1

[DGM]

These two answers are relevant and contain links to other related answers.

https://www.mathworks.com/matlabcentral/answers/1675544-how-do-i-find-the-x-and-y-coordinates-of-image-want-to-plot-this-figure

https://www.mathworks.com/matlabcentral/answers/875968-is-there-are-a-way-to-take-the-result-of-this-figure-with-out-input-data

Long story short, accuracy will be limited, and trying to write something that will automagically find the right curve is almost always going to be a waste of time, especially if you only have one plot to transcribe. Just manually transcribe the curve as described. I've attached an SVG file that's already had a spline fit to it. The SVG processing is as described in the answers above.

% using the following FEX tools:
% https://www.mathworks.com/matlabcentral/fileexchange/72225-load-svg-into-your-matlab-code

% filename of manually-fit svg file
fname = 'discharge_curves.svg';
% data range from original image axis labels
xrange = [0 4000];
yrange = [0 5];
% spline discretization parameter [0 1]
coarseness = 0.001;

% get plot box geometry
str = fileread(fname);
str = regexp(str,'((?<=<rect)(.*?)(?=\/>))','match');
pbx = regexp(str,'((?<=x=")(.*?)(?="))','match');
pby = regexp(str,'((?<=y=")(.*?)(?="))','match');
pbw = regexp(str,'((?<=width=")(.*?)(?="))','match');
pbh = regexp(str,'((?<=height=")(.*?)(?="))','match');
pbrect = [str2double(pbx{1}{1}) str2double(pby{1}{1}) ...
    str2double(pbw{1}{1}) str2double(pbh{1}{1})];

% get coordinates representing the curve
S = loadsvg(fname,coarseness,false);
x = S{1}(:,1); % assuming the first path is the correct one
y = S{1}(:,2);
% if there are multiple paths you want to extract
% you'll need to do do the rescaling, etc for each element of S

% rescale to fit data range
x = xrange(1) + diff(xrange)*(x-pbrect(1))/pbrect(3);
y = yrange(1) + diff(yrange)*(pbrect(4) - (y-pbrect(2)))/pbrect(4);

% get rid of nonunique points
[x,idx,~] = unique(x); 
y = y(idx);

% plot
plot(x,y); grid on; hold on
xlim(xrange)
ylim(yrange)

At this point, x and y will be relatively finely sampled. If you want to resample the curve on a coarser abcissa, you can use interp1().

Answer 2

[Image Analyst]

Lots of File Exchange submissions on digitizing a graph:

https://www.mathworks.com/matlabcentral/fileexchange/?term=tag%3A%22digitize%22