Extract data from contour plot of .png file?

Hi,

I have the attached contour plot figure in .png format. I need to extract the data points form the plot (x,y,z)?

Would someone please help me with the same?

Thanks in advance.

[Rik]

What steps did you try yourself? You should be able to extract the image field and the colorbar.

However, it is much easier if you could get the data from the source. Is it possible to get the data before it is converted to a png?

[Surendra Gupta]

Hi Rik,

Thank you for the response.

This is one of the old graphs that I have and I have missed all the data and the .fig file of the image

But now i need this data for my analysis.

Below is one of the source which I tried. However, I could not get proper data from it for my contour plot.

https://stackoverflow.com/questions/44434576/extracting-data-from-pixels-of-a-contour-plot-figure-in-python-or-matlab

[Rik]

That second answer looks like it should work. What exact code did you try, and what errors did you get?

[Surendra Gupta]

I have used the below code given in that link.. I have given my image and changed the colorLim.

%// Import the data:
imdata = importdata('ir_image.png');
Gray = rgb2gray(imdata.cdata);
colorLim = [300 1220]; %// this should be set manually
%// Get the area of the data:
f = figure('Position',get(0,'ScreenSize'));
imshow(imdata.cdata,'Parent',axes('Parent',f),'InitialMagnification','fit');
%// Get the area of the data:
title('Click with the cross on the most top left area of the *data*')
da_tp_lft = round(getPosition(impoint));
title('Click with the cross on the most bottom right area of the *data*') 
da_btm_rgt = round(getPosition(impoint));
dat_area = double(Gray(da_tp_lft(2):da_btm_rgt(2),da_tp_lft(1):da_btm_rgt(1)));
%// Get the area of the colorbar:
title('Click with the cross within the upper most color of the *colorbar*')
ca_tp_lft = round(getPosition(impoint));
title('Click with the cross within the bottom most color of the *colorbar*')
ca_btm_rgt = round(getPosition(impoint));
cmap_area = double(Gray(ca_tp_lft(2):ca_btm_rgt(2),ca_tp_lft(1):ca_btm_rgt(1)));
close(f)
%// Convert the colormap to data:
data = dat_area./max(cmap_area(:)).*range(colorLim)-abs(min(colorLim));

[Rik]

I hard-coded the coordinates for this example image. It looks like there is an issue with rescaling the data, but I will leave that for you to untangle. Note that this solution does not use the actual colorbar, but convert everything to grayscale first. That is probably done to avoid having to match the actual colors, which can be quite tricky for low resolution images like yours.

%// Import the data:
imdata = importdata('ir_image.png');
Gray = rgb2gray(imdata.cdata);
colorLim = [300 1220]; %// this should be set manually

%// Get the area of the data:
da_tp_lft = [25 13];
da_btm_rgt = [231 287];
dat_area = double(Gray(da_tp_lft(2):da_btm_rgt(2),da_tp_lft(1):da_btm_rgt(1)));

%// Get the area of the colorbar:
ca_tp_lft = [246 13];
ca_btm_rgt = [246 287];
cmap_area = double(Gray(ca_tp_lft(2):ca_btm_rgt(2),ca_tp_lft(1):ca_btm_rgt(1)));

%// Convert the colormap to data:
data = dat_area./max(cmap_area(:)).*range(colorLim)-abs(min(colorLim));

figure(1),clf(1)
imshow(data,[])
colorbar

[Surendra Gupta]

Thank you.

Answer 1

[Bjorn Gustavsson]

In case the conversion of the colormap to grayscale doesn't work well (i.e. multiple colours mapping to the same gray-level) I once had to solve this problem, and came up with a solution working reasonably well with the functions attached.

HTH

[Surendra Gupta]

Thank you for sharing the files. However, it is not taking my image as input file when I replaced the path of my image file in the place of 'img_in' in scan@Ddata.m file.

May I please know what could be the issue?

[Bjorn Gustavsson]

Ok, the scan2D2data function expects an rgb-image as input, not a string with path to an image-file. So call it something like this:

imfilename = 'your-png-filename.png';
im_rgb = imread(imfilename);
[M_out] = scan2D2data(img_in);

Then the function will interactively query you about the colour-map and the range the data, and the image-region of the data.

HTH

[jay shah]

Can you please explain in detail how to use the function? like how to select diagonal points and color map etc.