Extracting the indices in a 2D map associted with lines drawn over the map

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Sai Prasanth 2021-5-20

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评论： Sai Prasanth 2021-6-11

Hello,

I have a 2D map of some geophysical variable. Visually, to mark the regions of interest and draw cross-sections, I have drawn several lines that indicate the various regions of the map. Now, I would like to extract the indices associated with each of these lines. I would then like to average the geophysical variable once I know the indices (which is simple). I just don't know how to extract the indices given these lines.

Any guidance will be appreciated!

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Image Analyst 2021-5-20

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Use improfile() to draw your lines. Two of the outputs will be the vector of x and vector of y values for the line you drew.

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Image Analyst 2021-6-4

在 MATLAB Online 中打开

I don't understand what you're saying. Maybe if you say this before you draw:

axis('on', 'image');

you'll see that the cx and cy DO have coordinate values that correspond to the actual pixel location in the image. If you have some other calibration system, then you'll have to convert the pixel locations from a digital image into something else, like latitude or longitude or kilometers or whatever. To convert to kilometers - that's trivial. To get latitude and longitude, there might be something in the Mapping Toolbox that might help (I don't have it).

So getting back to your question: "The whole point of drawing these lines was to be able to pick the array indices so that I could access the values at those specific locations to draw a histogram/average etc." I believe the cx and cy are the indices. Of course they're fractional because the profile is along a straight line. If you wanted integer coordinates to line up with rows and columns, then you can simply cast to integer by rounding:

cx = round(cx); % Round to nearest integer.
cy = round(cy); % Round to nearest integer.

Now the line will be jaggy. If you want to get the intensity along the jaggy line, simply run along the line

for k = 1 : length(cx)
    intensity(k) = grayImage(cy(k), cx(k)); % Make sure y comes first, NOT x!
end

Sai Prasanth 2021-6-5

在 MATLAB Online 中打开

I guess, my problem would instantly be solved if there was a way to extract these indices from a line drawn on a plot - without the need for converting it to an image. Since improfile only works on images, the indices of an image are totally different.

Please see in the below code that creates a line on a simple pcolor plot - I would just like to know the indices that it passes through (without converting to an image).

f3 = figure('DefaultAxesFontSize',18, 'DefaultAxesTitleFontWeight','bold')
 set(gcf,'color','w');
 set(0,'DefaultAxesFontName','Times New Roman','DefaultTextFontName','Times New Roman','DefaultAxesFontWeight', 'bold'); 
 pcolor(wrfvars_time1.XLONG,wrfvars_time1.XLAT,log10(Int_icepath))
 xlabel('Longitude') 
 ylabel('Latitude') 
 title('log_{10}(Vertically Integrated Ice-Water) in g/m^2') 
 shading flat
 caxis([-2 5])
 xlim([-36 -25])
 ylim([6 16.15])
 colormap(map)
 colorbar; 
   
x1 = -34.49; 
x2 = -31.58; 
y1 = 11.01;
y2 = 11.67;  
plot([x1 x2], [y1 y2],'LineWidth',3,'Color','red');