How to I get the coordinates of a given pixel?

I was wondering if there was a method to get the X,Y coordinate of a pixel in an image? I have the pixels that I'd like to get the coordinates of, but as they aren't regions, a regionprops centroid doesn't help, and I need it to be done without user input, so I can't click on them with ginput.

[Matthew Eicholtz]

What do you mean by "I have the pixels that I'd like to get the coordinates of"? Meaning you have the linear indices and want to convert to (x,y) coordinates? Or something else?

[James Richards]

Sorry for the late reply. I'm not entirely certain myself (still quite new to matlab). I'm trying to get the X,Y coordinates of pixels found at the end points of ellipses. The following code is used to generate the ellipse from my regionprops-

phi = linspace(0,2*pi,50);
cosphi = cos(phi);
sinphi = sin(phi);

for k = 1:length(refinedStats)
    xbar = refinedStats(k).Centroid(1);
    ybar = refinedStats(k).Centroid(2);

      a = refinedStats(k).MajorAxisLength/2;
      b = refinedStats(k).MinorAxisLength/2;

      theta = pi*refinedStats(k).Orientation/180;
      R = [ cos(theta)   sin(theta)
           -sin(theta)   cos(theta)];

      xy = [a*cosphi; b*sinphi];
      xy = R*xy;

      x = xy(1,:) + xbar;
      y = xy(2,:) + ybar;

      plot(x,y,'r','LineWidth',2);
  end

And this code to get the endpoints of the ellipse-

l = length(refinedStats);
for n = 1:l
theta = pi*refinedStats(k).Orientation/180;
    %for xEnd
    xbar = refinedStats(n).Centroid(1);
    a = refinedStats(n).MajorAxisLength/2;
    xEnd = xbar + a * cos(theta);
    refinedStats(n).endPointX = xEnd;

      %for yEnd
      ybar = refinedStats(n).Centroid(1);
      yEnd = ybar + a * sin(theta);
      refinedStats(n).endPointY = yEnd;
  end

This gives me data in my 'refinedstats' structure like endPointX = 1003.87333157056 and endPointY= 989.636902390572. It's these numbers I'd like to get the coordinates of.

[Walter Roberson]

X coordinate 1003.87333157056 with Y coordinate 989.636902390572 would be found at index (989.636902390572, 1003.87333157056) in the array. Y then X.

Answer 1

[Image Analyst]

Use bilinear interpolation, like interp2() if you want the exact value of fractional coordinates. However, it may be good enough for you to do a much simpler operation of just rounding the coordinates to the nearest integer and just getting the array element

grayLevel = grayImage(round(y), round(x));

Remember, like Walter said above, it's (y,x) not (x,y) because the first index is rows and y is the row.

You might also like the impixel() function, but I don't find that any more convenient than simple indexing.

[Raka Mukherjee]

While finding the centroid of the connected components with the regionprops function we are getting two columns as centroids, the first column is x and the second column is y which means the first is row and the second is column,right? Can this x and y values be converted to latitude longitude coordinates? I have tried using pix2latlong followed by projinv but the coordinates are not matching with the orinal image.

Kindly help. Thanks.

[Image Analyst]

No, it's (x, y) not (row, column), which would be (y, x). That's one thing you always have to keep mindful of and a common cause of beginners' problems. It's described in the help documentation.

props = regionprops(mask, 'Centroid');
xy = vertcat(props.Centroid);
x = xy(:, 1); % Columns
y = xy(:, 2); % Rows

I can't help you with the conversion to geographical coordinates. Look into functions of the Mapping Toolbox.

[Pablo Velez]

"a common cause of beginners' problems" happend to me but I didn't find it in the documentation https://www.mathworks.com/help/matlab/ref/imread.html I could only infer it from the example, is that what you mean with "described in the help documentation" or am I missing something.

Also I don't undertand why they switch x y places, python matplotlib does the same, why?