MATLAB: Impixel fails when a function is called the second time.

impixel

Impixel doesn't work the second time.
CENTER = 0;
[X,Y,MAX]=getcenter(MA,x,y)
if (MAX > CENTER)
       [X,Y,MAX] = getcenter(MAX,X,Y);
       CENTER = MAX;
      end
function [X,Y,MAX] = getcenter(MA,X,Y)
value = impixel(MA,X,Y);  % FAILS HERE WHEN getcenter is called the second time.
     for i = -5:5
       for j = -5:5 
           temp = impixel(MA, X + i,Y + j);
           if temp > value
               value = temp;
               X = X + i;
               Y = Y + j;
           end
         end
   end
   MAX = value;
The function getcenter fails the second time around at impixel. Error using interp2>makegriddedinterp (line 237) Interpolation requires at least two sample points in each dimension. Consider using INTERP1 if X or Y have constant coordinates that can be eliminated to reduce the dimension.
Error in interp2 (line 128)
        F = makegriddedinterp({X, Y}, V, method,extrap);
Error in impixel (line 106)
        rgb = interp2(xx,yy,a,xi,yi,'*nearest');
Error in getcenter (line 2)
value = impixel(MA,X,Y);
Error in MAflow (line 17)
     [X,Y,MAX] = getcenter(MAX,X,Y);
ideas? thanks

Best Answer

The problem is not with impixel but with your code. For impixel with three inputs, the first input is an image, the next two the pixel coordinates.
Your getcenter function:
function [X,Y,MAX]=getcenter(MA,x,y);
     value = impixel(MA,X,Y);
     %...
     MAX = value;
end
So MA must be an image (what a bad name). x and y are coordinates. and the MAX returned by getcenter is a pixel intensity.
Your first call:
[X,Y,MAX]=getcenter(MA,x,y)
So the returned MAX is an intensity. Second call:
[X,Y,MAX] = getcenter(MAX,X,Y);
MAX is not an image, it's just a scalar. So of course, the call to impixel is going to fail. You probably meant to pass MA again. If you give your variable some more meaningful names, this wouldn't happen.
By the way, a lot more efficient getcenter function would be:
function [maxcol, maxrow, maxintensity] = getcenter(searchimage, col, row)
    %Note that your code only worked with greyscale images
    %and scalar coordinate inputs. 
    %same here
    assert(ndims(searchimage) == 2, 'Image is not greyscale');
    assert(numel(col) == 1 && numel(row) == 1, 'Coordinates are not scalar');
    %generate all combinations of coordinates within +/- 5 of input:
    %note that unlike your code, the below works properly even if the input coordinates are within 5 pixels of the image edge
    [searchcols, searchrows] = ndgrid(searchimage(max(col-5, 1):min(col+5, size(searchimage, 2), ...
                                      searchimage(max(row-4, 1):max(row+5, size(searchimage, 1));
    intensities = impixel(searchimage, searchcols(:), searchrows(:));  %get all pixel intensities at once
    [maxintensity, loc] = max(intensities);  %find the max intensity and linear index of location
    maxcol = searchcols(loc);
    maxrow = searchrows(loc);
end
Related Question