This function finds the 'first' pixel in an N-dimensional image, along any dimension. This is somewhat like measuring from the top row of an image towards the bottom row of the image, and recording the first collision with an "on" pixel.
FIND_NDIM Finds first/last nonzero element indices along a dimension.
I = FIND_NDIM(BW,DIM) returns the subscripts of the first nonzero
elements of BW along the dimension DIM. Output I will contain zeros in
element locations where no nonzero elements were found
I = FIND_NDIM(BW,DIM,'first) is the same as I = FIND_NDIM(BW,DIM).
I = FIND_NDIM(BW,DIM,'last') returns the subscripts of the last nonzero
elements of BW along the dimension DIM.
[I,MASK] = FIND_NDIM(BW,DIM,...) also returns a MASK the same size as
BW with only the first/last pixels in dimension DIM on.
If X = [0 1 0
1 1 0]
then find_ndim(X,1) returns [2 1 0], find_ndim(X,1,'last')
returns [2 2 0], and find_ndim(X,2) returns [2 1]';
I = imread('rice.png');
greyIm = imadjust(I - imopen(I,strel('disk',15)));
BW = imclearborder(bwareaopen(im2bw(greyIm, graythresh(greyIm)), 50));
topMostOnInds = find_ndim(BW, 1, 'first');
rightMostOnInds = find_ndim(BW, 2, 'last');
figure, imshow(BW), hold on
plot(1:size(BW,2), topMostOnInds, 'r.')
plot(rightMostOnInds, 1:size(BW,1), '.b')
A discussion of how this came about is at: