Question

Hello, I have an image as shown above. Is it possible for me to detect the center point of the cross and output the result using Matlab? Thanks.

Answer 1

Here you go. I’m assuming that you have the image toolbox because if you don’t then you probably shouldn’t be trying to do this sort of thing. However, all of these functions can be implemented with convolutions I believe. I did this process on the image you presented above and obtained the point (139,286) where 138 is the row and 268 is the column.

1.Convert the image to a binary image:

bw = bw2im(img, .25);

where img is the original image. Depending on the image you might have to adjust the second parameters (which ranges from 0 to 1) so that you only get the cross. Don’t worry about the cross not being fully connected because we’ll remedy that in the next step.

2.Dilate the image to join the parts. I had to do this twice because I had to set the threshold so low on the binary image conversion (some parts of your image were pretty dark). Dilation essentially just adds pixels around existing white pixels (I’ll also be inverting the binary image as I send it into bwmorph because the operations are made to act on white pixels which are the ones that have a value of 1).

bw2 = bwmorph(~bw, 'dilate', 2);

The last parameter says how many times to do the dilation operation.

3.Shrink the image to a point.

bw3 = bwmorph(bw2, 'shrink',Inf);

Again, the last parameter says how many times to perform the operation. In this case I put in Inf which shrinks until there is only one pixel that is white (in other words a 1).

4.Find the pixel that is still a 1.

[i,j] = find(bw3);

Here, i is the row and j is the column of the pixel in bw3 such that bw3(i,j) is equal to 1. All the other pixels should be 0 in bw3.

There might be other ways to do this with bwmorph, but I think that this way works pretty well. You might have to adjust it depending on the picture too. I can include images of each step if desired.