How to determine co-ordinates of a geometrical shape in an image?

I am new to image processing in Matlab. I have a bunch of images taken from experiments. There is a circular dot in each image which has different positions in different images. I want to find out the co-ordinates of these circles in the various images. Can someone please help me with this?

Thanks,

Arun

[Ashish Uthama]

Arun, post the picture online and add the links to your question.

Depending on the lighting condition, clarity etc, folks here might be able to direct you to a robust segmentation/detection approach.

[Arun Krishnan]

Thanks for responding Ashish. I sort of figured out what to do from this webpage (http://knol.google.com/k/learn-matlab-image-processing-edge-detection-function#). But this is one of my pictures. (http://imgur.com/YZdvs). I have a set of pictures similar to this one. I am trying to figure out by how much the red dot is moving in each of these pictures.

Thanks

Arun

Answer 1

[David Young]

If the dots are always circular (or almost circular), the Hough transform offers one approach. If you know the radius in advance, it's more reliable, but you can search across radii as well as for the centre position.

My own implementation is here: http://www.mathworks.com/matlabcentral/fileexchange/26978-hough-transform-for-circles, but there are several others on the file exchange which you can easily find, and you may need to experiment to find the best one for you.

Note that most forms of the Hough transform need to be preceded by an edge detection operation.