COT: colour thresholding. We separated yellow objects from the rest.
BLED: blob edge detection. We retrieved the bottom edges of blobs (pawns).
T: transformation. We transformed the image’s pixels into game field points (aka. pixels to meters).
And the last step CID: Circles Detection.
As you may have already noticed, pawns and tower of pawns are in fact circles when viewed from above. Therefore, the bottom edges we found with BLED are also circles’ segments when transformed into game field coordinates (step T). This is the property we’re exploiting below.
There is still too much information we do not need on the B&W image we got on the last step. That’s why we need to extract the features we do need. One way of accomplishing this is by performing a connected component analysis in binary images, aka blob labelling. However as you’ll will see, this method is not completely adapted to our needs, so a new approach is proposed: Blob Edge Detection.