Default type

Use adaptive thresholding to convert the image to black-n-white, rather than a fixed threshold level (computed from the average image brightness)

Normalize the image using cvNormalizeHist before applying fixed or adaptive thresholding.

Use additional criteria (like contour area, perimeter, square-like shape) to filter out false quads that are extracted at the contour retrieval stage

If it is on, then this check is performed before the main algorithm and if a chessboard is not found, the function returns 0 instead of wasting 0.3-1s on doing the full search.