0 means basic descriptors (64 elements each), 1 means extended descriptors (128 elements each)

Only features with keypoint.hessian larger than that are extracted. good default value is ~300-500 (can depend on the average local contrast and sharpness of the image). user can further filter out some features based on their hessian values and other characteristics

The number of layers within each octave (4 by default)

The number of octaves to be used for extraction. With each next octave the feature size is doubled (3 by default)