The MCvSURFParams type exposes the following members.
Zero means basic descriptors (64 elements each), Non-zero 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
The number of octaves to be used for extraction. With each next octave the feature size is doubled