Feature Detection and Description

SIFT

class SIFT : public Feature2D

Class for extracting keypoints and computing descriptors using the Scale Invariant Feature Transform (SIFT) algorithm by D. Lowe [Lowe04].

[Lowe04]

Lowe, D. G., “Distinctive Image Features from Scale-Invariant Keypoints”, International Journal of Computer Vision, 60, 2, pp. 91-110, 2004.

SIFT::SIFT

The SIFT constructors.

C++: SIFT::SIFT(int nfeatures=0, int nOctaveLayers=3, double contrastThreshold=0.04, double edgeThreshold=10, double sigma=1.6)

Parameters:

• nfeatures – The number of best features to retain. The features are ranked by their scores (measured in SIFT algorithm as the local contrast)
• nOctaveLayers – The number of layers in each octave. 3 is the value used in D. Lowe paper. The number of octaves is computed automatically from the image resolution.
• contrastThreshold – The contrast threshold used to filter out weak features in semi-uniform (low-contrast) regions. The larger the threshold, the less features are produced by the detector.
• edgeThreshold – The threshold used to filter out edge-like features. Note that the its meaning is different from the contrastThreshold, i.e. the larger the edgeThreshold, the less features are filtered out (more features are retained).
• sigma – The sigma of the Gaussian applied to the input image at the octave #0. If your image is captured with a weak camera with soft lenses, you might want to reduce the number.

SIFT::operator ()

Extract features and computes their descriptors using SIFT algorithm

C++: void SIFT::operator()(InputArray img, InputArray mask, vector<KeyPoint>& keypoints, OutputArray descriptors, bool useProvidedKeypoints=false)

Parameters:

• img – Input 8-bit grayscale image
• mask – Optional input mask that marks the regions where we should detect features.
• keypoints – The input/output vector of keypoints
• descriptors – The output matrix of descriptors. Pass cv::noArray() if you do not need them.
• useProvidedKeypoints – Boolean flag. If it is true, the keypoint detector is not run. Instead, the provided vector of keypoints is used and the algorithm just computes their descriptors.

SURF

class SURF : public Feature2D

Class for extracting Speeded Up Robust Features from an image [Bay06]. The class is derived from CvSURFParams structure, which specifies the algorithm parameters:

int extended

• 0 means that the basic descriptors (64 elements each) shall be computed
• 1 means that the extended descriptors (128 elements each) shall be computed

int upright

• 0 means that detector computes orientation of each feature.
• 1 means that the orientation is not computed (which is much, much faster). For example, if you match images from a stereo pair, or do image stitching, the matched features likely have very similar angles, and you can speed up feature extraction by setting upright=1.

double hessianThreshold

Threshold for the keypoint detector. Only features, whose hessian is larger than hessianThreshold are retained by the detector. Therefore, the larger the value, the less keypoints you will get. A good default value could be from 300 to 500, depending from the image contrast.

int nOctaves

The number of a gaussian pyramid octaves that the detector uses. It is set to 4 by default. If you want to get very large features, use the larger value. If you want just small features, decrease it.

int nOctaveLayers

The number of images within each octave of a gaussian pyramid. It is set to 2 by default.

[Bay06]

Bay, H. and Tuytelaars, T. and Van Gool, L. “SURF: Speeded Up Robust Features”, 9th European Conference on Computer Vision, 2006

SURF::SURF

The SURF extractor constructors.

C++: SURF::SURF()

C++: SURF::SURF(double hessianThreshold, int nOctaves=4, int nOctaveLayers=2, bool extended=true, bool upright=false )

Python: cv2.SURF([hessianThreshold[, nOctaves[, nOctaveLayers[, extended[, upright]]]]]) → <SURF object>

Parameters:

• hessianThreshold – Threshold for hessian keypoint detector used in SURF.
• nOctaves – Number of pyramid octaves the keypoint detector will use.
• nOctaveLayers – Number of octave layers within each octave.
• extended – Extended descriptor flag (true - use extended 128-element descriptors; false - use 64-element descriptors).
• upright – Up-right or rotated features flag (true - do not compute orientation of features; false - compute orientation).

SURF::operator()

Detects keypoints and computes SURF descriptors for them.

C++: void SURF::operator()(InputArray img, InputArray mask, vector<KeyPoint>& keypoints) const

C++: void SURF::operator()(InputArray img, InputArray mask, vector<KeyPoint>& keypoints, OutputArray descriptors, bool useProvidedKeypoints=false)

Python: cv2.SURF.detect(image[, mask]) → keypoints

C: void cvExtractSURF(const CvArr* image, const CvArr* mask, CvSeq** keypoints, CvSeq** descriptors, CvMemStorage* storage, CvSURFParams params)

Python: cv.ExtractSURF(image, mask, storage, params)-> (keypoints, descriptors)

Parameters:

• image – Input 8-bit grayscale image
• mask – Optional input mask that marks the regions where we should detect features.
• keypoints – The input/output vector of keypoints
• descriptors – The output matrix of descriptors. Pass cv::noArray() if you do not need them.
• useProvidedKeypoints – Boolean flag. If it is true, the keypoint detector is not run. Instead, the provided vector of keypoints is used and the algorithm just computes their descriptors.
• storage – Memory storage for the output keypoints and descriptors in OpenCV 1.x API.
• params – SURF algorithm parameters in OpenCV 1.x API.

The function is parallelized with the TBB library.

If you are using the C version, make sure you call cv::initModule_nonfree() from nonfree/nonfree.hpp.