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It is possible to create a custom SIFT descriptor extractor:

SIFT siftDetectorExtractor = SIFT(200, 3, 0.04, 15, 1.6);

Mat logo = imread("logoName.jpg");

vector<KeyPoint> keyPoints;
Mat sifts;
siftDetectorExtractor(logo, Mat(), keyPoints, sifts);

or use the detector-class:

Ptr<FeatureDetector> detector = Ptr<FeatureDetector>( new SIFT( <your arguments> ) );

Is there a way to specify custom SIFT detector parameters in OpenCV?

It seems that the FeatureDetector constructor does not take any parameter, whereas it seems possible to specify those parameters in the SIFT constructor.

I am working on logo detection. Some of the logos have very low texture information, so I would like to detect more keypoints when there are too few (I could increase the edgeThreshold of SIFT?).

Answer:

It is possible to create a custom SIFT descriptor extractor:

SIFT siftDetectorExtractor = SIFT(200, 3, 0.04, 15, 1.6);

Mat logo = imread("logoName.jpg");

vector<KeyPoint> keyPoints;
Mat sifts;
siftDetectorExtractor(logo, Mat(), keyPoints, sifts);

I forgot to call cv::initModule_nonfree(); before doing anything else.

How to install OpenCV in a custom directory on linux? I would like to install from the latest source code (version 3.0), but without touching the current version (4.6.0) installed on my computer.

I know I can use CMAKE_INSTALL_PREFIX to specify a custom dir, but how will pkg-config handle the two versions then?

cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local ..

How can I specify to pkg-config which version I want to use, and where it is installed?

I want C = A/B when B != 0, 0 otherwise. My test showed that the / operator does exactly what I want (with CV_32S mat).

It would be nice if this behavior were documented. How is divide() different from the / operator?

I forgot to call cv::initModule_nonfree(); before doing anything else.

I'm trying to detect SIFT keypoints, using this code:

#include "opencv2/features2d/features2d.hpp"
#include "opencv2/nonfree/nonfree.hpp"
#include "opencv2/nonfree/features2d.hpp"

[...]

Ptr<FeatureDetector> detector = FeatureDetector::create( "SIFT" );
vector<KeyPoint> keyPoints;
detector->detect( image, keyPoints );

The problem is that my detector pointer points to nowhere after that last line, and no error is shown in the console.

I'm using QtCreator and my .pro file contains:

LIBS += -lopencv_nonfree
CONFIG += link_pkgconfig
PKGCONFIG += opencv

so my makefile contains:

LIBS          = $(SUBLIBS) -lopencv_nonfree [...]  /usr/local/lib/libopencv_nonfree.so [...]

There is no error when loading the libraries.

I've installed opencv using those instructions.