Converting OpenCV

Hints for Converting OpenCV C/C++ code to JavaCV.

Created Mar 24, 2010 by shervin.emami

Hints for Converting OpenCV C/C++ code to JavaCV

JavaCV is designed so that most OpenCV functions can be used in JavaCV in the exact same way as they would in C/C++ code. However there are many functions that need to be treated differently, such as when a pointer to a struct needs to be passed to OpenCV. So here is a basic guide to help anyone port their OpenCV code to JavaCV.

How to use certain OpenCV functions in JavaCV

Here are examples for using the following functions in JavaCV:

OpenCV Code:	JavaCV Equivalent:
int i = cvRound(f);	int i = (int)Math.round(f);
cvNamedWindow("Out");	CanvasFrame canvas = new CanvasFrame("Out");
cvShowImage("Out", img);	canvas.showImage(img);
cvDestroyWindow("Out");	canvas.dispose();
CvCapture *cap = cvCreateFileCapture("lena.avi");	OpenCVFrameGrabber grabber = new OpenCVFrameGrabber("lena.avi"); grabber.start();
CvCapture *cap = cvCreateCameraCapture(0);	OpenCVFrameGrabber grabber = new OpenCVFrameGrabber(0); grabber.start();
IplImage *img = cvQueryFrame(cap); if (!img) return;	IplImage img = grabber.grab(); if (img == null) return;
cvReleaseImage(&img);	cvReleaseImage(img);
int key = cvWaitKey(10); if (key) return;	KeyEvent key = canvas.waitKey(10); if (key != null) return;
CvVideoWriter *videoWriter = cvCreateVideoWriter("out.avi", CV_FOURCC('M','J','P','G'), 15, cvSize(320,240), 1);	FrameRecorder recorder = new OpenCVFrameRecorder("out.avi", 320, 240); recorder.setVideoCodec(CV_FOURCC('M','J','P','G')); recorder.setFrameRate(15); recorder.setPixelFormat(1); recorder.start();
// Look for corners in 'img' that seem good for tracking, // storing them into 'corners' and 'count'. CvPoint2D32f corners[MAX_POINTS]; int count = MAX_POINTS; cvGoodFeaturesToTrack(img, eig, temp, corners, &count, CORNER_QUALITY, CORNER_MIN_DISTANCE, MASK, CORNER_BLOCK_SIZE, USE_HARRIS, K);	// Look for corners in 'img' that seem good for tracking, // storing them into 'corners' and 'count'. // This internally allocates MAX_POINTS points contiguously. // They are accessed using Pointer.position(i). CvPoint2D32f corners = new CvPoint2D32f(MAX_POINTS); int[] count = { MAX_POINTS }; cvGoodFeaturesToTrack(img, eig, temp, corners, count, CORNER_QUALITY, CORNER_MIN_DISTANCE, MASK, CORNER_BLOCK_SIZE, USE_HARRIS, K);
// Use Optical Flow to compare 'prev_img' and 'prev_points' // with 'img', generating 'current_points' and 'status' arrays. char status[MAX_POINTS]; CvPoint2D32f current_points[MAX_POINTS]; CvPoint2D32f prev_points[MAX_POINTS]; cvCalcOpticalFlowPyrLK(prev_img, img, prev_pyramid, pyramid, prev_points, current_points, count, cvSize(OPFLOW_WIN_SIZE, OPFLOW_WIN_SIZE), OPFLOW_PYR_LEVEL, status, null, cvTermCriteria(CV_TERMCRIT_ITER | CV_TERMCRIT_EPS, OPFLOW_MAX_ITERATIONS, OPFLOW_EPSILON), flags);	// Use Optical Flow to compare 'prev_img' and 'prev_points' // with 'img', generating 'current_points' and 'status' arrays. byte[] status = new byte[MAX_POINTS]; CvPoint2D32f current_points = new CvPoint2D32f(MAX_POINTS); CvPoint2D32f prev_points = new CvPoint2D32f(MAX_POINTS); cvCalcOpticalFlowPyrLK(prev_img, img, prev_pyramid, pyramid, prev_points, current_points, count, cvSize(OPFLOW_WIN_SIZE, OPFLOW_WIN_SIZE), OPFLOW_PYR_LEVEL, status, null, cvTermCriteria(CV_TERMCRIT_ITER | CV_TERMCRIT_EPS, OPFLOW_MAX_ITERATIONS, OPFLOW_EPSILON), flags);
// Allocate and fill array, given x[], y[], and z[] arrays. CvPoint3D32f points[nPoints]; for (int i = 0; i < nPoints; i++) { points[i] = cvPoint3D32f(x[i], y[i], z[i]); }	// Allocate and fill points array, given Point3d[] pts array. CvPoint3D32f points = new CvPoint3D32f(nPoints); for (int i = 0; i < nPoints; i++) { points.position(i).put(pts[i].x, pts[i].y, pts[i].z); } // The position is part of the state, but can be reset this way: targetPoints.position(0);
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