openCV get subimage in C

Question

I am using OpenCV for image manipulation in C. Please forgive me if this question is answered in the documentation, but I have found the OpenCV docs to be pretty badly formed and difficult to read.

I have an CvMat* that i have extracted from an image file as below:

CvMat* mat = cvLoadImageM((char*) filename, CV_LOAD_IMAGE_COLOR);

What I need to do is get a subimage of that by cropping out a certain bounded region. A logical command for this might be:

CvMat* subMat = cvGetSubImage(mat, minx, maxx, miny, maxy);

where minx, maxx, miny, and maxy define the boundaries of the cropped region. Is there a built in way to do this easily?

score 8 · Accepted Answer · answered Jul 30 '12 at 16:56

Take a look at http://nashruddin.com/OpenCV_Region_of_Interest_(ROI)/

In which the tutorial does the following on a Region of Interest:

cvSetImageROI(img1, cvRect(10, 15, 150, 250));
IplImage *img2 = cvCreateImage(cvGetSize(img1),
                           img1->depth,
                           img1->nChannels);
cvCopy(img1, img2, NULL);
cvResetImageROI(img1);

OpenCV has built in capabilities for setting the region which you care about and copying that region out of an image, just as you want to achieve.

score 4 · Answer 2 · answered Jun 25 '14 at 12:00

4

If you want a sub-pixel accurate rectangular section of a src image use cvGetRectSubPix or cv::getRectSubPix (this creates an individual copy of all the data, this is not a ROI!)

Example:

cv::Size size(dst_width,dst_height);
cv::Point2f center(src_centerx,src_center_y);
cv::Mat dst;
cv::getRectSubPix(src,size, center,dst,CV_8U);

answered Jun 25 '14 at 12:00

Oliver Zendel

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Oh, btw: for ROIs I use: `cv::Mat newRoi = srcMat(cv::Rect(roiX,roiY,roiW,roiH));` – Oliver Zendel Oct 19 '15 at 08:48

score 2 · Answer 3 · edited May 23 '17 at 10:32

Generally this is done by cropping an ROI (region of interest). This blog post goes into some detail on cropping:

/* load image */
IplImage *img1 = cvLoadImage("elvita.jpg", 1);

/* sets the Region of Interest
   Note that the rectangle area has to be __INSIDE__ the image */
cvSetImageROI(img1, cvRect(10, 15, 150, 250));

/* create destination image
   Note that cvGetSize will return the width and the height of ROI */
IplImage *img2 = cvCreateImage(cvGetSize(img1),
                               img1->depth,
                               img1->nChannels);

/* copy subimage */
cvCopy(img1, img2, NULL);

/* always reset the Region of Interest */
cvResetImageROI(img1);

To convert between IplImage (legacy OpenCV) and cvMat (OpenCV 2.x), simply use the cvMat constructor or look at this question for more methods.

http://stackoverflow.com/questions/14756505/croping-an-image-in-ios-uisng-opencv-face-detect I facing cropping problem in iphone with opencv. Can please have a look.. My ROI are getting wrong I gues.. — 2vision2, Feb 07 '13 at 19:02

openCV get subimage in C

3 Answers3