openCv crop image

Question

I running into problems with my openCv IplImage cropping. Assuming both tmp and img are IplImage* . Using the code:

printf("Orig dimensions: %dx%d\n", img->width, img->height);
cvSetImageROI(img, cvRect(0, 0,500,500));
tmp = cvCreateImage(cvGetSize(img),img->depth,img->nChannels);
cvCopy(img, tmp, NULL);
cvResetImageROI(img);
img = cvCloneImage(tmp);
printf("Orig dimensions after crop: %dx%d\n", tmp->width, tmp->height);

when I use the the cvRect above I'll get an image cropped with a size of 500 x500 as expected, however when i use the rect (400,400,500,500) I'll get an image with the size 500 X 320.

Answer 1

cvRect is defined as ( int x, int y, int width, int height ), not (int left, int top, int right, int bottom). So you are selecting a 500x500 region starting at the point (x,y) = (400,400). I am guessing your image height is 720 ;).

Answer 2

Image cropping using OpenCV

Mat image=imread("image.png",1);

int startX=200,startY=200,width=100,height=100

Mat ROI(image, Rect(startX,startY,width,height));

Mat croppedImage;

// Copy the data into new matrix
ROI.copyTo(croppedImage);

imwrite("newImage.png",croppedImage);

Answer 3

Try this.It's work.

                     IplImage *source_image;
                     IplImage *cropped_Image1;

                     cout << "Width:" << source_image->width << " pixels" << endl;
                     cout << "Height:" << source_image->height << " pixels" << endl;
                     int width = source_image->width;
                     int lenght = source_image->height;

                     cv::Rect roi;
                     roi.x = 1200; //1200     // 950
                     roi.y = 355; //350      //150 
                     roi.width = 2340; //2360          //2750
                     roi.height = 1425;  //1235 /2500         //2810   //2465 fully braille sheet


                     cropped_Image1 = cvCreateImage(cvSize(roi.width, roi.height), source_image->depth, source_image->nChannels);
                     cvSetImageROI(source_image, roi);
                     cvCopy(source_image, cropped_Image1);
                     cvResetImageROI(source_image);
                     cvShowImage("Cropped Image", cropped_Image1);
                     cvSaveImage("1_cropped.jpg", cropped_Image1);