OpenCV crop live feed from camera - c++

Question

How can I get properly one resolution feed from camera in OpenCV (640x320) but cut it into half and display only one half of the frame (320x240). So not to scale down, but to actually crop. I am using OpenCV 2.4.5, VS2010 and C++
This quite standard code gets 640x480 input resolution and I made some changes to crop resolution to 320x240. Should I use Mat instead of IplImage, and if so what would be the best way?
#include "stdafx.h"
#include <iostream>
#include "opencv2/core/core.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
using namespace std;
char key;
int main()
{
cvNamedWindow("Camera_Output", 1); //Create window
CvCapture* capture = cvCaptureFromCAM(1); //Capture using camera 1 connected to system
cvSetCaptureProperty( capture, CV_CAP_PROP_FRAME_WIDTH, 640 );
cvSetCaptureProperty( capture, CV_CAP_PROP_FRAME_HEIGHT, 480 );
while(1){ //Create loop for live streaming
IplImage* framein = cvQueryFrame(capture); //Create image frames from capture
/* sets the Region of Interest - rectangle area has to be __INSIDE__ the image */
cvSetImageROI(framein, cvRect(0, 0, 320, 240));
/* create destination image - cvGetSize will return the width and the height of ROI */
IplImage *frameout = cvCreateImage(cvGetSize(framein), framein->depth, framein->nChannels);
/* copy subimage */
cvCopy(framein, frameout, NULL);
/* always reset the Region of Interest */
cvResetImageROI(framein);
cvShowImage("Camera_Output", frameout); //Show image frames on created window
key = cvWaitKey(10); //Capture Keyboard stroke
if (char(key) == 27){
break; //ESC key loop will break.
}
}
cvReleaseCapture(&capture); //Release capture.
cvDestroyWindow("Camera_Output"); //Destroy Window
return 0;
}

Answer 1

I think you don't check whether you are getting a CvCapture. On my system with only one camera your code doesn't work because you query camera 1. But the first camera should be 0 Thus change this code.
CvCapture* capture = cvCaptureFromCAM(1); //Capture using camera 1 connected to system
to (note I change 1 to 0):
CvCapture* capture = cvCaptureFromCAM(0); //Capture using camera 1 connected to system
if (! capture ){
/*your error handling*/
}
Further than that your code seems to be working for me. You might also check the other pointer values whether you are not getting NULL.

Answer 2

You can easily crop a video by calling the following function.
cvSetMouseCallback("image", mouseHandler, NULL);
The mouseHandler function is like that.
void mouseHandler(int event, int x, int y, int flags, void* param){
if (event == CV_EVENT_LBUTTONDOWN && !drag)
{
/* left button clicked. ROI selection begins */
select_flag=0;
point1 = Point(x, y);
drag = 1;
}
if (event == CV_EVENT_MOUSEMOVE && drag)
{
/* mouse dragged. ROI being selected */
Mat img1 = img.clone();
point2 = Point(x, y);
rectangle(img1, point1, point2, CV_RGB(255, 0, 0), 3, 8, 0);
imshow("image", img1);
}
if (event == CV_EVENT_LBUTTONUP && drag)
{
point2 = Point(x, y);
rect = Rect(point1.x,point1.y,x-point1.x,y-point1.y);
drag = 0;
roiImg = img(rect);
}
if (event == CV_EVENT_LBUTTONUP)
{
/* ROI selected */
select_flag = 1;
drag = 0;
}
}
For the details you can visit the following link.:How to Crop Video from Webcam using OpenCV

Answer 3

this is easy in python... but the key idea is that cv2 arrays can be referenced and sliced. all you need is a slice of framein.
the following code takes a slice from (0,0) to (320,240). note that numpy arrays are indexed with column priority.
# Required modules
import cv2
# Constants for the crop size
xMin = 0
yMin = 0
xMax = 320
yMax = 240
# Open cam, decode image, show in window
cap = cv2.VideoCapture(0) # use 1 or 2 or ... for other camera
cv2.namedWindow("Original")
cv2.namedWindow("Cropped")
key = -1
while(key < 0):
success, img = cap.read()
cropImg = img[yMin:yMax,xMin:xMax] # this is all there is to cropping
cv2.imshow("Original", img)
cv2.imshow("Cropped", cropImg)
key = cv2.waitKey(1)
cv2.destroyAllWindows()

Answer 4

Working Example of cropping Faces from live camera
void CropFaces::DetectAndCropFaces(Mat frame, string locationToSaveFaces) {
std::vector<Rect> faces;
Mat frame_gray;
// Convert to gray scale
cvtColor(frame, frame_gray, COLOR_BGR2GRAY);
// Equalize histogram
equalizeHist(frame_gray, frame_gray);
// Detect faces
face_cascade.detectMultiScale(frame_gray, faces, 1.1, 3,
0 | CASCADE_SCALE_IMAGE, Size(30, 30));
// Iterate over all of the faces
for (size_t i = 0; i < faces.size(); i++) {
// Find center of faces
Point center(faces[i].x + faces[i].width / 2, faces[i].y + faces[i].height / 2);
Mat face = frame_gray(faces[i]);
std::vector<Rect> eyes;
Mat croppedRef(frame, faces[i]);
cv::Mat cropped;
// Copy the data into new matrix
croppedRef.copyTo(cropped);
string fileName = locationToSaveFaces+ "\\face_" + to_string(faces[i].x) + ".jpg";
resize(cropped, cropped, Size(65, 65));
imwrite(fileName, cropped);
}
// Display frame
imshow("DetectAndSave", frame);
}
void CropFaces::PlayVideoForCropFaces(string locationToSaveFaces) {
VideoCapture cap(0); // Open default camera
Mat frame;
face_cascade.load("haarcascade_frontalface_alt.xml"); // load faces
while (cap.read(frame)) {
DetectAndCropFaces(frame, locationToSaveFaces); // Call function to detect faces
if (waitKey(30) >= 0) // pause
break;
}
}