Crop image from camera does not work correctly

Question

I am using openCV to capture and save the output image with fixed size (92 by 112). It will capture the frames of video and crop them. However, the output image did not show correct size (some time are 147 by 147, 140 by 140...). What is problem in my code. There are crop image code and whole code. Thanks in advance

CROP image

crop = frame(roi_b);
resize(crop, res, Size(92, 112), 0, 0, INTER_LINEAR); // This will be needed later while saving images        

cvtColor(crop, gray, CV_BGR2GRAY); // Convert cropped image to Grayscale

Let see whole code to view the meaning of parameters

#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"

#include <iostream>
#include <stdio.h>

using namespace std;
using namespace cv;

// Function Headers
void detectAndDisplay(Mat frame);

// Global variables
// Copy this file from opencv/data/haarscascades to target folder
string face_cascade_name = "haarcascade_frontalface_alt.xml";
CascadeClassifier face_cascade;
string window_name = "Capture - Face detection";
int filenumber; // Number of file to be saved
string filename;

// Function main
int main(void)
{
    VideoCapture capture(0);

    if (!capture.isOpened())  // check if we succeeded
        return -1;

    // Load the cascade
    if (!face_cascade.load(face_cascade_name))
    {
        printf("--(!)Error loading\n");
        return (-1);
    };

    // Read the video stream
    Mat frame;

    for (;;)
    {
        capture >> frame;

        // Apply the classifier to the frame
        if (!frame.empty())
        {
            detectAndDisplay(frame);
        }
        else
        {
            printf(" --(!) No captured frame -- Break!");
            break;
        }

        int c = waitKey(10);

        if (27 == char(c))
        {
            break;
        }
    }

    return 0;
}

// Function detectAndDisplay
void detectAndDisplay(Mat frame)
{
    std::vector<Rect> faces;
    Mat frame_gray;
    Mat crop;
    Mat res;
    Mat gray;
    string text;
    stringstream sstm;

    cvtColor(frame, frame_gray, COLOR_BGR2GRAY);
    equalizeHist(frame_gray, frame_gray);

// Detect faces
    face_cascade.detectMultiScale(frame_gray, faces, 1.1, 2, 0 | CASCADE_SCALE_IMAGE, Size(30, 30));

// Set Region of Interest
    cv::Rect roi_b;
    cv::Rect roi_c;

    size_t ic = 0; // ic is index of current element
    int ac = 0; // ac is area of current element

    size_t ib = 0; // ib is index of biggest element
    int ab = 0; // ab is area of biggest element

    for (ic = 0; ic < faces.size(); ic++) // Iterate through all current elements (detected faces)

    {
        roi_c.x = faces[ic].x;
        roi_c.y = faces[ic].y;
        roi_c.width = (faces[ic].width);
        roi_c.height = (faces[ic].height);

        ac = roi_c.width * roi_c.height; // Get the area of current element (detected face)

        roi_b.x = faces[ib].x;
        roi_b.y = faces[ib].y;
        roi_b.width = (faces[ib].width);
        roi_b.height = (faces[ib].height);

        ab = roi_b.width * roi_b.height; // Get the area of biggest element, at beginning it is same as "current" element

        if (ac > ab)
        {
            ib = ic;
            roi_b.x = faces[ib].x;
            roi_b.y = faces[ib].y;
            roi_b.width = (faces[ib].width);
            roi_b.height = (faces[ib].height);
        }

        crop = frame(roi_b);
        resize(crop, res, Size(92, 112), 0, 0, INTER_LINEAR); // This will be needed later while saving images        

        cvtColor(crop, gray, CV_BGR2GRAY); // Convert cropped image to Grayscale

        // Form a filename
        filename = "";
        stringstream ssfn;
        ssfn << filenumber << ".pgm";
        filename = ssfn.str();
        filenumber++;

        imwrite(filename, gray);

        Point pt1(faces[ic].x, faces[ic].y); // Display detected faces on main window - live stream from camera
        Point pt2((faces[ic].x + faces[ic].height), (faces[ic].y + faces[ic].width));
        rectangle(frame, pt1, pt2, Scalar(0, 255, 0), 2, 8, 0);
    }

// Show image
    sstm << "Crop area size: " << roi_b.width << "x" << roi_b.height << " Filename: " << filename;
    text = sstm.str();

    putText(frame, text, cvPoint(30, 30), FONT_HERSHEY_COMPLEX_SMALL, 0.8, cvScalar(0, 0, 255), 1, CV_AA);
    imshow("original", frame);

    if (!crop.empty())
    {
        imshow("detected", crop);
    }
    else
        destroyWindow("detected");
}

Answer 1

You get it wrong in the 'resize image' part. The cv::resize is defined as

void resize(InputArray src, OutputArray dst, Size dsize, double fx=0, double fy=0, int interpolation=INTER_LINEAR )

thus in your code, when you call

resize(crop, res, Size(92, 112), 0, 0, INTER_LINEAR);

the resulted resized image is stored in 'res', not in 'crop'.

The correct code for the crop part is:

crop = frame(roi_b);
resize(crop, res, Size(92, 112), 0, 0, INTER_LINEAR); // This will be needed later while saving images

cvtColor(res, gray, CV_BGR2GRAY); // Convert cropped image to Grayscale

and 'gray' is the grayscale resulted of your resized cropped image.