Capturing grey screen from camera using OpenCV

Question

I have a problem with capturing image from my web cam (built in hp dv5). The only result is grey screen. Here's the code:

#include "StdAfx.h"
#include "cv.h"
#include "highgui.h"
#include <stdio.h>  // A Simple Camera Capture Framework
int main() { 
    CvCapture* capture;
    for (int i = -1;i < 100;i++) {
        capture = cvCaptureFromCAM( i );   
        if( !capture ) {
            fprintf( stderr, "ERROR: capture is NULL \n" ); 

        } else {
            break;
        }
    }
    //cvSetCaptureProperty( capture, CV_CAP_PROP_FPS,15);
    //cvSetCaptureProperty( capture, CV_CAP_PROP_FRAME_WIDTH, 160 );
    //cvSetCaptureProperty( capture, CV_CAP_PROP_FRAME_HEIGHT, 120 );
    // Create a window in which the captured images will be presented   
    cvNamedWindow( "mywindow", CV_WINDOW_AUTOSIZE );    // Show the image captured from the camera in the window and repeat   
    while( 1 ) {     // Get one frame     
        IplImage* frame = cvQueryFrame( capture );
        cvGrabFrame(capture);
        frame = cvRetrieveFrame(capture);

        if( !frame ) {
            fprintf( stderr, "ERROR: frame is null...\n" );
            getchar();
            break;
        } else {
            fprintf( stderr, "OK\n" );  
        }
        cvShowImage( "mywindow", frame );     // Do not release the frame!      
        //If ESC key pressed, Key=0x10001B under OpenCV 0.9.7(linux version),     //remove higher bits using AND operator     
        int c = cvWaitKey(100);
    }    // Release the capture device housekeeping   
    cvReleaseCapture( &capture );   
    cvDestroyWindow( "mywindow" );   
    return 0; 
}

It's modified code from OpenCV's wiki. I know that finding the camera in this way its crazy but it didn't work with -1 or 0. I added some additional properties (commented already) but it didn't work anyway. Thanks in advance :) Greetings, Chris

Answer 1

I encapsulate the image capturing into a class 'Capture', maybe you could give it a try. Below are main.cpp, capture.h and capture.cpp, enjoy it :)

main.cpp

#include "capture.h"
#include <string>
using namespace std;

    int main(int argc, char* argv[]) {

        // --- read file --- //
        //Capture capture("test.wmv", windowSize, windowSize);

        // --- read camera --- //
        CvSize windowSize = cvSize(640, 480);
        Capture capture(2, windowSize, windowSize);

        while (1) {

            capture.captureNext();

            for (int i = 0; i < capture.channelNum; ++i) {

                ostringstream oss;
                oss << i;
                string winName = "WINDOW-" + (oss.str());

                cvShowImage(winName.c_str(), capture.channelframeList[i]);

            }

            int c = cvWaitKey(30);
            if ( (char) c == 27 ) { // 'Esc' to terminate

                break;
            }

        }

        return 0;

    }

caputure.h

#ifndef _CAPTURE_H_
#define _CAPTURE_H_

#include <cv.h>
#include <cxcore.h>
#include <highgui.h>
#include <string>
#include <vector>
#include <iostream>
using namespace std;

enum VIDEO_TYPE {
    CAMMERA = 0,
    VIDEOFILE
};

class Capture {

public:

    Capture(int num, CvSize dispSize, CvSize resolutionSize);
    Capture(string fileName, CvSize dispSize, CvSize resolutionSize);

    int channelNum;
    vector<IplImage*> channelframeList;

    void captureNext();

    ~Capture();

private:

    string m_fileName;
    vector<CvCapture*> m_channelList;

    CvSize m_resolutioSize;
    CvSize m_displaySize;

    void initChannelList(VIDEO_TYPE type);
    void initChannelImgList(CvSize sz);
    IplImage* getNextVideoFrame(CvCapture* pCapture);


};

#endif

capture.cpp

#include "capture.h"

Capture::Capture(int num, CvSize dispSize, CvSize resolutionSize) {

    channelNum = num;
    m_fileName = "";
    m_resolutioSize = resolutionSize;

    m_channelList.resize(channelNum);
    channelframeList.resize(channelNum);

    initChannelList(CAMMERA);
    initChannelImgList(dispSize);

}

Capture::Capture(string fileName, CvSize dispSize, CvSize resolutionSize) {

    channelNum = 1;
    m_fileName = fileName;
    m_resolutioSize = resolutionSize;

    m_channelList.resize(channelNum);
    channelframeList.resize(channelNum);

    initChannelList(VIDEOFILE);
    initChannelImgList(dispSize);

}

void Capture::captureNext() {

    for (int i = 0; i < channelNum; ++i) {

        IplImage* nextFrame = getNextVideoFrame(m_channelList[i]);
        IplImage* channelFrame = channelframeList[i];
        cvResize(nextFrame, channelFrame);

    }
}

void Capture::initChannelList(VIDEO_TYPE type) {

    if (type == CAMMERA) {

        for (int i = 0; i < channelNum; ++i) {

            m_channelList[i] = cvCreateCameraCapture(i);

            //set resolution
            cvSetCaptureProperty(m_channelList[i], CV_CAP_PROP_FRAME_WIDTH, m_resolutioSize.width);
            cvSetCaptureProperty(m_channelList[i], CV_CAP_PROP_FRAME_HEIGHT, m_resolutioSize.height);

            if ( !(m_channelList[i]) ) {
                cout << "failed to initialize video capture" << endl;
                exit(EXIT_FAILURE);

            }
        }

    } else if (type == VIDEOFILE) {

        const char* fileNameChar = m_fileName.c_str();
        m_channelList[0] = cvCreateFileCapture(fileNameChar);

        if ( !(m_channelList[0]) ) {
            cout << "failed to initialize video capture" << endl;
            exit(EXIT_FAILURE);
        }

    }

}

void Capture::initChannelImgList(CvSize sz) {

    for (int i = 0; i < channelNum; ++i)
        channelframeList[i] = cvCreateImage(sz, 8, 3);

}

IplImage* Capture::getNextVideoFrame(CvCapture* pCapture) {

    IplImage* nextFrame = cvQueryFrame(pCapture);

    if (!nextFrame) {
        cout << "failed to get a video frame" << endl;
        exit(EXIT_FAILURE);
    }

    return nextFrame;

}

Capture::~Capture() {

    for (int i = 0; i < channelNum; ++i) {
        cvReleaseImage( &(channelframeList[i]) );
        cvReleaseCapture(&m_channelList[i]);
    }

}

Answer 2

I was getting a grey screen when trying to use the camera and the Python wrapper for OpenCV inside Windows.

Tracing through with the debugger I found that it was spotting 2 camera interfaces called "Google Camera Adapter 0" and "Google Camera Adapter 1".

I was able to fix the camera input by:

1. Going to "Add/Remove programs" and uninstalling "Google Talk Plugin"
2. Unplugging the Webcam and plugging it back in again so that new drivers were installed.

It now works fine for me.

(Note that I do not know whether step 1 was important, and it may cause other things to break, so I would recommend you try step 2 first...)

Answer 3

That's the wrong approach to solve your problem. So here I go: first, isolate which camera_id is currently working. Which one is it? -1 , 0 , 1 ?

Second: please check the return of the functions, I beg you! If something is wrong, you'll never know.

CvCapture* capture = NULL;
if ((capture = cvCaptureFromCAM(-1)) == NULL)
{
    fprintf(stderr, "ERROR: capture is NULL \n"); 
    return -1;
}

cvNamedWindow("mywindow", CV_WINDOW_AUTOSIZE);

cvQueryFrame(capture); // Sometimes needed to get correct data

while (1) 
{     
    IplImage* frame = cvQueryFrame(capture); // check return
    {
        fprintf( stderr, "ERROR: cvQueryFrame failed\n");
        break;
    }

    // At this point you already have the frame! There's no need to
    // repeat the thing 10x with cvGrabFrame and cvRetrieveFrame. 
    // You are probably sabotaging yourself doing this multiple times.

    cvShowImage("mywindow", frame); // Do not release the frame!

    int key = cvWaitKey(10);
    if (key  == 0x1b)
        break;
}    

cvReleaseCapture(&capture);   
cvDestroyWindow("mywindow");   

return 0;