Question

I am using OpenCV via Matlab to detect faces in a video and then do some processing using Matlab. At the moment I do face detection on the IplImage-structured frames (queried by cvQueryFrame) of the video. I save each of the queried frames as a jpg and then use the face coordinates to get the ROI for the required processing. See the portion of code outlining this below.

% After reading in frame from video..
for i=1:size
    img = calllib('highgui210','cvQueryFrame',cvCapture);
    calllib('cxcore210','cvFlip',img,img,1);
    calllib('highgui210', 'cvSaveImage', 'ThisFrame.jpg', img, ptr); 
% Rest of the processing comes here..

This being the case, I feel that there should be an easier and less-crude way to convert an ‘IplImage’ image to a matrix or array in Matlab. Is this a possibility? If yes,how is this done?

Some pointers in this direction would be much appreciated!

Answer 1

Try mexing the following code:

/*
 * Usage:
 *   img = IplImage2mxArray( cvImgPtr, releaseFlag );
 */
void mexFunction( int nout, mxArray* pout[], int nin, const mxArray* pin[]) {
    if ( nin != 2 )
        mexErrMsgTxt("wrong number of inputs");
    if ( nout != 1 )
        mexErrMsgTxt("wrong number of outputs");
    IplImage* cvImg = (IplImage*)mxGetData(pin[0]); // get the pointer
    // allocate the output
    mwSize imgDims[3] = {cvImg->height,  cvImg->width, cvImg->nChannels};
    pout[0] = mxCreateNumericArray( 3, imgDims, mxDOUBLE_CLASS, mxREAL );
    if ( pout[0] == NULL )
        mexErrMsgTxt("out of memeory");
    double* imgP = mxGetPr(pout[0]);
    double divVal = pow(2.0, cvImg->depth) - 1;
    double releaseFlag = mxGetScalar( pin[1] );
    for ( int x = 0 ; x < cvImg->width; x++ ) {
        for ( int y = 0 ; y < cvImg->height; y++ ) {
            CvScalar s = cvGet2D(cvImg, y, x);
            for (int c = 0; c < cvImg->nChannels; c++) {
                imgP[ y + x * cvImg->height + c * cvImg->height * cvImg->width ] = s.val[c] / divVal;
            }
        }
    }
    if ( releaseFlag != 0 ) {
        cvReleaseImage( &cvImg );
    }
}

You’ll end up with a function (mex) IplImage2mxArray, use it in Matlab:

>> cvImg = calllib('highgui210','cvQueryFrame',cvCapture);
>> img = IplImage2mxArray( cvImg, false ); % no cvReleaseImage after cvQueryFrame

Due to internal opencv representations the channels of img might be permuted (BGR instread of RGB). Also note that img might contain four channels – an additional alpha channel.

-Shai