c++/openCV : cvConvert 返回空值

我在 c++ 中找到了一个代码，可以找到两个图像之间的差异。它正常加载图像，但在之后

cvConvert(img1_temp, img1); 

img1 保持为 NULL，最终结果（我打印的结果）是 nan。我使用的输入图像是 png。知道为什么会这样吗？

int main(int argc, char** argv)
{
    if(argc!=3)
            return -1;
    // default settings
    double C1 = 6.5025, C2 = 58.5225;
    IplImage
            *img1=NULL, *img2=NULL, *img1_img2=NULL,
            *img1_temp=NULL, *img2_temp=NULL,
            *img1_sq=NULL, *img2_sq=NULL,
            *mu1=NULL, *mu2=NULL,
            *mu1_sq=NULL, *mu2_sq=NULL, *mu1_mu2=NULL,
            *sigma1_sq=NULL, *sigma2_sq=NULL, *sigma12=NULL,
            *ssim_map=NULL, *temp1=NULL, *temp2=NULL, *temp3=NULL;

    /***************************** INITS **********************************/
    img1_temp = cvLoadImage(argv[1]);
    img2_temp = cvLoadImage(argv[2]);
    if(img1_temp==NULL || img2_temp==NULL)
            return -1;
    int x=img1_temp->width, y=img1_temp->height;
    int nChan=img1_temp->nChannels, d=IPL_DEPTH_32F;
    CvSize size = cvSize(x, y);
    img1 = cvCreateImage( size, d, nChan);
    img2 = cvCreateImage( size, d, nChan);
    cvConvert(img1_temp, img1);
    cvConvert(img2_temp, img2);
    cvReleaseImage(&img1_temp);
    cvReleaseImage(&img2_temp);
    img1_sq = cvCreateImage( size, d, nChan);
    img2_sq = cvCreateImage( size, d, nChan);
    img1_img2 = cvCreateImage( size, d, nChan);
    cvPow( img1, img1_sq, 2 );
    cvPow( img2, img2_sq, 2 );
    cvMul( img1, img2, img1_img2, 1 );
    mu1 = cvCreateImage( size, d, nChan);
    mu2 = cvCreateImage( size, d, nChan);
    mu1_sq = cvCreateImage( size, d, nChan);
    mu2_sq = cvCreateImage( size, d, nChan);
    mu1_mu2 = cvCreateImage( size, d, nChan);

    sigma1_sq = cvCreateImage( size, d, nChan);
    sigma2_sq = cvCreateImage( size, d, nChan);
    sigma12 = cvCreateImage( size, d, nChan);
    temp1 = cvCreateImage( size, d, nChan);
    temp2 = cvCreateImage( size, d, nChan);
    temp3 = cvCreateImage( size, d, nChan);
    ssim_map = cvCreateImage( size, d, nChan);
    /*************************** END INITS **********************************/
// PRELIMINARY COMPUTING
    cvSmooth( img1, mu1, CV_GAUSSIAN, 11, 11, 1.5 );
    cvSmooth( img2, mu2, CV_GAUSSIAN, 11, 11, 1.5 );
    cvPow( mu1, mu1_sq, 2 );
    cvPow( mu2, mu2_sq, 2 );
    cvMul( mu1, mu2, mu1_mu2, 1 );

    cvSmooth( img1_sq, sigma1_sq, CV_GAUSSIAN, 11, 11, 1.5 );
    cvAddWeighted( sigma1_sq, 1, mu1_sq, -1, 0, sigma1_sq );
    cvSmooth( img2_sq, sigma2_sq, CV_GAUSSIAN, 11, 11, 1.5 );
    cvAddWeighted( sigma2_sq, 1, mu2_sq, -1, 0, sigma2_sq );
    cvSmooth( img1_img2, sigma12, CV_GAUSSIAN, 11, 11, 1.5 );
    cvAddWeighted( sigma12, 1, mu1_mu2, -1, 0, sigma12 );

    //////////////////////////////////////////////////////////////////////////
    // FORMULA
    // (2*mu1_mu2 + C1)
    cvScale( mu1_mu2, temp1, 2 );
    cvAddS( temp1, cvScalarAll(C1), temp1 );
    // (2*sigma12 + C2)
    cvScale( sigma12, temp2, 2 );
    cvAddS( temp2, cvScalarAll(C2), temp2 );
    // ((2*mu1_mu2 + C1).*(2*sigma12 + C2))
    cvMul( temp1, temp2, temp3, 1 );
    // (mu1_sq + mu2_sq + C1)
    cvAdd( mu1_sq, mu2_sq, temp1 );
    cvAddS( temp1, cvScalarAll(C1), temp1 );
    // (sigma1_sq + sigma2_sq + C2)
    cvAdd( sigma1_sq, sigma2_sq, temp2 );
    cvAddS( temp2, cvScalarAll(C2), temp2 );
    // ((mu1_sq + mu2_sq + C1).*(sigma1_sq + sigma2_sq + C2))
    cvMul( temp1, temp2, temp1, 1 );
    // ((2*mu1_mu2 + C1).*(2*sigma12 + C2))./((mu1_sq + mu2_sq + C1).*(sigma1_sq +    sigma2_sq + C2))
    cvDiv( temp3, temp1, ssim_map, 1 );

    CvScalar index_scalar = cvAvg( ssim_map );
    std::cout << "(R, G & B SSIM index)" << std::endl ;
    std::cout << index_scalar.val[2] * 100 << "%" << std::endl ;
    std::cout << index_scalar.val[1] * 100 << "%" << std::endl ;
    std::cout << index_scalar.val[0] * 100 << "%" << std::endl ;
    return 0;
}