opencv Mat memory access error

Question

I'm trying to write a function to compensate lens vignetting in an OpenCV project. The problem is that when I attempt to access the first pixel in the Mat, I get a memory access error. It's probably a silly error, but I'd appreciate it if you could point it out to me. Here's the function:

void EckOpCam::FlatField(cv::Mat & parmI)
{
// workaround for access violation error
cv::Mat I = parmI;
const int channels = I.channels();
if (flatField.rows == 0)
    MakeFlatFieldMat(I.rows);
// accept only char type matrices
ASSERT(I.depth() == CV_8U);
// make sure the matrices are the same size
ASSERT(I.cols == flatField.cols);
ASSERT(channels == flatField.channels());


// Try the really slow way...
int nRows = I.rows;
int nCols = I.cols;
unsigned int ipix0, ipix1, ipix2;   // B,G,r values of pixel in I
float fpix0, fpix1,fpix2;           // B,G,r values of pixel in flatField
float pix0, pix1, pix2;
switch (channels)
{
case 1:
    for (int y = 0; y < nRows; ++y)
        for (int x = 0; x < nCols; ++x)
        {
            {
                ipix0 = I.at<cv::Vec3b>(x, y)[0];
                fpix0 = flatField.at<cv::Vec3f>(x, y)[0];
                pix0 = ipix0 * fpix0;
                I.at<cv::Vec3b>(x, y)[0] = (pix0 > 255) ? 255 : uchar(pix0);
            }
        }
    break;
case 3:
    for (int y = 0; y < nRows; ++y)
        for (int x = 0; x < nCols; ++x)
        {
            {
                ipix0 = static_cast<uchar>(I.at<cv::Vec3b>(x, y)[0]); //<<< memory access error!
                ipix1 = static_cast<uchar>(I.at<cv::Vec3b>(x, y)[1]);
                ipix2 = static_cast<uchar>(I.at<cv::Vec3b>(x, y)[2]);
                fpix0 = flatField.at<cv::Vec3f>(x, y)[0];
                fpix1 = flatField.at<cv::Vec3f>(x, y)[1];
                fpix2 = flatField.at<cv::Vec3f>(x, y)[2];
                pix0 = ipix0 * fpix0;
                pix1 = ipix1 * fpix1;
                pix2 = ipix2 * fpix2;
                I.at<cv::Vec3b>(x, y)[0] = (pix0 > 255) ? 255 : uchar(pix0);
                I.at<cv::Vec3b>(x, y)[1] = (pix1 > 255) ? 255 : uchar(pix1);
                I.at<cv::Vec3b>(x, y)[2] = (pix2 > 255) ? 255 : uchar(pix2);
            }
        }
}

}

The error is: Exception thrown: read access violation. cv::Vec::operator returned 0x240C37EC4A8.

The address given is exactly the address for I.data. What's wrong? Thanks for your help.

Answer 1

Um... Never mind. It turned out that the exception was real and not a problem with this code. I am using sample code from Point Grey. It converts from a Point Grey ImagePtr to a Mat using a Mat constructor that just copies the pointer to the data. The ImagePtr went out of scope, so the data was invalid when I attempted to use it. Mat.CopyTo() solved the problem. I highly recommend reading this if you need a fast way to solve a similar problem.