From IPLImage to Mat

Question

I am well acquainted with the IPL-image format used in OpenCV 1.1. However i am using the latest 2.4 version and want to switch to the C++ interface of OpenCV. Here is the method by which i access the pixels in an image:

int step = img->widthStep;
int height = img->height;
int width = img->width;
unsigned char* data = (unsigned char*) img->imageData;

for (int i=0; i<height; i++)
{
    for (int j=0; j<step; j+=3)          // 3 is the number of channels.
    {
        if (data[i*step + j] > 200)      // For blue
            data[i*step + j] = 255;

        if (data[i*step + j + 1] > 200)  // For green
            data[i*step + j + 1] = 255;

        if (data[i*step + j + 2] > 200)  // For red
            data[i*step + j + 2] = 255;
    }
} 

I need help for converting this exact code block with the Mat structure. I find several functions here and there but it will be very helpful if i get the exact conversion of the above few lines as a whole.

Answer 1

// Mat mat; // a bgr, CV_8UC3 mat

for (int i=0; i<mat.rows; i++)
{
    // get a new pointer per row. this replaces fumbling with widthstep, etc.
    // also a pointer to a Vec3b pixel, so no need for channel offset, either
    Vec3b *pix = mat.ptr<Vec3b>(i); 
    for (int j=0; j<mat.cols; j++)
    {
        Vec3b & p = pix[j];
        if ( p[0] > 200 ) p[0] = 255;
        if ( p[1] > 200 ) p[1] = 255;
        if ( p[2] > 200 ) p[2] = 255;
    }
}