Fastest way to extract individual pixel data?

Question

I have to get information about the scalar value of a lot of pixels on a gray-scale image using OpenCV. It will be traversing hundreds of thousands of pixels so I need the fastest possible method. Every other source I've found online has been very cryptic and hard to understand. Is there a simple line of code that should just hand a simple integer value representing the scalar value of the first channel (brightness) of the image?

Answer 1

for (int row=0;row<image.height;row++) {
    unsigned char *data = image.ptr(row);
    for (int col=0;col<image.width;col++) {
       // then use *data for the pixel value, assuming you know the order, RGB etc           
       // Note 'rgb' is actually stored B,G,R
       blue= *data++;
       green = *data++;
       red = *data++;
    }
}

You need to get the data pointer on each new row because opencv will pad the data to 32bit boundary at the start of each row

Answer 2

With regards to Martin's post, you can actually check if the memory is allocated continuously using the isContinuous() method in OpenCV's Mat object. The following is a common idiom for ensuring the outer loop only loops once if possible:

#include <opencv2/core/core.hpp>

using namespace cv;

int main(void)
{

    Mat img = imread("test.jpg");
    int rows = img.rows;
    int cols = img.cols;

    if (img.isContinuous())
    {
        cols = rows * cols; // Loop over all pixels as 1D array.
        rows = 1;
    }

    for (int i = 0; i < rows; i++)
    {
        Vec3b *ptr = img.ptr<Vec3b>(i);
        for (int j = 0; j < cols; j++)
        {
            Vec3b pixel = ptr[j];
        }
    }

    return 0;
}