Bug with pixel access in OpenCV 2.x

Question

I\'m having trouble trying to find out how to access rgb pixel in the new version (2.x) of OpenCV. I tried using a mix of the old and the new method but without success.

Answer 1

Inside your loop, you can do:

img.at<Vec3b>(j,i)[0] = 0;    // Blue Channel
img.at<Vec3b>(j,i)[1] = 0;    // Green Channel
img.at<Vec3b>(j,i)[2] = 0;    // Red Channel

Is this what you wanted or I understood incorrectly?

Answer 2

I tested out your code, and I found the bug. You multiplied the column index by 3 (i * 3), but it's also necessary to multiply the row index by 3 (j * img.cols * 3).

I replaced j * img.cols with j * img.cols * 3:

for (int j = 0; j < img.rows; j++)
{
    for (int i = 0; i < img.cols; i++)
    {
        img.data[j * img.cols * 3 + i*3 + 0] = (uchar)0; //B
        //img.data[j * img.cols * 3 + i*3 + 1] = (uchar)0; //G
        //img.data[j * img.cols * 3 + i*3 + 2] = (uchar)0; //R
    }
}

Let's try an example.

Example image (from MIT pedestrian dataset):

Result using OP's code:

Result using the revised code (with j * img.cols * 3):