Get a pixel by Mat::at

Question

I'm trying to get a pixel from a Mat object. To test I try to draw a diagonal line on a square and expect to get a perfect line crossing from the top left to the down right vertex.

for (int i =0; i<500; i++){
     //I just hard-coded the width (or height) to make the problem more obvious

  (image2.at<int>(i, i)) = 0xffffff;
     //Draw a white dot at pixels that have equal x and y position.
}

The result, however, is not as expected. Here is a diagonal line drawn on a color picture. Here is it on a grayscale picture. Anyone sees the problem?

Answer 1

The problem is that you are trying to access each pixel as int (32 bit per pixel image), while your image is a 3-channel unsigned char (24 bit per pixel image) or a 1-channel unsigned char (8 bit per pixel image) for the grayscale one. You can try to access each pixel like this for the grayscale one

for (int i =0; i<image2.width; i++){
  image2.at<unsigned char>(i, i) = 255;
}

or like this for the color one

for (int i =0; i<image2.width; i++){     
      image2.at<Vec3b>(i, i)[0] = 255;
      image2.at<Vec3b>(i, i)[1] = 255;
      image2.at<Vec3b>(i, i)[2] = 255;
}

Answer 2

(image2.at<int>(i, i)) = 0xffffff;

It looks like your color image is 24bit, but your addressing pixels in terms of int which seems to be 32 bit.