Pixels Not Set Properly In OpenCV - c++

Question

I am trying to make an image that is completely black except for a white rectangle at the centre of the image. However, on my first attempt, I got a weird result so I changed my code to nail down the problem.
So with for loops, I tried to set all the horizontal pixels at the centre to white to draw a white line across the image. Below is my code.
//--Block Mask--//
block_mask = cv::Mat::zeros(image_height, image_width, CV_8UC3);
int img_height = block_mask.rows;
int img_width = block_mask.cols;
for (int row = (img_height / 2); row < ((img_height / 2) + 1); row++)
{
for (int column = 0; column < img_width; column++)
{
block_mask.at<uchar>(row, column) = 255;
}
}
cv::namedWindow("Block Mask", CV_WINDOW_AUTOSIZE);
cv::imshow("Block Mask", block_mask);
img_height = 1080
img_width = 1920
image_height and image_width are defined from another image.
With this code I expected to see a white line drawn across the entire image, however, the white line extends only part way across the image. See the image below.
To troubleshoot I made a variable to count the iterations of the inner for loop and it counted up to 1920 as I expected it to. This leaves me wondering if it is something to do with the image being displayed? When simply setting individual pixels (not in loops) to white past where the line comes to, no results can be seen either.
I am at a loss as to what is going on here so any help, or perhaps a better way of achieving this, would be greatly appreciated.

Answer 1

Solved: The image block_mask is a three channel BGR image as it was created with the type CV_8UC3. However, when setting the pixel values to white the type uchar was used. Moreover, this was set to a signal integer type of value 255.
To properly set the colour of each pixel all three channels must be set. This can be achieved using a cv::Vec3b type variable that contains values for each channel and can be individually set. This can be done by:
cv::Vec3b new_pixel_colour;
new_pixel_colour[0] = 255; //Blue channel
new_pixel_colour[1] = 255; //Green channel
new_pixel_colour[2] = 255; //Red channel
From here, pixels can be assigned with this variable to change their colour, making sure to change the type in the .at operator to cv::Vec3b also. The corrected code is below.
//--Block Mask--//
block_mask = cv::Mat::zeros(image_height, image_width, CV_8UC3);
cv::Vec3b new_pixel_colour;
new_pixel_colour[0] = 255; //Blue channel
new_pixel_colour[1] = 255; //Green channel
new_pixel_colour[2] = 255; //Red channel
int img_height = block_mask.rows;
int img_width = block_mask.cols;
for (int row = (img_height / 2); row < ((img_height / 2) + 1); row++)
{
for (int column = 0; column < img_width; column++)
{
block_mask.at<cv::Vec3b>(row, column) = new_pixel_colour;
}
}
cv::namedWindow("Block Mask", CV_WINDOW_AUTOSIZE);
cv::imshow("Block Mask", block_mask);
An alternative solution for drawing is using the in-buit drawing functions of OpenCV. Specifically, for drawing a rectangle the OpenCV function cv::rectangle() can be used. A tutorial on basic drawing in OpenCV can be found here: https://docs.opencv.org/master/d3/d96/tutorial_basic_geometric_drawing.html