change image brightness with c++ - c++

Question

i want to change brightness of image, only accessing pixel value.
not using opencv function(ex. convertTo)
input : image , num
num means constant value for brightness
here is my code and result looks wierd.
Is there any problem?
original
result
cv::Mat function(cv::Mat img, int num){
cv::Mat output;
output = cv::Mat::zeros(img.rows, img.cols, img.type());
for (int i = 0; i < img.rows; i++)
{
for (int j = 0; j < img.cols; j++)
{
for (int c = 0; c < img.channels(); c++)
{
output.at<cv::Vec3b>(i, j)[c] = img.at<cv::Vec3b>(i, j)[c] + num;
if (output.at<cv::Vec3b>(i, j)[c] > 255){
output.at<cv::Vec3b>(i, j)[c] = 255;
}
else if (output.at<cv::Vec3b>(i, j)[c] < 0)
{
output.at<cv::Vec3b>(i, j)[c] = 0;
}
}
}
}
cv::imshow("output", output);
cv::waitKey(0);
return img;
}

Answer 1

not using opencv function
that's somewhat silly, since your code is already using opencv's data structures.
trying to do so, you also reinvented the wheel, albeit a slightly square one ...
Check for overflow before assigning to output.
yea that's the problem. correct way to do it would be: assign the sum to something larger than uchar, and then check
else if (output.at<cv::Vec3b>(i, j)[c] < 0)
this will never happen, try to understand why
but please note, that your whole code (triple loop, omg !!!) could be rewritten as a simple:
Mat output = img + Scalar::all(num);
(faster, safer, & this will also saturate correctly !)