Using saturate_cast or not - c++

Question

This is a simple program to change contrast and brightness of an image. I have noticed that there is a an another program with one simple difference:saturate_cast is added to code.
And I don't realize what is the reason of doing this and there is no need to converting to unsigned char or uchar both code (with saturate_cast<uchar> and to not use this) are outputting the same result. I appreciate if anyone help.
Here it is code :
#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui/highgui.hpp"
#include <iostream>
#include "Source.h"
using namespace cv;
double alpha;
int beta;
int main(int, char** argv)
{
/// Read image given by user
Mat image = imread(argv[1]);
Mat image2 = Mat::zeros(image.size(), image.type());
/// Initialize values
std::cout << " Basic Linear Transforms " << std::endl;
std::cout << "-------------------------" << std::endl;
std::cout << "* Enter the alpha value [1.0-3.0]: ";std::cin >> alpha;
std::cout << "* Enter the beta value [0-100]: "; std::cin >> beta;
for (int x = 0; x < image.rows; x++)
{
for (int y = 0; y < image.cols; y++)
{
for (int c = 0; c < 3; c++)
{
image2.at<Vec3b>(x, y)[c] =
saturate_cast<uchar>(alpha*(image.at<Vec3b>(x, y)[c]) + beta);
}
}
/// Create Windows
namedWindow("Original Image", 1);
namedWindow("New Image", 1);
/// Show stuff
imshow("Original Image", image);
imshow("New Image", image2);
/// Wait until user press some key
waitKey();
return 0;
}

Answer 1

Since the result of your expression may go outside the valid range for uchar, i.e. [0,255], you'd better always use saturate_cast.
In your case, the result of the expression: alpha*(image.at<Vec3b>(x, y)[c]) + beta is a double, so it's safer to use saturate_cast<uchar> to clamp values correctly.
Also, this improves readability, since it's easy to see that you want a uchar out of an expression.
Without using saturate_cast you may have unexpected values:
uchar u1 = 257; // u1 = 1, why a very bright value is set to almost black?
uchar u2 = saturate_cast<uchar>(257); // u2 = 255, a very bright value is set to white

Answer 2

inline unsigned char saturate_cast_uchar(double val) {
val += 0.5; // to round the value
return unsigned char(val < 0 ? 0 : (val > 0xff ? 0xff : val));
}
if val lies between 0 to 255 than this function will return rounded value,
if val lies outside the range [0, 255] than it will return lower or upper boundary value.