The add function and the + operator can be used to add the elements of two matrices, or a matrix and a scalar, as seen in the following examples:

Mat image = imread("Test.png"); 
Mat overlay = imread("Overlay.png"); 
Mat result; 
add(image, overlay, result); 

You can replace the last line in the preceding code with the following:

result = image + overlay; 

The following image demonstrates the resulting image of an add operation of two images:

In case, you want to add a single scalar value to all elements of a Mat object, you can simply use something similar to the following:

result = image + 80; 

If the preceding code is executed on a grayscale image, the result will be brighter than the source image. Note that if the image has three channels, you must use a three-item vector instead of a single value. For instance, to be able to make an RGB image brighter, you can use the following:

result = image + Vec3b(80, 80, 80); 

Here's an image depicting the brighter result image, when the preceding code is executed on it:

In the preceding example codes, simply increase the added value to get an even brighter image.