Why "cout" works weird for "unsigned char"?

I have the following code:

cvtColor (image, image, CV_BGRA2RGB);
Vec3b bottomRGB;
bottomRGB=image.at<Vec3b>(821,1232);

When I display bottomRGB[0], it displays a value greater than 255. What is the reason for this?

As you have commented, the reason is that you use cout to print its content directly. Here I will try to explain to you why this will not work.

cout << bottomRGB[0] << endl;

Why "cout" works weird for "unsigned char"?

It will not work because here bottomRGB[0] is a unsigned char (with value 218), cout actually will print some garbage value (or nothing) as it is just a non-printable ASCII character which is getting printed anyway. Note that ASCII character corresponding to 218 is non-printable. Check out here for the ASCII table.

P.S. You can check whether bottomRGB[0] is printable or not using isprint() as:

cout << isprint(bottomRGB[0]) << endl; // will print garbage value or nothing

It will print 0 (or false) indicating the character is non-printable

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For your example, to make it work, you need to type cast it first before cout:

cout << (int) bottomRGB[0] << endl; // correctly printed (218 for your example)