Pixel problems in image processing: unsigned char and Char

Before doing image processing, has been not too concerned about this problem, the image of each pixel point of gray values, always think that char can also, unsigned char can also be. Although they are all 8 bits, the range of numbers represented is not the same: char: -128~127, unsigned char:0~255. It is clear that unsigned char is the right choice.

You can define this: 1 struct {
2 char R;
3 char g;
4 char b;
5}pixel_t;
6
You can also define this:
8 struct {
9 unsigned char R;
unsigned char g;
One unsigned char b;
}pixel_ut;

If you don't use the above definition for arithmetic operations on pixels, but only for assignment operations, OK, no problem. But as soon as arithmetic is done, the hidden bugs come along.

Consider the following:
2 struct pixel_t pix1;
3 struct pixel_t pix2;
4 struct pixel_ut pix3;
5 struct pixel_ut pix4;
6
7 int ans0, ans1;
8
9 PIX1.R = 127;
Ten PIX2.R = 129;
One
PIX3.R = 127;
PIX4.R = 129;

Ans0 = PIX2.R-PIX1.R;//ans0 =?

Ans1 = pix4.r-pix3.r//ans1 =? 

If you think Ans0 and ans1 are equal, then you are wrong, they are not only unequal, but they are far apart.  
Ans0 = -254; 
ans1 = 2; 

The reason is: 
When we assign 129 to PIX2.R, because the type of PIX2.R is char, the PIX2.R The value is not 129, but  
-127. So when we do the operation, there is an error.