OpenCV文档说，“uchar”是“无符号整数”数据类型。怎么样？ [英] OpenCV documentation says that "uchar" is "unsigned integer" datatype. How?

问题描述

我对openCV文档提到了 此处 。

I got confused with the openCV documentation mentioned here.

根据文档，如果我使用uchar ，该图片的像素可以存储无符号整数值，但如果我使用以下代码创建图片：

As per the documentation, if i create an image with "uchar", the pixels of that image can store unsigned integer values but if i create an image using the following code:

Mat image;
image = imread("someImage.jpg" , 0); // Read an image in "UCHAR" form

或执行

image.create(10, 10, CV_8UC1);
for(int i=0; i<image.rows; i++)
{
    for(int j=o; j<image.cols; j++)
    {
        image.at<uchar>(i,j) = (uchar)255;
    }
}

然后如果我尝试使用

cout<<"  "<<image.at<uchar>(i,j);

然后我得到一些wierd结果在终端，但如果我使用下面的语句，然后我可以得到值in between 0-255。

then i get some wierd results at terminal but if i use the following statement then i can get the values inbetween 0-255.

cout<<"  "<<(int)image.at<uchar>(i,j); // with TYPECAST

问题：为什么我需要做typecast

Question: Why do i need to do typecast to get print the values in range 0-255 if the image itself can store "unsigned integer" values.

推荐答案

如果您尝试使用无符号整数值，则可以打印范围为0-255的值。查找 uchar 的定义（如果使用的是Visual Studio，则为F12），那么最终会出现在OpenCV的 core / types_c.h ：

If you try to find definition of uchar (which is pressing F12 if you are using Visual Studio), then you'll end up in OpenCV's core/types_c.h:

#ifndef HAVE_IPL
   typedef unsigned char uchar;
   typedef unsigned short ushort;
#endif

这是标准和合理的定义无符号整数8位类型的方法>8位无符号整数），因为标准确保 char 总是需要1个字节的内存。这意味着：

which standard and reasonable way of defining unsigned integral 8bit type (i.e. "8-bit unsigned integer") since standard ensures that char always requires exactly 1 byte of memory. This means that:

cout << "  " << image.at<uchar>(i,j);

使用重载的运算符<< 需要 unsigned char （ char ），以字符形式打印传递的值，而不是数字。

uses the overloaded operator<< that takes unsigned char (char), which prints passed value in form of character, not number.

但是，显式转换会导致使用另一个版本的<< ：

Explicit cast, however, causes another version of << to be used:

cout << "  " << (int) image.at<uchar>(i,j);

，因此它会打印数字。此问题与您使用OpenCV的事实无关。

and therefore it prints numbers. This issue is not related to the fact that you are using OpenCV at all.

简单示例：

char           c = 56; // equivalent to c = '8'
unsigned char uc = 56;
int            i = 56;
std::cout << c << " " << uc << " " << i;

输出： 8 8 56

如果它是一个模板的事实让你感到困惑，那么这种行为也等同于：

And if the fact that it is a template confuses you, then this behavior is also equivalent to:

template<class T>
T getValueAs(int i) { return static_cast<T>(i); }

typedef unsigned char uchar;

int main() {
    int i = 56;
    std::cout << getValueAs<uchar>(i) << " " << (int)getValueAs<uchar>(i);
}

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