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社区首页 >问答首页 >在Linux上从C++屏幕上获取像素颜色

问在Linux上从C++屏幕上获取像素颜色
EN

Code Review用户
提问于 2020-03-27 06:03:01
回答 1查看 1.7K关注 0票数 2

在很长一段时间里，我一直在研究这段代码，主要是为了理解它。这是需要审查的主要原因。第二个原因很简单，我想以你能找到的任何方式来优化它。

附带注意:我已经尽可能地缩短了代码。

汇编以下代码：

代码语言：javascript
复制
g++-8 -O2 -std=c++17 -Wall -Wextra -Werror -Wpedantic -pedantic-errors getPixelColor.cpp -o getPixelColor $(pkg-config --cflags --libs x11)
代码语言：javascript
复制
#include <iostream>
#include <X11/Xlib.h>
#include <cmath> // round()

// https://stackoverflow.com/a/59663458/1997354
inline __attribute__((always_inline)) \
unsigned long MyXGetPixel(XImage * ximage, const int & x, const int & y)
{
    return (*ximage->f.get_pixel)(ximage, x, y);
}

void MyGetPixelColor(Display * my_display, const int & x_coord, const int & y_coord, XColor * pixel_color)
{
    XImage * screen_image = XGetImage(
        my_display,
        XRootWindow(my_display, XDefaultScreen(my_display)),
        x_coord, y_coord,
        1, 1,
        AllPlanes,
        XYPixmap
    );
    pixel_color->pixel = MyXGetPixel(screen_image, 0, 0);
    XFree(screen_image);
    XQueryColor(my_display, XDefaultColormap(my_display, XDefaultScreen(my_display)), pixel_color);
}

// usage
int main()
{

Display * my_display = XOpenDisplay(NULL);
XColor screen_pixel_color;
// in this example let's take [0,0] coordinates
MyGetPixelColor(my_display, 0, 0, & screen_pixel_color);
XCloseDisplay(my_display);

unsigned short raw_r_value = screen_pixel_color.red;
unsigned short raw_g_value = screen_pixel_color.green;
unsigned short raw_b_value = screen_pixel_color.blue;

std::cout
    << "R = " << round(raw_r_value / 256.0) << std::endl
    << "G = " << round(raw_g_value / 256.0) << std::endl
    << "B = " << round(raw_b_value / 256.0) << std::endl;

}

未缩短的全码

代码语言：javascript
复制
#include <iostream>
#include <X11/Xlib.h>
#include <cmath> // round()
//#include <cstring>
//#include <cstdlib>
//#include <gdk/gdk.h>
//#include <gtk/gtk.h>
//#include <X11/Xutil.h>
//#include </usr/include/gtk-3.0/gtk/gtk.h>
//#include <ncurses.h>

class BasicCoordinates
{
private:
    long long X;
    long long Y;

public:
    BasicCoordinates(const long long & _X, const long long & _Y)
    {
        X = _X;
        Y = _Y;
    }
    long long getX()
    {
        return X;
    }
    long long getY()
    {
        return Y;
    }

};

bool StringContainsInteger(const std::string & str)
// true : if the string contains an integer number (possibly starting with a sign)
// false: if the string contains some other character(s)
{
    std::string::size_type str_len = str.length();
    if (str_len == 0) return false;

    bool sign_present = (str[0] == '-' || str[0] == '+');
    if (str_len == 1 && sign_present) return false;

    for (std::string::size_type i = 0; i < str_len; i++)
    {
        if (i == 0 && sign_present) continue;
        if (! std::isdigit((unsigned char) str[i])) return false;
    }

    return true;
}


// https://stackoverflow.com/a/59663458/1997354
inline __attribute__((always_inline)) \
unsigned long MyXGetPixel(XImage * ximage, const int & x, const int & y)
{
    return (*ximage->f.get_pixel)(ximage, x, y);
}

void MyGetPixelColor(Display * my_display, const int & x_coord, const int & y_coord, XColor * pixel_color)
{
    XImage * screen_image = XGetImage(
        my_display,
        XRootWindow(my_display, XDefaultScreen(my_display)),
        x_coord, y_coord,
        1, 1,
        AllPlanes,
        XYPixmap
    );
    pixel_color->pixel = MyXGetPixel(screen_image, 0, 0);
    XFree(screen_image);
    XQueryColor(my_display, XDefaultColormap(my_display, XDefaultScreen(my_display)), pixel_color);
}




int main(const int argc, const char * argv[])
{

    const int given_arguments_count = argc - 1;
    if (given_arguments_count != 2)
    {
        std::cerr
            << "Fatal error occurred while checking\n"
            << "the number of given arguments\n"
            << "--------------------------------------\n"
            << "In the function : " << __func__ << std::endl
            << "At the command  : " << "given_arguments_count\n"
            << "Given arguments : " << given_arguments_count << std::endl
            << "Error message   : " << "This program is expecting exactly 2 arguments\n"
            << "                  being the X Y coordinates of a pixel on the screen\n";
        return 1;
    }

    const std::string cli_argument_1 = argv[1];
    if (! StringContainsInteger(cli_argument_1))
    {
        std::cerr
            << "Fatal error occurred while checking\n"
            << "if the first argument contains a number\n"
            << "----------------------------------------\n"
            << "In the function : " << __func__ << std::endl
            << "At the command  : " << "StringContainsInteger\n"
            << "Input string    : " << cli_argument_1 << std::endl
            << "Error message   : " << "The first argument is not an integer number\n";
        return 1;
    }

    const std::string cli_argument_2 = argv[2];
    if (! StringContainsInteger(cli_argument_2))
    {
        std::cerr
            << "Fatal error occurred while checking\n"
            << "if the second argument contains a number\n"
            << "----------------------------------------\n"
            << "In the function : " << __func__ << std::endl
            << "At the command  : " << "StringContainsInteger\n"
            << "Input string    : " << cli_argument_2 << std::endl
            << "Error message   : " << "The second argument is not an integer number\n";
        return 1;
    }

    long long x_coord;
    try
    {
        x_coord = std::stoll(cli_argument_1);
    }
    catch (const std::exception & input_exception)
    {
        std::cerr
            << "Fatal error occurred while converting\n"
            << "the first argument to an integer variable\n"
            << "-------------------------------------\n"
            << "In the function : " << __func__ << std::endl
            << "At the command  : " << input_exception.what() << std::endl
            << "Input string    : " << cli_argument_1 << std::endl
            << "Error message   : " << "The first number argument is too big an integer\n";
        return 1;
    }

    long long y_coord;
    try
    {
        y_coord = std::stoll(cli_argument_2);
    }
    catch (const std::exception & input_exception)
    {
        std::cerr
            << "Fatal error occurred while converting\n"
            << "the second argument to an integer variable\n"
            << "--------------------------------------\n"
            << "In the function : " << __func__ << std::endl
            << "At the command  : " << input_exception.what() << std::endl
            << "Input string    : " << cli_argument_2 << std::endl
            << "Error message   : " << "The second number argument is too big an integer\n";
        return 1;
    }


    BasicCoordinates * pixel_coords = new BasicCoordinates(x_coord, y_coord);
    std::cout << "X = " << pixel_coords->getX() << std::endl;
    std::cout << "Y = " << pixel_coords->getY() << std::endl << std::endl;









        Display * my_display = XOpenDisplay(NULL);
        Screen * my_screen = XDefaultScreenOfDisplay(my_display);

        const int screen_width = my_screen->width;
        const int screen_height = my_screen->height;

        if (x_coord >= screen_width)
        {
            std::cerr
                << "X coord bigger than the screen with\n"; // TEMP

            return 1;
        }

        if (y_coord >= screen_height)
        {
            std::cerr
                << "Y coord bigger than the screen height\n"; // TEMP

            return 1;
        }



        XWindowAttributes root_window_attributes;
        XGetWindowAttributes(my_display, DefaultRootWindow(my_display), & root_window_attributes);


        XColor screen_pixel_color;


        MyGetPixelColor(my_display, x_coord, y_coord, & screen_pixel_color);

        XCloseDisplay(my_display);



        unsigned short raw_r_value = screen_pixel_color.red;
        unsigned short raw_g_value = screen_pixel_color.green;
        unsigned short raw_b_value = screen_pixel_color.blue;
/*
        std::cout
            << "Raw Values" << std::endl
            << "R = " << raw_r_value << std::endl
            << "G = " << raw_g_value << std::endl
            << "B = " << raw_b_value << std::endl;

        std::cout << std::endl;
*/
        std::cout
//            << "Normalized" << std::endl
            << "R = " << round(raw_r_value / 256.0) << std::endl
            << "G = " << round(raw_g_value / 256.0) << std::endl
            << "B = " << round(raw_b_value / 256.0) << std::endl;



        return 0;

}
beginner
linux
c++17
x11
c++
EN

回答 1

Code Review用户

发布于 2020-03-27 15:59:27

使用XGetPixel()而不是编写自己的函数

只要有可能，就应该使用X库提供的API。实现细节可能会随着时间的推移而改变，因此您自己的XGetPixel()副本在将来可能不再正确。当然，这在Xlib中是不太可能的，但总的来说，这是很好的实践。

不要不必要地经过const引用，

不要通过const引用传递小变量。您通常只在传递值的成本大于通过引用传递(例如std::string、std::vector等)的重要类型时才这样做。对于像int、float之类的东西，您不应该需要一个const引用。所以：

代码语言：javascript
复制
void MyGetPixelColor(Display *my_display, int x_coord, int y_coord, XColor *pixel_color)
{ ... }

适当时

按值返回

返回某些值的函数最好是return该值，而不是接受将被写入的指针。所以我会重写你的代码：

代码语言：javascript
复制
Xcolor MyGetPixelColor(Display *my_display, int x_coord, int y_coord)
{
    Xcolor pixel_color;
    XImage *screen_image = ...;
    pixel_color->pixel = XGetPixel(screen_image, 0, 0);
    XFree(screen_image);
    XQueryColor(my_display, XDefaultColormap(my_display, XDefaultScreen(my_display)), pixel_color);

    return pixel_color;
}

这不会导致Xcolor的额外副本，因为C++编译器将在这里使用返回值优化。

使用\n而不是std::endl

避免使用std::endl，它等同于\n，但它也会刷新输出，这可能会影响性能。有关更多详细信息，请参阅这个问题。

票数 1
EN
页面原文内容由Code Review提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：

https://codereview.stackexchange.com/questions/239490

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