如何使用 xlib 正确截屏？

Question

我正在尝试捕获屏幕图像以用于截屏。因此我需要一个快速的解决方案，并且不能依赖诸如 import 或 xwd 之类的 shell 程序。

这是我到目前为止编写的代码，但它失败并给了我一个垃圾图像，它似乎只是显示了几张奇怪颜色的图像的碎片。

关于我做错了什么有什么想法吗？

#include <X11/Xlib.h>
#include <X11/X.h>

#include <cstdio>
#include <CImg.h>
using namespace cimg_library;

int main()
{
   Display *display = XOpenDisplay(NULL);
   Window root = DefaultRootWindow(display);

   XWindowAttributes gwa;

   XGetWindowAttributes(display, root, &gwa);
   int width = gwa.width;
   int height = gwa.height;


   XImage *image = XGetImage(display,root, 0,0 , width,height,AllPlanes, ZPixmap);

   unsigned char *array = new unsigned char[width * height * 3];

   unsigned long red_mask = image->red_mask;
   unsigned long green_mask = image->green_mask;
   unsigned long blue_mask = image->blue_mask;

   for (int x = 0; x < width; x++)
      for (int y = 0; y < height ; y++)
      {
         unsigned long pixel = XGetPixel(image,x,y);

         unsigned char blue = pixel & blue_mask;
         unsigned char green = (pixel & green_mask) >> 8;
         unsigned char red = (pixel & red_mask) >> 16;

         array[(x + width * y) * 3] = red;
         array[(x + width * y) * 3+1] = green;
         array[(x + width * y) * 3+2] = blue;
      }

   CImg<unsigned char> pic(array,width,height,1,3);
   pic.save_png("blah.png");

   printf("%ld %ld %ld\n",red_mask>> 16, green_mask>>8, blue_mask);

   return 0;
}

Answer 1

您对array 在内存中的布局方式有误，您可以通过在循环之前声明img 并将此printf 添加到您的内部循环中来发现：

printf("%ld %ld %u %u %u\n",x,y,pic.offset(x,y,0),pic.offset(x,y,1),pic.offset(x,y,2));

这会产生（在我的 1920x1200 屏幕上）：

0 0 0 2304000 4608000
0 1 1920 2305920 4609920
0 2 3840 2307840 4611840

以此类推，表示红/绿/蓝子图像保持“在一起”，而不是单个像素的三个颜色分量彼此相邻。

内置的 CImg 访问器将使您的代码工作：

pic(x,y,0) = red;
pic(x,y,1) = green;
pic(x,y,2) = blue;

Answer 2

你可以使用 libpng

int code = 0;
FILE *fp;
png_structp png_ptr;
png_infop png_info_ptr;
png_bytep png_row;

// Open file
fp = fopen ("test.png", "wb");
if (fp == NULL){
    fprintf (stderr, "Could not open file for writing\n");
    code = 1;
}

// Initialize write structure
png_ptr = png_create_write_struct (PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
if (png_ptr == NULL){
    fprintf (stderr, "Could not allocate write struct\n");
    code = 1;
}

// Initialize info structure
png_info_ptr = png_create_info_struct (png_ptr);
if (png_info_ptr == NULL){
    fprintf (stderr, "Could not allocate info struct\n");
    code = 1;
 }

// Setup Exception handling
if (setjmp (png_jmpbuf (png_ptr))){
    fprintf(stderr, "Error during png creation\n");
   code = 1;
}

png_init_io (png_ptr, fp);

// Write header (8 bit colour depth)
png_set_IHDR (png_ptr, png_info_ptr, width, height,
     8, PNG_COLOR_TYPE_RGB, PNG_INTERLACE_NONE,
     PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);

// Set title
char *title = "Screenshot";
if (title != NULL){
    png_text title_text;
    title_text.compression = PNG_TEXT_COMPRESSION_NONE;
    title_text.key = "Title";
    title_text.text = title;
    png_set_text (png_ptr, png_info_ptr, &title_text, 1);
}

png_write_info (png_ptr, png_info_ptr);

// Allocate memory for one row (3 bytes per pixel - RGB)
png_row = (png_bytep) malloc (3 * width * sizeof (png_byte));

// Write image data
int x, y;
for (y = 0; y < height; y++){
    for (x = 0; x < width; x++){
        unsigned long pixel = XGetPixel (image, x, y);
        unsigned char blue = pixel & blue_mask;
        unsigned char green = (pixel & green_mask) >> 8; 
        unsigned char red = (pixel & red_mask) >> 16;
        png_byte *ptr = &(png_row[x*3]);
        ptr[0] = red;
        ptr[1] = green;
        ptr[2] = blue;
    }
    png_write_row (png_ptr, png_row);
}

// End write
png_write_end (png_ptr, NULL);

// Free
fclose (fp);
if (png_info_ptr != NULL) png_free_data (png_ptr, png_info_ptr, PNG_FREE_ALL, -1);
if (png_ptr != NULL) png_destroy_write_struct (&png_ptr, (png_infopp)NULL);
if (png_row != NULL) free (png_row);

Answer 3

图像必须以 R1R2R3R4R5R6......G1G2G3G4G5G6.......B1B2B3B4B5B6 的形式存储在内存中。 cimg storage