使用 X11 的 Xcomposite 扩展获取覆盖或不可见或最小化的窗口的屏幕截图

Question

我有以下起始代码来获取 X 窗口的屏幕截图（该窗口可以被覆盖、不可见或最小化）。

#include <stdlib.h>
#include <stdio.h>

#include <X11/Xlib.h>
#include <X11/X.h>
#include <X11/extensions/Xcomposite.h>
#include <X11/extensions/Xrender.h>

int
main ()
{
  Display *display = XOpenDisplay (NULL);
  XID xid = 90177543; // xdotool search --name "World of Warcraft" | head -1

  // Check if Composite extension is enabled
  int event_base_return;
  int error_base_return;
  if (XCompositeQueryExtension (display, &event_base_return, &error_base_return))
    printf ("COMPOSITE IS ENABLED!\n");

  // Requests the X server to direct the hierarchy starting at window to off-screen storage
  XCompositeRedirectWindow (display, xid, CompositeRedirectAutomatic);
  // Preventing the backing pixmap from being freed when the window is hidden/destroyed
  // If you want the window contents to still be available after the window has been destroyed,
  // or after the window has been resized (but not yet redrawn), you can increment the backing
  // pixmaps ref count to prevent it from being deallocated.
  Pixmap pixmap = XCompositeNameWindowPixmap (display, xid);

  // Get window attributes
  XWindowAttributes attr;
  Status s = XGetWindowAttributes (display, xid, &attr);
  if (s == 0)
    printf ("Fail to get window attributes!\n");

  // Extract the data
  XRenderPictFormat *format = XRenderFindVisualFormat (display, attr.visual);
  int width = attr.width;
  int height = attr.height;
  int depth = attr.depth;

  // What we need to do now is to create an XRender Picture for the window,
  // which we'll need to draw it with the Render extension.
  // A picture is a basically a handle to a server side struct with some
  // additional information about a drawable (in this case a window),
  // such as its format, which clipping region should be used when
  // drawing it (if any), whether it should be tiled etc.
  XRenderPictureAttributes pa;
  pa.subwindow_mode = IncludeInferiors;
  Picture picture = XRenderCreatePicture (display, xid, format, CPSubwindowMode, &pa);

  // We now have all the information we need in order to be able to draw the window
  // using the Xrender extension, and we've created and prepared a source picture
  // for the window for this purpose.
  // The Xrender function we'll use to draw the window is XRenderComposite().

  //XRenderComposite (display, PictOpSrc, picture, None, ???destination???, 0,0, 0,0, 0,0, width, height);

  XFreePixmap (display, pixmap);
  XCompositeUnredirectWindow (display, xid, CompositeRedirectAutomatic);

  return 0;
}

[编译代码gcc file.c -lX11 -lXcomposite -lXrender并运行./a.out]

[您可以使用命令获取桌面窗口的有效 XID xdotool search --name "Title of a window" | head -1]

现在我有两个问题：

1. BadMatch (invalid parameter attributes)调用该XRenderFindVisualFormat函数时出现错误。我不知道为什么。XID 为 90177543 的窗口存在。
2. 我不知道如何获取 的缓冲区Picture picture以将其另存为 PNG 图像。我不想使用 QT 库来做到这一点。

你能帮助我吗？

Answer 1

我能够重现您的错误，如下所示：

$ ./xidtest 
COMPOSITE IS ENABLED!
X Error of failed request:  BadWindow (invalid Window parameter)
  Major opcode of failed request:  142 (Composite)
  Minor opcode of failed request:  1 ()
  Resource id in failed request:  0x5600007
  Serial number of failed request:  9
  Current serial number in output stream:  12

当使用我知道存在的窗口 ID（0x440000B 或 71303179，使用 xwininfo 找到）进行设置时，我没有收到错误：

$ ./xidtest 
COMPOSITE IS ENABLED!

我的结论是，错误是您提供的窗口 ID 无效。