Get a screenshot of a window that is cover or not visible or minimized with Xcomposite extension for X11

Question

I have the follow starting code to get a screenshot of a X window (the window can be covered, not visibled, or minimized).

#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;
}

[Compile the code with gcc file.c -lX11 -lXcomposite -lXrender and run with ./a.out]

[You can get a valid XID of a window of your Desktop with the command xdotool search --name "Title of a window" | head -1]

Now I have two problem:

1. I get a BadMatch (invalid parameter attributes) error when I call the XRenderFindVisualFormat function. I dont know why. The window with XID 90177543 exists.
2. I don't know how to get the buffer of the Picture picture to save it as a PNG image. I don't want use QT library to do this.

Can you help me?

Answer 1

I am able to reproduce your error as follows:

$ ./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

When set using a window id I knew to exist (0x440000B or 71303179, found using xwininfo), I received no error:

$ ./xidtest 
COMPOSITE IS ENABLED!

I conclude that the error is the window id you have provided is invalid.