Going deep into X11

This is at the shell script level inside "xscreencapture".

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xwd -screen -display $HOST_IP:0 -root>image.xwd;

This is at the "xwd" utility level from source.

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image = XGetImage (dpy, window, x, y, width, height, AllPlanes, format);

This is at the Xwindows level as "C" source.

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This function specifically supports rudimentary screen dumps.
XImage *XGetImage(display, d, x, y, width, height, plane_mask, format)
        Display *display;
        Drawable d;
        int x, y;
        unsigned int width, height;
        unsigned long plane_mask;
        int format;
Arguments
display  Specifies the connection to the X server.
d  Specifies the drawable.
x
y  Specify the x and y coordinates, 
which are relative to the origin of the drawable
and define the upper-left corner of the rectangle.
width
height  Specify the width and height of the subimage,
 which define the dimensions of the rectangle.
plane_mask  Specifies the plane mask.
format  Specifies the format for the image.
 You can pass XYPixmap or ZPixmap .

The point of this is to allow programs to look at themselves and adjust their reactions to what is happening on the screen, whether virtual or real. It allows Alice Infinity to control the mouse and run programs.