How to set Xlib window background transparent?

Question

I want to create a window that has transparent background. How to do it?

I use XSetBackground(display, gc, 0), the background is black. I change the depth of the screen to 32. The result is still black. Here is my code:

    display = XOpenDisplay(getenv("DISPALY"));
    screen = DefaultScreen(display);
    depth = DefaultDepth(display,screen);
    printf("depth: %d\n", depth);
    rootwindow = RootWindow(display,screen);

    XVisualInfo vinfo;
    XMatchVisualInfo(display, DefaultScreen(display), 32, TrueColor, &vinfo);

    XSetWindowAttributes attr;
    attr.colormap = XCreateColormap(display, DefaultRootWindow(display), vinfo.visual, AllocNone);
    attr.border_pixel = 0;
    attr.background_pixel = 0; 

    window = XCreateWindow(display, DefaultRootWindow(display), 0, 0, 1440, 900, 0, vinfo.depth, InputOutput,
                    vinfo.visual, CWColormap | CWBorderPixel | CWBackPixel, &attr);

    gc = XCreateGC (display, window, 0, NULL);
   XSetBackground(display, gc, 0L);

Answer 1

You need to

• make sure window depth is 32
• set value of alpha bits for transparent areas
• make sure you are running composite manager that can handle transparency correctly

See my question "How to upload 32bit pixmap to server" as an example of how to set alpha channel value

upd: also make sure your window is created with associated colormap (I don't know the reason behind this, but I was not able to get 32 bit window displayed correctly without colormap)