Create window from arbitrary pixmap as background

[Lucas de Sena]

Hi, I am using libX11/Xlib C bindings to write a routine that, given two
arbitrary pixmaps (one of them is a bitmap mask), creates an icon window
with the pixmap as background pixmap and Xshape'd with the bitmap as
shape bounding.

First, I was using XCreateWindow(3) with a CWBackPixmap attribute.
However, I got BadMatch error from the CreateWindow request.  That is
probably because of window's and pixmap's unmatching depths.

Then, I tried to create the icon window using the pixmap's depth, but a
BadMatch error still occurs.  That is probably because the default visual
does not support the given depth.  If that is the case, I do not know how
to get a proper visual.

Here is my current (broken) solution in C99:

        Display *display;
        Pixmap icon, mask;
        Window win;
        unsigned int width, height, depth;
        char const **xpmdata;

        if (icon == None) {
                /* fallback to default icon; that works */
                XpmAttributes xpmattr = { 0 };
                int status;

                status = XpmCreatePixmapFromData(
                        display, DefaultRootWindow(display),
                        xpmdata, &icon, &mask, &xpmattr
                );
                if (status != XpmSuccess || icon == None)
                        return None;
                width = xpmattr.width;
                height = xpmattr.height;
                depth = xpmattr.depth;
        } else {
                Status success;

                success = XGetGeometry(
                        display, icon, &(Window){0}, &(int){0},
                        &width, &height, &(unsigned){0}, &depth
                );
                if (!success)
                        return None;
        }
        win = XCreateWindow(
                display, root, 0, 0, width, height, 0,
                depth, InputOutput, CopyFromParent,
                CWBackPixmap | CWOverrideRedirect,
                &(XSetWindowAttributes){
                        .background_pixmap = icon,
                        .override_redirect = True,
                }
        );
        XShapeCombineMask(display, win, ShapeBounding, 0, 0, mask, ShapeSet);
        XFreePixmap(display, icon);
        XFreePixmap(display, mask);
        return win;
        
What is the proper solution?

I checked how a few iconifying window managers deal with clients
providing icon pixmap and mask on XWMHints(3).  But as far as I could
understand, fvwm and twm ignores pixmaps with non-default depths; and
windowmaker creates an XImage from the pixmap and then creates a
default-depth pixmap back from it.  Is the latter approach the most
correct (or only) one?

-- 
Lucas de Sena