c - How to create semi transparent white window in XLib

Question

I would like to create a semi transparent white window in XLib, but the window is not semi translucent, it remains fully opaque. I use the compton compositor and there are transparent windows in the system, so the problem is in the code:

#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <stdio.h>

int main(int argc, char* argv[])
{
    Display* display = XOpenDisplay(NULL);

    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 = 0x80ffffff;

    Window win = XCreateWindow(display, DefaultRootWindow(display), 0, 0, 300, 200, 0, vinfo.depth, InputOutput, vinfo.visual, CWColormap | CWBorderPixel | CWBackPixel, &attr);
    XSelectInput(display, win, StructureNotifyMask);
    GC gc = XCreateGC(display, win, 0, 0);

    Atom wm_delete_window = XInternAtom(display, "WM_DELETE_WINDOW", 0);
    XSetWMProtocols(display, win, &wm_delete_window, 1);

    XMapWindow(display, win);

    int keep_running = 1;
    XEvent event;

    while (keep_running) {
        XNextEvent(display, &event);

        switch(event.type) {
            case ClientMessage:
                if (event.xclient.message_type == XInternAtom(display, "WM_PROTOCOLS", 1) && (Atom)event.xclient.data.l[0] == XInternAtom(display, "WM_DELETE_WINDOW", 1))
                    keep_running = 0;

                break;

            default:
                break;
        }
    }

    XDestroyWindow(display, win);
    XCloseDisplay(display);
    return 0;
}

Answer 1

X11 expects pre-multiplied colours, i.e. real opaque colours need to be multiplied by the alpha value (and scaled accordingly, i.e. divided by 256 when channel widths is 8 bits). This format is easier to work with when you need to combine many levels. See formulas here. There's less computation when everything is pre-multiplied.

So you need to multiply each of your R, G and B channels by the alpha value (0x80) and divide by 256.

Setting the background to 0x80808080 gives the desired result:

Note the result is different from what @patthoyts suggests: here only the window proper is semi-transparent, the WM decoration stays opaque; there both the window proper and the decoration are made transparent by the WM (and the WM does the necessary colour blending).