X11: Translucency not working when the window's parent is not the root window

Roy_SRC · 01-19-2017, 08:02 PM

I am trying to get translucency to work when creating X windows using C and Xlib. Here's my code:

Code:

#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <Imlib2.h>

int main(int argc, char **argv)
{
    Imlib_Image img;
    Display *dpy;
    Window root;
    int scnNum;

    int width, height;
    const char *filename = argv[1];

    dpy = XOpenDisplay(NULL);
    if (!dpy) return 0;

    scnNum = DefaultScreen(dpy);
    root = DefaultRootWindow(dpy);

    XVisualInfo vinfo;
    Status ret = XMatchVisualInfo(dpy, scnNum, 32, TrueColor, &vinfo);
    
    // Create a top-level window (with no alpha channel)
    Window win = XCreateSimpleWindow(dpy, root,
                                     0, 0, 500, 500, 0,
                                     WhitePixel(dpy, 0), BlackPixel(dpy, 0));
    
    // Draw an image on the window
    img = imlib_load_image("sampleImage.jpg");

    imlib_context_set_image(img);
    width = imlib_image_get_width();
    height = imlib_image_get_height();
    
    Pixmap pix = XCreatePixmap(dpy, win, width, height, DefaultDepth(dpy, scnNum));
    
    imlib_context_set_display(dpy);
    imlib_context_set_visual(DefaultVisual(dpy, scnNum));
    imlib_context_set_colormap(DefaultColormap(dpy, scnNum));
    imlib_context_set_drawable(pix);
    
    imlib_render_image_on_drawable(0, 0);
    XSetWindowBackgroundPixmap(dpy, win, pix);
    
    // Map window on display
    XMapWindow(dpy, win);
    
    // Create a child window with alpha channel
    XSetWindowAttributes attrChild;
    attrChild.colormap = XCreateColormap(dpy, root, vinfo.visual, AllocNone);
    attrChild.border_pixel = 1;
    attrChild.background_pixel = 0x20423444;
    attrChild.background_pixmap = None;
    
    Window childWin = XCreateWindow(dpy, win,
                                    10, 10, 200, 200, 1,
                                    vinfo.depth,
                                    InputOutput,
                                    vinfo.visual,
                                    CWColormap | CWBorderPixel | CWBackPixel | CWBackPixmap,
                                    &attrChild);
      
    // Map child window on display
    XMapWindow(dpy, childWin);
    
    // Event loop
    while (1) {
      XEvent event;
      XNextEvent(dpy, &event);
    }

    XCloseDisplay(dpy);
    return 0;
}

First, I run xcompmgr -c

Then, I run my program and this is what I get:

Click image for larger version

Name: withWin.jpg
Views: 56
Size: 38.1 KB
ID: 24019

I don't get any translucency. But here's the strange part: if I change the parent of childWin to be root:

Code:

    Window childWin = XCreateWindow(dpy, root,
                                    10, 10, 200, 200, 1,
                                    vinfo.depth,
                                    InputOutput,
                                    vinfo.visual,
                                    CWColormap | CWBorderPixel | CWBackPixel | CWBackPixmap,
                                    &attrChild);

This is what I get:

Click image for larger version

Name: withRoot.jpg
Views: 53
Size: 39.5 KB
ID: 24018

I get some kind of translucency. I want childWin to be a child of win and I want to preserve translucency, but I have no idea what I'm doing wrong.

Thanks!