How to draw on root window with XCB?

Question

I have a code with XLib using which works properly:

    Display *display = XOpenDisplay(NULL);
    int screen_num = DefaultScreen(display);
    Window root_win = RootWindow ( display, screen_num );

    // Create a GC (Graphics Context) for the line
    XGCValues gc_val;
    gc_val.function           = GXxor;
    gc_val.plane_mask         = AllPlanes;
    gc_val.foreground         = WhitePixel(display, screen_num);
    gc_val.background         = BlackPixel(display, screen_num);
    gc_val.line_width         = 4;
    gc_val.line_style         = LineSolid;
    gc_val.cap_style          = CapButt;
    gc_val.join_style         = JoinMiter;
    gc_val.fill_style         = FillOpaqueStippled;
    gc_val.fill_rule          = WindingRule;
    gc_val.graphics_exposures = False;
    gc_val.clip_x_origin      = 0;
    gc_val.clip_y_origin      = 0;
    gc_val.clip_mask          = None;
    gc_val.subwindow_mode     = IncludeInferiors;

    GC gc_line = XCreateGC(display, root_win, GCFunction | GCPlaneMask |  GCForeground | GCBackground | GCLineWidth | GCLineStyle |
                GCCapStyle  | GCJoinStyle  |  GCFillStyle  |  GCFillRule  |  GCGraphicsExposures |
                GCClipXOrigin |  GCClipYOrigin  |  GCClipMask  | GCSubwindowMode, &gc_val);

    //XSetForeground(display, gc_line, some_color);
    XDrawRectangle(display, root_win, gc_line, 50, 50, 400, 400);

    XFlush(display);

... and I need to do the same with XCB.

I wrote as much as possible similar code with XCB:

xcb_connection_t    *c;
xcb_screen_t        *screen;
xcb_generic_event_t *e;
uint32_t             mask = 0;
xcb_gcontext_t    gc = { 0 };    /* the returned default graphic context */
xcb_drawable_t draw;

xcb_rectangle_t rectangles[] = {
  {0, 0, 100, 100},
};

c = xcb_connect (NULL, NULL);

/* get the first screen */
screen = xcb_setup_roots_iterator (xcb_get_setup (c)).data;

gc = xcb_generate_id (c);

/* root window */
draw = screen->root;

mask = XCB_GC_FOREGROUND | XCB_GC_BACKGROUND | XCB_GC_FUNCTION | XCB_GC_CAP_STYLE | XCB_GC_JOIN_STYLE | XCB_GC_FILL_STYLE | XCB_GC_FILL_RULE | XCB_GC_GRAPHICS_EXPOSURES
        | XCB_GC_CLIP_ORIGIN_X | XCB_GC_CLIP_ORIGIN_Y | XCB_GC_CLIP_MASK | XCB_GC_SUBWINDOW_MODE;
uint32_t values[] = {
    screen->black_pixel,
    screen->white_pixel,
    XCB_GX_XOR,
    XCB_CAP_STYLE_BUTT,
    XCB_JOIN_STYLE_MITER,
    XCB_FILL_STYLE_OPAQUE_STIPPLED,
    XCB_FILL_RULE_WINDING,
    1,
    0,
    0,
    XCB_NONE,
    XCB_SUBWINDOW_MODE_INCLUDE_INFERIORS
};

xcb_create_gc (c, gc, draw, mask, values);
xcb_map_window (c, draw);
xcb_flush(c);

while (1)
{
    xcb_poly_rectangle (c, draw, gc, 1, rectangles);
    xcb_flush(c);
}

But at start of the program I don't see the rectangle.

What I am doing wrong?

Answer 1

I don't really know but I after searching and modifying your code I was able to draw on a root window with this simple code:

#include <stdlib.h>
#include <stdio.h>
#include <xcb/xcb.h>

int
main ()
{
  xcb_connection_t    *c;
  xcb_screen_t        *screen;
  xcb_generic_event_t *e;
  uint32_t             mask = 0;
  xcb_gcontext_t    gc = { 0 };    /* the returned default graphic context */
  xcb_drawable_t draw;

  xcb_rectangle_t rectangles[] = {
    { 200, 200, 400, 400 },
  };

  c = xcb_connect (NULL, NULL);

  /* get the first screen */
  screen = xcb_setup_roots_iterator ( xcb_get_setup ( c ) ).data;

  gc = xcb_generate_id ( c );

  /* root window */
  draw = screen->root;

  mask = XCB_GC_FUNCTION | XCB_GC_FOREGROUND | XCB_GC_BACKGROUND |  XCB_GC_LINE_WIDTH| XCB_GC_LINE_STYLE | XCB_GC_GRAPHICS_EXPOSURES;
  uint32_t values[] = {
      XCB_GX_XOR,
      screen->white_pixel,
      screen->black_pixel,
      1,
      XCB_LINE_STYLE_ON_OFF_DASH,
      0
  };

  xcb_create_gc ( c, gc, draw, mask, values );
  xcb_poly_rectangle (c, draw, gc, 3, rectangles);
  xcb_map_window (c, draw);
  xcb_flush(c);
  pause();
  return 0;
}

I think that your problem is how to update on your need the drawing on the root window... (maybe). Show you code if you find a solution.

I use for the test:

 Xephyr -br -noreset -screen "1024x640" :1&
 DISPLAY=:1.0 ./mylittletest

Answer 2

I have some progress.

Looks like the problem in GC parameters combination.

Сode that is shown in the screenshot:

xcb_connection_t    *c;
xcb_screen_t        *screen;
xcb_drawable_t       win;
xcb_gcontext_t       foreground;
uint32_t             mask = 0;

xcb_rectangle_t rectangles[] = {
  {50, 50, 600, 400},
};

c = xcb_connect (NULL, NULL);

/* get the first screen */
screen = xcb_setup_roots_iterator (xcb_get_setup (c)).data;

/* root window */
win = screen->root;

foreground = xcb_generate_id (c);
mask = XCB_GC_FOREGROUND | XCB_GC_LINE_WIDTH | XCB_GC_SUBWINDOW_MODE;
uint32_t values[] = {
    screen->black_pixel,
    4,
    XCB_SUBWINDOW_MODE_INCLUDE_INFERIORS
};

xcb_create_gc (c, foreground, win, mask, values);

while(1)
{
    xcb_poly_rectangle (c, win, foreground, 1, rectangles);
    xcb_flush ( c );
}