Cairo+glitz/Setup glitz with glx

Note: The glitz backend for cairo has been dropped in favor of GL surface.

glitz is an OpenGL image compositing library partially designed to act as a cairo backend. Getting glitz to work can be a bit tricky especially due to the lack of documentation. Lack of good documentation and samples for Xlib/glx makes it even worse, but I find that common with anything related to Xlib. This will be a simple tutorial on how to get glitz to work. I assume you have a basic understanding of Xlib and glx before reading this.

Creating a window

First we open a X display as usual.

Code:

1.  Display* display = XOpenDisplay(NULL);
2.  unsigned int screen = DefaultScreen(display);

When creating the X window a XVisualInfo is needed. glitz requires a glitz_drawable_format_t which is related to the XVisualInfo. These two should match but I have seen cases where non matching similar formats has been used. I found it simpler to search for a suitable drawable format and extract the visual info from it than the other way around.

Code:

1.  // A template format is specified.
2.  glitz_drawable_format_t templ;
3.  templ.samples = 1;
4.  templ.depth_size = 24;
5.  templ.doublebuffer = 1;
6.  
7.  // Only the fields specified by this mask will be matched.
8.  unsigned long format_mask = GLITZ_FORMAT_SAMPLES_MASK | GLITZ_FORMAT_DOUBLEBUFFER_MASK | GLITZ_FORMAT_DEPTH_SIZE_MASK;
9.  
10.  // glitz_glx_find_window_format searches the available formats for one matching the template. The zero in the end specifies
11.  // which format to use, zero means the first matching. It is possible to iterate over the available formats by increasing
12.  // this value until NULL is returned.
13.  glitz_drawable_format_t* dformat = glitz_glx_find_window_format (display, screen, format_mask, &templ, 0);
14.  if (!dformat) {
15.         fprintf (stderr, "Error: couldn't find window format\n");
16.         return 1;
17.  }
18.  
19.  // As the name implies glitz_glx_get_visual_info_from_format extracts a XVisualInfo from a glitz_drawable_format_t.
20.  XVisualInfo* vinfo = glitz_glx_get_visual_info_from_format (display, screen, dformat);
21.  if (!vinfo) {
22.         fprintf (stderr, "Error: no visual info from format\n");
23.         return 1;
24.  }

Next up is actually creating the window.

Code:

1.  // Reference to the root-window.
2.  Window root = DefaultRootWindow(display);
3.  
4.  // Creates the colormap
5.  Colormap cmap = XCreateColormap(display, root, vinfo->visual, AllocNone);
6.  
7.  // Window attributes
8.  XSetWindowAttributes swa;
9.  swa.colormap = cmap;
10.  swa.event_mask = ExposureMask;
11.  unsigned long mask = CWColormap | CWEventMask;
12.  
13.  // Create a window using the earlier found XVisualInfo.
14.  Window window = XCreateWindow(display, root, 0, 0, 800, 600, 0, vinfo->depth, InputOutput, vinfo->visual, mask, &swa);
15.  XMapWindow(display, window);
16.  
17.  // Make it possible to "correctly" close the window.
18.  Atom wm_delete_window = XInternAtom(display, "WM_DELETE_WINDOW", 0);
19.  XSetWMProtocols(display, window, &wm_delete_window, 1);
20.  
21.  XSync(display, false);

Creating a drawable surface

After the window is created we create a glitz drawable which is used to create glitz surfaces. Behind the scenes this is kind of the OpenGL context and drawable usually created when using Xlib and OpenGL.

Code:

1.  // Create the glitz drawable. If this is the first time it is called it will create the opengl context and make it the current.
2.  glitz_drawable_t* drawable = glitz_glx_create_drawable_for_window (display, screen, dformat, window, 800, 600);
3.  
4.  // A glitz_format_t must be used when creating a surface. Not that this is not the same format as glitz_drawable_format_t used earlier.
5.  // Similar to before a template is created and a matching format is searched for.
6.  glitz_format_t *format, templ;
7.  glitz_drawable_buffer_t buffer;
8.  
9.  templ.color = dformat->color;
10.  format = glitz_find_format (drawable,
11.         GLITZ_FORMAT_FOURCC_MASK     |
12.         GLITZ_FORMAT_RED_SIZE_MASK   |
13.         GLITZ_FORMAT_GREEN_SIZE_MASK |
14.         GLITZ_FORMAT_BLUE_SIZE_MASK  |
15.         GLITZ_FORMAT_ALPHA_SIZE_MASK,
16.         &templ,
17.         0
18.  );
19.  
20.  if (!format) {
21.         fprintf (stderr, "Error: couldn't find surface format\n");
22.         return NULL;
23.  }
24.  
25.  // Create a surface with above format.
26.  glitz_surface_t* surface = glitz_surface_create (drawable, format, 800, 600, 0, NULL);
27.  if (!surface) {
28.         fprintf (stderr, "Error: couldn't create glitz surface\n");
29.         return NULL;
30.  }
31.  
32.  // The surface buffer must be bound to the drawable. The surface buffer is the buffer which rendering
33.  // calls is made to. If a doublebuffered surface is selected the backbuffer will used.
34.  if (dformat->doublebuffer) {
35.         buffer = GLITZ_DRAWABLE_BUFFER_BACK_COLOR;
36.  } else {
37.         buffer = GLITZ_DRAWABLE_BUFFER_FRONT_COLOR;
38.  }
39.  
40.  // Bind the surface buffer to the drawable.
41.  glitz_surface_attach (surface, drawable, buffer);

Next part: cairo+glitz/Setup cairo.