mutter/clutter/clutter-stage.c

/*
 * Clutter.
 *
 * An OpenGL based 'interactive canvas' library.
 *
 * Authored By Matthew Allum  <mallum@openedhand.com>
 *
 * Copyright (C) 2006 OpenedHand
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 */

#include "config.h"

#include "clutter-stage.h"
#include "clutter-main.h"
#include "clutter-color.h"
#include "clutter-marshal.h"
#include "clutter-enum-types.h"
#include "clutter-private.h" 	/* for DBG */

#include <GL/glx.h>
#include <GL/gl.h>

/* the stage is a singleton instance */
static ClutterStage *stage_singleton = NULL;

G_DEFINE_TYPE (ClutterStage, clutter_stage, CLUTTER_TYPE_GROUP);

#define CLUTTER_STAGE_GET_PRIVATE(obj) \
(G_TYPE_INSTANCE_GET_PRIVATE ((obj), CLUTTER_TYPE_STAGE, ClutterStagePrivate))

struct _ClutterStagePrivate
{
  Window        xwin;  
  Pixmap        xpixmap;
  gint          xwin_width, xwin_height; /* FIXME target_width / height */
  
  GLXPixmap     glxpixmap;
  GLXContext    gl_context;
  
  ClutterColor  color;
  
  guint         want_fullscreen : 1;
  guint         want_offscreen  : 1;
  guint         hide_cursor     : 1;
};

enum
{
  PROP_0,
  
  PROP_COLOR,
  PROP_FULLSCREEN,
  PROP_OFFSCREEN,
  PROP_HIDE_CURSOR
};

enum
{
  INPUT_EVENT,
  BUTTON_PRESS_EVENT,
  BUTTON_RELEASE_EVENT,
  KEY_PRESS_EVENT,
  KEY_RELEASE_EVENT,
  MOTION_EVENT,
  
  LAST_SIGNAL
};

static guint stage_signals[LAST_SIGNAL] = { 0 };

static ClutterElementClass *parent_class = NULL;

static void
sync_fullscreen (ClutterStage *stage)
{
  Atom                 atom_WINDOW_STATE, atom_WINDOW_STATE_FULLSCREEN;

  atom_WINDOW_STATE 
    = XInternAtom(clutter_xdisplay(), "_NET_WM_STATE", False);
  atom_WINDOW_STATE_FULLSCREEN 
    = XInternAtom(clutter_xdisplay(), "_NET_WM_STATE_FULLSCREEN",False);

  if (stage->priv->want_fullscreen)
    {
      clutter_element_set_size (CLUTTER_ELEMENT(stage),
				DisplayWidth(clutter_xdisplay(), 
					     clutter_xscreen()),
				DisplayHeight(clutter_xdisplay(), 
					      clutter_xscreen()));

      if (stage->priv->xwin != None)
	XChangeProperty(clutter_xdisplay(), stage->priv->xwin,
			atom_WINDOW_STATE, XA_ATOM, 32,
			PropModeReplace,
			(unsigned char *)&atom_WINDOW_STATE_FULLSCREEN, 1);
    }
  else
    {
      /* FIXME */
      if (stage->priv->xwin != None)
	XDeleteProperty(clutter_xdisplay(), 
			stage->priv->xwin, atom_WINDOW_STATE);
    }
}

static void
sync_cursor_visible (ClutterStage *stage)
{
  if (stage->priv->xwin == None)
    return;
  
  if (stage->priv->hide_cursor)
    {
      XColor col;
      Pixmap pix;
      Cursor curs;

      pix = XCreatePixmap (clutter_xdisplay(), 
			   stage->priv->xwin, 1, 1, 1);
      memset (&col, 0, sizeof (col));
      curs = XCreatePixmapCursor (clutter_xdisplay(), 
				  pix, pix, &col, &col, 1, 1);
      XFreePixmap (clutter_xdisplay(), pix);
      XDefineCursor(clutter_xdisplay(), stage->priv->xwin, curs);
    }
  else
    {
      XUndefineCursor(clutter_xdisplay(), stage->priv->xwin);
    }
}

static void
frustum (GLfloat left,
	 GLfloat right,
	 GLfloat bottom,
	 GLfloat top,
	 GLfloat nearval,
	 GLfloat farval)
{
  GLfloat x, y, a, b, c, d;
  GLfloat m[16];

  x = (2.0 * nearval) / (right - left);
  y = (2.0 * nearval) / (top - bottom);
  a = (right + left) / (right - left);
  b = (top + bottom) / (top - bottom);
  c = -(farval + nearval) / ( farval - nearval);
  d = -(2.0 * farval * nearval) / (farval - nearval);

#define M(row,col)  m[col*4+row]
  M(0,0) = x;     M(0,1) = 0.0F;  M(0,2) = a;      M(0,3) = 0.0F;
  M(1,0) = 0.0F;  M(1,1) = y;     M(1,2) = b;      M(1,3) = 0.0F;
  M(2,0) = 0.0F;  M(2,1) = 0.0F;  M(2,2) = c;      M(2,3) = d;
  M(3,0) = 0.0F;  M(3,1) = 0.0F;  M(3,2) = -1.0F;  M(3,3) = 0.0F;
#undef M

  glMultMatrixf (m);
}

static void
perspective (GLfloat fovy,
	     GLfloat aspect,
	     GLfloat zNear,
	     GLfloat zFar)
{
  GLfloat xmin, xmax, ymin, ymax;

  ymax = zNear * tan (fovy * M_PI / 360.0);
  ymin = -ymax;
  xmin = ymin * aspect;
  xmax = ymax * aspect;

  frustum (xmin, xmax, ymin, ymax, zNear, zFar);
}

static void
sync_gl_viewport (ClutterStage *stage)
{
  /* Set For 2D */
#if 0
  glViewport (0, 0, stage->priv->xwin_width, stage->priv->xwin_height);
  glMatrixMode (GL_PROJECTION);
  glLoadIdentity ();
  glOrtho (0, stage->priv->xwin_width, stage->priv->xwin_height, 0, -1, 1);
  glMatrixMode (GL_MODELVIEW);
  glLoadIdentity ();

#endif
  /* For 3D */

  glViewport (0, 0, stage->priv->xwin_width, stage->priv->xwin_height);
  glMatrixMode (GL_PROJECTION);
  glLoadIdentity ();
  perspective (60.0f, 1.0f, 0.1f, 100.0f);
  glMatrixMode (GL_MODELVIEW);
  glLoadIdentity ();

  /* Then for 2D like transform */

  /* camera distance from screen, 0.5 * tan (FOV) */
#define DEFAULT_Z_CAMERA 0.866025404f

  glTranslatef (-0.5f, -0.5f, -DEFAULT_Z_CAMERA);
  glScalef (1.0f / stage->priv->xwin_width, 
	    -1.0f / stage->priv->xwin_height, 1.0f / stage->priv->xwin_width);
  glTranslatef (0.0f, -stage->priv->xwin_height, 0.0f);
}

static void
clutter_stage_show (ClutterElement *self)
{
  if (clutter_stage_get_xwindow (CLUTTER_STAGE(self)))
    XMapWindow (clutter_xdisplay(), 
		clutter_stage_get_xwindow (CLUTTER_STAGE(self)));
}

static void
clutter_stage_hide (ClutterElement *self)
{
  if (clutter_stage_get_xwindow (CLUTTER_STAGE(self)))
    XUnmapWindow (clutter_xdisplay(), 
		  clutter_stage_get_xwindow (CLUTTER_STAGE(self)));
}

static void
clutter_stage_unrealize (ClutterElement *element)
{
  ClutterStage        *stage;
  ClutterStagePrivate *priv;

  stage = CLUTTER_STAGE(element);
  priv = stage->priv;

  if (priv->want_offscreen)
    {
      if (priv->glxpixmap)
	{
	  glXDestroyGLXPixmap (clutter_xdisplay(), priv->glxpixmap);
	  priv->glxpixmap = None;
	}

      if (priv->xpixmap)
	{
	  XFreePixmap (clutter_xdisplay(), priv->xpixmap);
	  priv->xpixmap = None;
	}
    }
  else
    {
      if (clutter_stage_get_xwindow (CLUTTER_STAGE(element)))
	{
	  XDestroyWindow (clutter_xdisplay(), priv->xwin);
	  priv->xwin = None;
	}
    }
}

static void
clutter_stage_realize (ClutterElement *element)
{
  ClutterStage        *stage;
  ClutterStagePrivate *priv;

  stage = CLUTTER_STAGE(element);

  priv = stage->priv;

  if (priv->want_offscreen)
    {
      priv->xpixmap = XCreatePixmap (clutter_xdisplay(),
				     clutter_root_xwindow(),
				     priv->xwin_width, priv->xwin_height,
				     clutter_xvisual()->depth);

      priv->glxpixmap = glXCreateGLXPixmap(clutter_xdisplay(),
					   clutter_xvisual(),
					   priv->xpixmap);
      sync_fullscreen (stage);  

      if (priv->gl_context)
	glXDestroyContext (clutter_xdisplay(), priv->gl_context);

      /* indirect */
      priv->gl_context = glXCreateContext (clutter_xdisplay(), 
					   clutter_xvisual(), 
					   0, 
					   False);
      
      glXMakeCurrent(clutter_xdisplay(), priv->glxpixmap, priv->gl_context);
    }
  else
    {
      priv->xwin = XCreateSimpleWindow(clutter_xdisplay(),
				       clutter_root_xwindow(),
				       0, 0,
				       priv->xwin_width, priv->xwin_height,
				       0, 0, 
				       WhitePixel(clutter_xdisplay(), 
						  clutter_xscreen()));

      XSelectInput(clutter_xdisplay(), 
		   priv->xwin, 
		   StructureNotifyMask
		   |ExposureMask
		   |KeyPressMask
		   |KeyReleaseMask
		   |ButtonPressMask
		   |ButtonReleaseMask
		   |PropertyChangeMask);

      sync_fullscreen (stage);  
      sync_cursor_visible (stage);  

      if (priv->gl_context)
	glXDestroyContext (clutter_xdisplay(), priv->gl_context);

      priv->gl_context = glXCreateContext (clutter_xdisplay(), 
					   clutter_xvisual(), 
					   0, 
					   True);
      
      glXMakeCurrent(clutter_xdisplay(), priv->xwin, priv->gl_context);
    }

  sync_gl_viewport (stage);
}

static void
clutter_stage_paint (ClutterElement *self)
{
  parent_class->paint (self);
}

static void
clutter_stage_allocate_coords (ClutterElement    *self,
			       ClutterElementBox *box)
{
  /* Do nothing, just stop group_allocate getting called */

  /* TODO: sync up with any configure events from WM ??  */
  return;
}

static void
clutter_stage_request_coords (ClutterElement    *self,
			      ClutterElementBox *box)
{
  ClutterStage        *stage;
  ClutterStagePrivate *priv;
  gint                 new_width, new_height;

  stage = CLUTTER_STAGE (self);
  priv = stage->priv;

  new_width  = ABS(box->x2 - box->x1);
  new_height = ABS(box->y2 - box->y1); 

  if (new_width != priv->xwin_width || new_height != priv->xwin_height)
    {
      priv->xwin_width  = new_width;
      priv->xwin_height = new_height;

      if (priv->xwin)
	XResizeWindow (clutter_xdisplay(), 
		       priv->xwin, 
		       priv->xwin_width, 
		       priv->xwin_height);

      if (priv->xpixmap)
	{
	  /* Need to recreate to resize */
	  clutter_element_unrealize(self);
	  clutter_element_realize(self);
	}

      sync_gl_viewport (stage);
    }

  if (priv->xwin) /* Do we want to bother ? */
    XMoveWindow (clutter_xdisplay(), 
		 priv->xwin,
		 box->x1,
		 box->y1);
}

static void 
clutter_stage_dispose (GObject *object)
{
  ClutterStage *self = CLUTTER_STAGE (object);

  if (self->priv->xwin)
    clutter_stage_unrealize (CLUTTER_ELEMENT (self));

  G_OBJECT_CLASS (clutter_stage_parent_class)->dispose (object);
}

static void 
clutter_stage_finalize (GObject *object)
{
  G_OBJECT_CLASS (clutter_stage_parent_class)->finalize (object);
}


static void
clutter_stage_set_property (GObject      *object, 
			    guint         prop_id,
			    const GValue *value, 
			    GParamSpec   *pspec)
{
  ClutterStage        *stage;
  ClutterStagePrivate *priv;

  stage = CLUTTER_STAGE(object);
  priv = stage->priv;

  switch (prop_id) 
    {
    case PROP_COLOR:
      clutter_stage_set_color (stage, g_value_get_boxed (value));
      break;
    case PROP_OFFSCREEN:
      if (priv->want_offscreen != g_value_get_boolean (value))
	{
	  clutter_element_unrealize (CLUTTER_ELEMENT(stage));
	  priv->want_offscreen = g_value_get_boolean (value);
	  clutter_element_realize (CLUTTER_ELEMENT(stage));
	}
      break;
    case PROP_FULLSCREEN:
      if (priv->want_fullscreen != g_value_get_boolean (value))
	{
	  priv->want_fullscreen = g_value_get_boolean (value);
	  sync_fullscreen (stage);
	}
      break;
    case PROP_HIDE_CURSOR:
      if (priv->hide_cursor != g_value_get_boolean (value))
	{
	  priv->hide_cursor = g_value_get_boolean (value);
	  sync_cursor_visible (stage);
	}
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
  }
}

static void
clutter_stage_get_property (GObject    *object, 
			      guint       prop_id,
			      GValue     *value, 
			      GParamSpec *pspec)
{
  ClutterStage        *stage;
  ClutterStagePrivate *priv;
  ClutterColor         color;

  stage = CLUTTER_STAGE(object);
  priv = stage->priv;

  switch (prop_id) 
    {
    case PROP_COLOR:
      clutter_stage_get_color (stage, &color);
      g_value_set_boxed (value, &color);
      break;
    case PROP_OFFSCREEN:
      g_value_set_boolean (value, priv->want_offscreen);
      break;
    case PROP_FULLSCREEN:
      g_value_set_boolean (value, priv->want_fullscreen);
      break;
    case PROP_HIDE_CURSOR:
      g_value_set_boolean (value, priv->hide_cursor);
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    } 
}

static void
clutter_stage_class_init (ClutterStageClass *klass)
{
  GObjectClass        *gobject_class = G_OBJECT_CLASS (klass);
  ClutterElementClass *element_class = CLUTTER_ELEMENT_CLASS (klass);

  parent_class = g_type_class_peek_parent (klass);

  element_class->realize    = clutter_stage_realize;
  element_class->unrealize  = clutter_stage_unrealize;
  element_class->show       = clutter_stage_show;
  element_class->hide       = clutter_stage_hide;
  element_class->paint      = clutter_stage_paint;

  element_class->request_coords  = clutter_stage_request_coords;
  element_class->allocate_coords = clutter_stage_allocate_coords;

  gobject_class->dispose      = clutter_stage_dispose;
  gobject_class->finalize     = clutter_stage_finalize;
  gobject_class->set_property = clutter_stage_set_property;
  gobject_class->get_property = clutter_stage_get_property;

  /**
   * ClutterStage:fullscreen
   *
   * Whether the stage should be fullscreen or not.
   */
  g_object_class_install_property
    (gobject_class, PROP_FULLSCREEN,
     g_param_spec_boolean ("fullscreen",
			   "Fullscreen",
			   "Make Clutter stage fullscreen",
			   FALSE,
			   G_PARAM_READWRITE));

  g_object_class_install_property
    (gobject_class, PROP_OFFSCREEN,
     g_param_spec_boolean ("offscreen",
			   "Offscreen",
			   "Make Clutter stage offscreen",
			   FALSE,
			   G_PARAM_READWRITE));


  g_object_class_install_property
    (gobject_class, PROP_HIDE_CURSOR,
     g_param_spec_boolean ("hide-cursor",
			   "Hide Cursor",
			   "Make Clutter stage cursor-less",
			   FALSE,
			   G_PARAM_READWRITE));

  g_object_class_install_property
    (gobject_class, PROP_COLOR,
     g_param_spec_boxed ("color",
			 "Color",
			 "The color of the stage",
			 CLUTTER_TYPE_COLOR,
			 G_PARAM_READWRITE));

  stage_signals[INPUT_EVENT] =
    g_signal_new ("input-event",
		  G_TYPE_FROM_CLASS (gobject_class),
		  G_SIGNAL_RUN_LAST,
		  G_STRUCT_OFFSET (ClutterStageClass, input_event),
		  NULL, NULL,
		  clutter_marshal_VOID__BOXED,
		  G_TYPE_NONE, 1,
		  CLUTTER_TYPE_EVENT);
  stage_signals[BUTTON_PRESS_EVENT] =
    g_signal_new ("button-press-event",
		  G_TYPE_FROM_CLASS (gobject_class),
		  G_SIGNAL_RUN_LAST,
		  G_STRUCT_OFFSET (ClutterStageClass, button_press_event),
		  NULL, NULL,
		  clutter_marshal_VOID__BOXED,
		  G_TYPE_NONE, 1,
		  CLUTTER_TYPE_EVENT);
  stage_signals[BUTTON_RELEASE_EVENT] =
    g_signal_new ("button-release-event",
		  G_TYPE_FROM_CLASS (gobject_class),
		  G_SIGNAL_RUN_LAST,
		  G_STRUCT_OFFSET (ClutterStageClass, button_release_event),
		  NULL, NULL,
		  clutter_marshal_VOID__BOXED,
		  G_TYPE_NONE, 1,
		  CLUTTER_TYPE_EVENT);
  stage_signals[BUTTON_PRESS_EVENT] =
    g_signal_new ("key-press-event",
		  G_TYPE_FROM_CLASS (gobject_class),
		  G_SIGNAL_RUN_LAST,
		  G_STRUCT_OFFSET (ClutterStageClass, key_press_event),
		  NULL, NULL,
		  clutter_marshal_VOID__BOXED,
		  G_TYPE_NONE, 1,
		  CLUTTER_TYPE_EVENT);
  stage_signals[BUTTON_RELEASE_EVENT] =
    g_signal_new ("key-release-event",
		  G_TYPE_FROM_CLASS (gobject_class),
		  G_SIGNAL_RUN_LAST,
		  G_STRUCT_OFFSET (ClutterStageClass, key_release_event),
		  NULL, NULL,
		  clutter_marshal_VOID__BOXED,
		  G_TYPE_NONE, 1,
		  CLUTTER_TYPE_EVENT);
  stage_signals[MOTION_EVENT] =
    g_signal_new ("motion-event",
		  G_TYPE_FROM_CLASS (gobject_class),
		  G_SIGNAL_RUN_LAST,
		  G_STRUCT_OFFSET (ClutterStageClass, motion_event),
		  NULL, NULL,
		  clutter_marshal_VOID__BOXED,
		  G_TYPE_NONE, 1,
		  CLUTTER_TYPE_EVENT);
  
  g_type_class_add_private (gobject_class, sizeof (ClutterStagePrivate));
}

static void
clutter_stage_init (ClutterStage *self)
{
  ClutterStagePrivate *priv;
  
  self->priv = priv = CLUTTER_STAGE_GET_PRIVATE (self);

  priv->want_offscreen = FALSE;
  priv->want_fullscreen = FALSE;
  priv->hide_cursor = FALSE;

  priv->xwin_width  = 100;
  priv->xwin_height = 100;

  priv->color.red   = 0xff;
  priv->color.green = 0xff;
  priv->color.blue  = 0xff;
  priv->color.alpha = 0xff;
  
  clutter_element_set_size (CLUTTER_ELEMENT (self), 640, 480);
}

/**
 * clutter_stage_get_default:
 *
 * Returns the main stage.  #ClutterStage is a singleton, so
 * the stage will be created the first time this function is
 * called (typically, inside clutter_init()); all the subsequent
 * calls to clutter_stage_get_default() will return the same
 * instance, with its reference count increased.
 *
 * Return value: the main #ClutterStage.  Use g_object_unref()
 *   when finished using it.
 */
ClutterElement *
clutter_stage_get_default (void)
{
  ClutterElement *retval = NULL;
  
  if (!stage_singleton)
    {
      stage_singleton = g_object_new (CLUTTER_TYPE_STAGE, NULL);
      retval = CLUTTER_ELEMENT (stage_singleton);
    }
  else
    {
      retval = CLUTTER_ELEMENT (stage_singleton);
      g_object_ref (retval);
    }

  return retval;
  
}


/**
 * clutter_stage_get_xwindow
 * @stage: A #ClutterStage
 *
 * Get the stages underlying x window ID.
 *
 * Return Value: Stage X Window XID
 **/
Window
clutter_stage_get_xwindow (ClutterStage *stage)
{
  return stage->priv->xwin;
}

/**
 * clutter_stage_set_color
 * @stage: A #ClutterStage
 * @color: A #ClutterColor
 * 
 * Set the stage color.
 **/
void
clutter_stage_set_color (ClutterStage       *stage,
			 const ClutterColor *color)
{
  ClutterStagePrivate *priv;

  g_return_if_fail (CLUTTER_IS_STAGE (stage));
  g_return_if_fail (color != NULL);
  
  priv = stage->priv;
  priv->color.red = color->red;
  priv->color.green = color->green;
  priv->color.blue = color->blue;
  priv->color.alpha = color->alpha;

  if (CLUTTER_ELEMENT_IS_VISIBLE (CLUTTER_ELEMENT (stage)))
    clutter_element_queue_redraw (CLUTTER_ELEMENT (stage));
  
  g_object_notify (G_OBJECT (stage), "color");
}

/**
 * clutter_stage_get_color
 * @stage: A #ClutterStage
 * @color: return location for a #ClutterColor
 * 
 * Request the stage color.
 */
void
clutter_stage_get_color (ClutterStage *stage,
			 ClutterColor *color)
{
  ClutterStagePrivate *priv;
  
  g_return_if_fail (CLUTTER_IS_STAGE (stage));
  g_return_if_fail (color != NULL);

  priv = stage->priv;
  
  color->red = priv->color.red;
  color->green = priv->color.green;
  color->blue = priv->color.blue;
  color->alpha = priv->color.alpha;
}

static void
snapshot_pixbuf_free (guchar   *pixels,
		      gpointer  data)
{
  g_free(pixels);
}

/**
 * clutter_stage_snapshot
 * @stage: A #ClutterStage
 * @x: x coordinate of the first pixel that is read from stage
 * @y: y coordinate of the first pixel that is read from stage
 * @width: Width dimention of pixels to be read, or -1 for the
 *   entire stage width
 * @height: Height dimention of pixels to be read, or -1 for the
 *   entire stage height
 *
 * Gets a pixel based representation of the current rendered stage.
 *
 * Return value: pixel representation as a  #GdkPixbuf
 **/
GdkPixbuf*
clutter_stage_snapshot (ClutterStage *stage,
			gint          x,
			gint          y,
			gint          width,
			gint          height)
{
  guchar    *data;
  GdkPixbuf *pixb, *fpixb;
  ClutterElement *element;

  g_return_val_if_fail (CLUTTER_IS_STAGE (stage), NULL);
  g_return_val_if_fail (x >= 0 && y >= 0, NULL);

  element = CLUTTER_ELEMENT (stage);

  if (width < 0)
    width = clutter_element_get_width (element);

  if (height < 0)
    height = clutter_element_get_height (element);

  data = g_malloc0 (sizeof (guchar) * width * height * 3);

  glReadPixels (x, 
		clutter_element_get_height (element) 
		- y - height,
		width, 
		height, GL_RGB, GL_UNSIGNED_BYTE, data);

  pixb = gdk_pixbuf_new_from_data (data,
				   GDK_COLORSPACE_RGB, 
				   FALSE, 
				   8, 
				   width, 
				   height,
				   width * 3,
				   snapshot_pixbuf_free,
				   NULL);

  if (pixb == NULL)
    return NULL;

  fpixb = gdk_pixbuf_flip (pixb, TRUE);

  g_object_unref (pixb);

  return fpixb;
}

/**
 * clutter_stage_get_element_at_pos:
 * @stage: a #ClutterStage
 * @x: the x coordinate
 * @y: the y coordinate
 *
 * If found, retrieves the element that the (x, y) coordinates.
 *
 * Return value: the #ClutterElement at the desired coordinates,
 *   or %NULL if no element was found.
 */
ClutterElement*
clutter_stage_get_element_at_pos (ClutterStage *stage,
				  gint          x,
				  gint          y)
{
  ClutterElement *found = NULL;
  GLuint          buff[64] = {0};
  GLint           hits, view[4];
 
  glSelectBuffer(64, buff);
  glGetIntegerv(GL_VIEWPORT, view);
  glRenderMode(GL_SELECT);

  glInitNames();

  glPushName(0);
 
  glMatrixMode(GL_PROJECTION);
  glPushMatrix();
  glLoadIdentity();

  /* This is gluPickMatrix(x, y, 1.0, 1.0, view); */
  glTranslatef((view[2] - 2 * (x - view[0])),
	       (view[3] - 2 * (y - view[1])), 0);
  glScalef(view[2], -view[3], 1.0);

  perspective (60.0f, 1.0f, 0.1f, 100.0f); 

  glMatrixMode(GL_MODELVIEW);

  clutter_element_paint (CLUTTER_ELEMENT (stage));

  glMatrixMode(GL_PROJECTION);
  glPopMatrix();

  hits = glRenderMode(GL_RENDER);

  if (hits != 0)
    {
#if 0
      gint i
      for (i = 0; i < hits; i++)
	g_print ("Hit at %i\n", buff[i * 4 + 3]);
#endif
  
      found = clutter_group_find_child_by_id (CLUTTER_GROUP (stage), buff[3]);
    }
  
  sync_gl_viewport (stage);

  return found;
}