Edmon Gween, actor, on his deathbed An actor's a guy who if you ain't talkin' about him, ain't listening.

When building a User Interface with Clutter, the visible part of the UI — that is, what is displayed on the screen — is commonly referred to as "the scene graph". Like every graph, a scene graph is composed by nodes. Every node on the Clutter scene graph is an actor. Every actor has a single relationship with the others: it can be the parent of another actor, or a child of another actor. The stage is an actor that can have children but cannot have any parent. Actors have different attributes: a position, a size, a scale factor, a rotation angle on each axis (relative to a specific center on the normal plane for that axis), an opacity factor. The scene graph is not fixed: it can be changed, not only by adding or removing actors, but also by changing the parent-child relationship: it is possible, for instance, to move an entire section of the scene graph from one parent actor to another.

You want to know when the position or the size, or both, of an actor change, for instance to update an unrelated actor or some internal state.

You can use the notify signal, detailed with the coordinate or the dimension you want to know has changed: g_signal_connect (actor, "notify::x", G_CALLBACK (on_x_changed), NULL); g_signal_connect (actor, "notify::height", G_CALLBACK (on_height_changed), NULL); g_signal_connect (actor, "notify::depth", G_CALLBACK (on_depth_changed), NULL); If you want to know if any of the coordinates or dimensions of an actor have been changed, except for depth, you can use the allocation-changed signal: g_signal_connect (actor, "allocation-changed", G_CALLBACK (on_allocation_changed), NULL); The signature for the handler of the "notify" signal is: void on_notify (GObject *gobject, GParamSpec *pspec, gpointer user_data); While the signature for the handler of the "allocation-changed" signal is: void on_allocation_changed (ClutterActor *actor, const ClutterActorBox *allocation, ClutterAllocationFlags flags, gpointer user_data);

Any change the position and size of an actor will cause a change in the allocation of the actor itself. This will update the values of the x, y, width and height properties as well. The first technique allows a greater deal of granularity, allowing you to know what exactly changed. Inside the callback for the signal you can query the value of the property: void on_x_changed (GObject *gobject, GParamSpec *pspec, gpointer user_data) { gint x_value = 0; /* Round the X coordinate to the nearest pixel */ x_value = floorf (clutter_actor_get_x (CLUTTER_ACTOR (gobject))) + 0.5; g_print ("The new X coordinate is '%d' pixels\n", x_value); } The second technique is more indicated if you want to get notification that any of the positional or dimensional attributes changed, except for the depth: void on_allocation_changed (ClutterActor *actor, const ClutterActorBox *allocation, ClutterAllocationFlags flags, gpointer user_data) { g_print ("The bounding box is now: (%.2f, %.2f) (%.2f x %.2f)\n", clutter_actor_box_get_x (allocation), clutter_actor_box_get_y (allocation), clutter_actor_box_get_width (allocation), clutter_actor_box_get_height (allocation)); } All actors will update these properties when their size or position change. Note that the stage, on the other hand, will not notify on position changes, so it is not possible to use the x and y properties to know that the platform-specific window embedding the stage has been moved — if the platform supports a windowing system. In order to achieve that you will have to use backend-specific API to extract the surface used by the stage and then platform-specific API to retrieve its coordinates.

You want to override the way an actor paints itself without creating a subclass.

You can use the paint signal to invoke a callback that will be executed before the actor's paint implementation: g_signal_connect (actor, "paint", G_CALLBACK (on_paint), NULL); You can paint something after the actor's paint implementation by using the g_signal_connect_after() function instead of g_signal_connect(): g_signal_connect_after (actor, "paint", G_CALLBACK (on_paint_after), NULL); The signature for the handler of the "paint" signal is: void on_paint (ClutterActor *actor, gpointer user_data);

The paint cycle in Clutter works its way recursively from the stage through every child. Whenever an Actor is going to be painted it will be positioned in a new frame of reference according to the list of transformations (scaling, rotation and additional translations). After that, the "paint" signal will be emitted. The "paint" signal is defined as run-last, that is the signal handlers connected to it using g_signal_connetc() will be called first; then the default handler defined by the Actor's sub-class will be called; finally, all the signal handlers connected to the signal using g_signal_connect_after() will be called. This allows pre- and post-default paint handlers, and it also allows completely overriding the way an Actor draws itself by default; for instance: void on_paint (ClutterActor *actor) { do_my_paint (actor); g_signal_stop_emission_by_name (actor, "paint"); } The code above will prevent the default paint implementation of the actor from running.

You want an actor to be transparent so that other actors are visible through it.

Change the actor's opacity so that it is partially (or even fully) transparent: /* 25% transparency */ clutter_actor_set_opacity (actor, 191); /* 50% transparency */ clutter_actor_set_opacity (actor, 122); /* completely transparent */ clutter_actor_set_opacity (actor, 0); Any actor covered or overlapped by the transparent actor should be visible through it; the Discussion section gives some examples of how visible you can expect the covered or overlapped actor to be.

Opacity is a property of every ClutterActor. It is a float on a scale from 0 (invisible) to 255 (completely opaque). Actors with 0 < opacity < 255 will have a varying amount of solidity on the stage, so other actors may be visible through them. For example, below are 4 yellow rectangles overlapping a white rectangle on a blue stage: The effect of different opacities levels on an actor's appearance The rectangles have the following opacities: top-left: 255 (0% transparency) top-right: 191 (25% transparency) bottom-right: 122 (50% transparency) bottom-left: 61 (75% transparency) Notice how both the stage and the white rectangle are visible through the yellow rectangles. As opacity is a property of every actor, it can be animated like any other GObject property, using any of the approaches in the animation API. The following sections cover some other considerations when working with actor opacity.

If a container has its opacity set, any children of the container have their opacity combined with their parent's opacity. For example, if a parent has an opacity of 122 (50% transparent) and the child also has an opacity of 122, the child's effective opacity is 25% (opacity = 61, and it is 75% transparent). To demonstrate the visual effect of this, here are three rectangles with the same color but different opacity settings, inside parents which also have different opacity settings: How a container's opacity affects the opacity of its children The left-hand rectangle has opacity = 255 and is in a ClutterGroup with opacity = 255. This means it is fully opaque. The middle rectangle has opacity = 255 and is in a ClutterGroup with opacity = 122. Notice that the parent opacity makes the rectangle appear darker, as the stage colour is showing through from behind. The right-hand rectangle has opacity = 122 and is in a ClutterGroup with opacity = 122. Notice that the rectangle appears to be even darker, as the stage colour is showing through both the rectangle and its parent. Similarly, ClutterColor also contains an alpha property which governs the transparency of the color. Where an actor can have a color set (e.g. ClutterRectangle) the alpha value of the color also affects the transparency of the actor, for example:

Each actor has two more aspects which affect its apparent opacity: An actor's depth can have an effect if the stage has fog (a depth cueing effect) turned on. As an actor's depth increases, the actor apparently "recedes" from view and gradually blends into the colour of the stage. This produces an effect similar to making the actor transparent. See the ClutterStage documentation for more details about fog. Depth also needs to be considered if you want one actor to be visible through another: the actor you want to see through a transparent actor must be "deeper" than (or at the same depth as) the transparent actor. The depth order governs how actors within a ClutterContainer implementation are placed with respect to each other. Depth ordering is not the same thing as depth: depth ordering records relationships between actors at the same depth. If you have two overlapping actors actorA and actorB in a container, and you want actorA (opaque) to be visible through actorB (transparent), you should ensure that actorB is "above" actorA in the depth ordering. You could do this as follows: /* * raise actorB so it is above actorA in the depth order; * NB actorA and actorB both need to be in the same container * for this to work */ clutter_actor_raise (actorB, actorA); clutter_actor_raise(), clutter_actor_lower() and related ClutterActor functions set depth ordering on actors; see also ClutterContainer's clutter_container_raise_child() and clutter_container_lower_child() functions.

You want to create a ClutterActor subclass, but don't want it to be rectangular; for example, you want a star-shaped actor.

Use Cogl primitives to draw the actor. Below is an example of the pick and paint implementations for a star-shaped StarActor class (an extension of ClutterActor). Like ClutterRectangle, it has a private struct internally, which contains a ClutterColor denoting the color it should be painted. This is used to set the Cogl source color. priv->color; clutter_actor_get_allocation_box (actor, &allocation); clutter_actor_box_get_size (&allocation, &width, &height); tmp_alpha = clutter_actor_get_paint_opacity (actor) * color.alpha / 255; cogl_path_new (); cogl_set_source_color4ub (color.red, color.green, color.blue, tmp_alpha); /* create and store a path describing a star */ cogl_path_move_to (width * 0.5, 0); cogl_path_line_to (width, height * 0.75); cogl_path_line_to (0, height * 0.75); cogl_path_move_to (width * 0.5, height); cogl_path_line_to (0, height * 0.25); cogl_path_line_to (width, height * 0.25); cogl_path_line_to (width * 0.5, height); cogl_path_fill (); } static void star_actor_pick (ClutterActor *actor, const ClutterColor *pick_color) { if (!clutter_actor_should_pick_paint (actor)) return; ClutterActorBox allocation = { 0, }; gfloat width, height; clutter_actor_get_allocation_box (actor, &allocation); clutter_actor_box_get_size (&allocation, &width, &height); cogl_path_new (); cogl_set_source_color4ub (pick_color->red, pick_color->green, pick_color->blue, pick_color->alpha); /* create and store a path describing a star */ cogl_path_move_to (width * 0.5, 0); cogl_path_line_to (width, height * 0.75); cogl_path_line_to (0, height * 0.75); cogl_path_move_to (width * 0.5, height); cogl_path_line_to (0, height * 0.25); cogl_path_line_to (width, height * 0.25); cogl_path_line_to (width * 0.5, height); cogl_path_fill (); } ]]> If you need more information about how to implement your own ClutterActor, see the Clutter reference manual. Note that the code in these two functions is virtually identical: the Discussion section suggests how to remove this redundancy.

The above is one approach to creating a non-rectangular actor. But it's also possible to get a similar effect by subclassing an existing actor (like ClutterRectangle) and giving it a non-rectangular appearance. You could do this by making the underlying rectangle transparent and then drawing on top of it (e.g. using Cairo or Cogl). However, if you then made such an actor reactive, events like mouse button presses would be triggered from anywhere on the underlying rectangle. This is true even if the visible part of the actor only partially fills the rectangle (underneath, it's still a rectangle). The advantage of using Cogl paths is that the reactive area of the actor is defined by the Cogl path. So if you have a star-shaped actor, only clicks (or other events) directly on the star will have any effect on it.

In the example shown, cogl_path_move_to() and cogl_path_line_to() are used. These take absolute x and y coordinates as arguments, relative to the GL 'modelview' transform matrix; in the case of an actor's paint implementation, relative to the bounding box for the actor. So if an actor has width and height of 50 pixels, and you used cogl_move_to (25, 25) in its paint implementation, the "pen" moves to the centre of the actor, regardless of where the actor is positioned on the stage. Similarly, using cogl_path_line_to() creates a line segment from the current pen position to the absolute coordinates (x, y) specified. The Cogl API also provides various "rel" variants of the path functions (e.g. cogl_path_rel_line_to()), which create path segments relative to the current pen position (i.e. pen_x + x, pen_y + y). It's important to note that the path isn't drawn until you call cogl_path_stroke() (to draw the path segments) or cogl_path_fill() (to fill the area enclosed by the path). The path is cleared once it's been drawn. Using the *_preserve variants of these functions draws the path and retains it (so it could be drawn again).

Note that the Cogl primitives API provides other types of path segment beyond straight lines that we didn't use here, including: Bezier curves (cogl_path_curve_to()) Arcs (cogl_path_arc()) Polygons (cogl_path_polygon()) Rectangles (cogl_path_rectangle()) Rectangles with rounded corners (cogl_path_round_rectangle()) Ellipses (cogl_path_ellipse()) If you need more flexibility than is available in the Cogl path API, you can make direct use of the CoglVertexBuffer API instead. This is a lower-level API, but could potentially be used to draw more complex shapes.

The disadvantage of the code above is that the paths are stored in two places: once for pick, and once for paint. It would make sense to store the path in one place and reference it from both of these functions to prevent duplication. Clutter provides a ClutterPath API for storing generic path descriptions. It can be used to describe paths which translate to Cogl or Cairo paths, and can also be used to describe animation paths. We can use a ClutterPath instance stored inside the actor to define the path for pick and paint; then, inside those functions, we translate the ClutterPath into Cogl path function calls (NB ClutterPath is effectively a declarative method for defining a path, while the Cogl path API is imperative). First we add a path member to the private struct for the StarActor class (using standard GObject mechanisms). The init implementation for StarActor creates an empty path: static void star_actor_init (StarActor *self) { self->priv = STAR_ACTOR_GET_PRIVATE (self); self->priv->path = clutter_path_new (); clutter_actor_set_reactive (CLUTTER_ACTOR (self), TRUE); } One consideration is that the path coordinates need to fit inside the actor's bounding box. So as the actor's allocation changes, path also needs to change. We can do this by implementing allocate for the StarActor class: priv->path; gfloat width, height; clutter_actor_box_get_size (box, &width, &height); /* create and store a path describing a star */ clutter_path_clear (path); clutter_path_add_move_to (path, width * 0.5, 0); clutter_path_add_line_to (path, width, height * 0.75); clutter_path_add_line_to (path, 0, height * 0.75); clutter_path_add_move_to (path, width * 0.5, height); clutter_path_add_line_to (path, 0, height * 0.25); clutter_path_add_line_to (path, width, height * 0.25); clutter_path_add_line_to (path, width * 0.5, height); CLUTTER_ACTOR_CLASS (star_actor_parent_class)->allocate (actor, box, flags); } ]]> This clears then adds segments to the ClutterPath stored with the StarActor instance. The positioning and lengths of the segments are relative to the size of the actor when its allocation changes. The pick and paint functions now reference the ClutterPath (only the pick is shown below); and to turn the path into drawing operations, we implement a star_actor_convert_clutter_path_node() function which takes a ClutterPathNode and converts it into its Cogl equivalent: type) { case CLUTTER_PATH_MOVE_TO: knot = node->points[0]; cogl_path_move_to (knot.x, knot.y); break; case CLUTTER_PATH_LINE_TO: knot = node->points[0]; cogl_path_line_to (knot.x, knot.y); break; default: break; } } static void star_actor_pick (ClutterActor *actor, const ClutterColor *pick_color) { if (!clutter_actor_should_pick_paint (actor)) return; ClutterActorBox allocation = { 0, }; gfloat width, height; ClutterPath *path = STAR_ACTOR (actor)->priv->path; clutter_actor_get_allocation_box (actor, &allocation); clutter_actor_box_get_size (&allocation, &width, &height); cogl_path_new (); cogl_set_source_color4ub (pick_color->red, pick_color->green, pick_color->blue, pick_color->alpha); clutter_path_foreach (path, star_actor_convert_clutter_path_node, NULL); cogl_path_fill (); } ]]> The conversion function only covers ClutterPathNode types encountered in this actor. Instead of converting to Cogl path operations, another alternative would be to use the clutter_path_to_cairo_path() function to write directly from the ClutterPath onto a Cairo context.