Innovative Interfaces with Clutter

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Use Clutter to develop OpenGL applications with rich 2-D and 3-D interfaces that include object rotation, scaling, texturing and more.

Figure 2. The simple “Hello World” Clutter Program Running on Ubuntu

Now, let's take a look at a more useful Clutter program. Our program will use the Clutter GStreamer library to display and control a video file on the stage. Connect GStreamer's video output to a video texture, which is then displayed on the stage and can be manipulated as an actor.

There are three main actors on the stage: the play button, the pause button and the GStreamer video. When the pause button is pressed, the video pauses, and it will continue playing when the play button is pressed. The program is shown in Listing 2, and the window is shown in Figure 3.

After the initial setup, define a signal for responding to mouse clicks. Clutter uses signals to respond to events in the same way GTK+ does. The signal dictates that when any button on the mouse is clicked, the specified function is called.

Next, create the buttons by creating rectangle actors for the button shape and text actors for the button text. Remember that Clutter is not widget-based, and there are no default button widgets as part of the API.

To create the video, you need a video texture. A video texture is a physical plane on which GStreamer can display the video. The video texture can be manipulated on the stage like an ordinary actor.

You also need two other GStreamer elements to play the video: a playbin and a pipeline. Clutter-Gstreamer will play any file type that GStreamer can play. After creating the playbin, set the location of the video to play and add the playbin to the pipeline.

Then, set the position of the video texture, add it to the stage and tell GStreamer to start playing the video.

Finally, create the mouseClick function. This function is called when the mouse is clicked anywhere on the stage. Check to see if the left mouse button was clicked inside one of the buttons and if it was, change the size and color of the buttons to give visual feedback of the click. Tell GStreamer to start or stop the video depending on which button was pressed.

Figure 3. Clutter Video Player Playing the Monty Python Skit “How Not to Be Seen”

An important feature of Clutter is its animation API. Animating any actors on the stage is easy with Clutter. Using the animation API, you can add smooth animations and effects to your Clutter application.

In the next example, let's take the GStreamer video texture and manipulate it in three dimensions. The GStreamer texture is rotating on the y-axis constantly.

The program is shown in Listing 3, and the window is shown in Figure 4.

After the initial setup, create a timeline. Timelines are used in Clutter to control animation and time events. The example timeline lasts for 100 frames at ten frames per second, and the timeline is set to loop forever.

Next create an alpha (see The Alpha Functions sidebar) for the animation, assign it to your timeline and give it a smooth step decreasing function. In simplest terms, the alpha is used to control the speed of the animation. The smooth step function causes the animation to speed up, then slow down, come to a halt, and then start up again. Clutter has several functions built in that you can use with the alpha, including sine, exponential and ramp functions.

Next, define the Rotation behavior the animation uses. Clutter uses behavior effects to describe animations. The Opacity behavior, for example, can change the visual alpha of an actor, making it transparent or opaque. Other behaviors include Scale, Path, Depth, B-Spline and Ellipse.

In this example, we tell our Rotation behavior to rotate over the x-axis, rotate clockwise, start rotating at an angle of zero, end at 360, and finally, tell it to use the alpha created earlier, respectively. After that, the rotational center, or the point the actor rotates around, is set to the approximate center of the GStreamer texture and the Rotation behaviour is applied to the video texture.

In addition to the normal startup steps, the timeline must be started.

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Incorrect statements about Elisa

Matt Layman's picture

Alex,

Thanks for posting an article about clutter. It is a great toolkit and worth talking about. However, your introduction makes statements that are simply incorrect.

You reference the Elisa project. First, it's interesting because your article was written in October yet Elisa changed its name to Moovida back in June of 2009.

Second, and more importantly, Elisa/Moovida has never used the clutter toolkit. They use the Pigment toolkit which is developed by Fluendo, the same company that develops Moovida (so the Moovida project is pretty invested into using their own toolkit). I'm sure that they work hard to make their toolkit useful for their purposes and wouldn't want to have their work mis-credited to some other toolkit.

I'm keenly aware that Moovida uses Pigment because I develop code for the Entertainer Media Center, another media center application that *does* use clutter. I probably wouldn't be working on this other project if there was already a larger media center application that used clutter.

I realize that your article is mostly tailored to be a HOWTO for using clutter, but I would expect that as a journalist for Linux Journal, you would research and verify your statements. I hope that my journalistic critique is not received negatively. I just want to give people credit where credit is due. The Fluendo developers deserve that much.

Thanks,
Matt

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