Cast Remote Display API: Processing

JUL 10, 2015

Posted by Leon Nicholls, Developer Programs Engineer

Remote Display on Google Cast allows your app to display both on your mobile and Cast device at the same time. Processing is a programming language that allows artists and hobbyists to create advanced graphics and interactive exhibitions. By putting these two things together we were able to quickly create stunning visual art and display it on the big screen just by bringing our phone to the party or gallery. This article describes how we added support for the Google Cast Remote Display APIs to Processing for Android and how you can too.

An example app from the popular Processing toxiclibs library on Cast. Download the code and run it on your own Chromecast!

A little background

Processing has its own IDE and has many contributed libraries that hide the technical details of various input, output and rendering technologies. Users of Processing with just basic programming skills can create complicated graphical scenes and visualizations.

To write a program in the Processing IDE you create a “sketch” which involves adding code to life-cycle callbacks that initialize and draw the scene. You can run the sketch as a Java program on your desktop. You can also enable support for Processing for Android and then run the same sketch as an app on your Android mobile device. It also supports touch events and sensor data to interact with the generated apps.

Instead of just viewing the graphics on the small screen of the Android device, we can do better by projecting the graphics on a TV screen. Google Cast Remote Display APIs makes it easy to bring graphically intensive apps to Google Cast receivers by using the GPUs, CPUs and sensors available on the mobile devices you already have.

How we did it

Adding support for Remote Display involved modifying the Processing for Android Mode source code. To compile the Android Mode you first need to compile the source code of the Processing IDE. We started with the source code of the current stable release version 2.2.1 of the Processing IDE and compiled it using its Ant build script (detailed instructions are included along with the code download). We then downloaded the Android SDK and source code for the Android Mode 0232. After some minor changes to its build config to support the latest Android SDK version, we used Ant to build the Android Mode zip file. The zip file was unzipped into the Processing IDE modes directory.

We then used the IDE to open one of the Processing example sketches and exported it as an Android project. In the generated project we replaced the processing-core.jar library with the source code for Android Mode. We also added a Gradle build config to the project and then imported the project into Android Studio.

The main Activity for a Processing app is a descendent of the Android Mode PApplet class. The PApplet class uses a GLSurfaceView for rendering 2D and 3D graphics. We needed to change the code to use that same GLSurfaceView for the Remote Display API.

It is a requirement in the Google Cast Design Checklist for the Cast button to be visible on all screens. We changed PApplet to be an ActionBarActivity so that we can show the Cast button in the action bar. The Cast button was added by using a MediaRouteActionProvider. To only list Google Cast devices that support Remote Display, we used a MediaRouteSelector with an App ID we obtained from the Google Cast SDK Developer Console for a Remote Display Receiver.

Next, we created a class called PresentationService that extends CastRemoteDisplayLocalService. The service allows the app to keep the remote display running even when it goes into the background. The service requires a CastPresentation instance for displaying its content. The CastPresentation instance uses the GLSurfaceView from the PApplet class for its content view. However, setting the CastPresentation content view requires some changes to PApplet so that the GLSurfaceView isn’t initialized in its onCreate, but waits until the service onRemoteDisplaySessionStarted callback is invoked.

When the user selects a Cast device in the Cast button menu and the MediaRouter onRouteSelected event is called, the service is started with CastRemoteDisplayLocalService.startService. When the user disconnects from a Cast device using the Cast button, MediaRouter onRouteUnselected event is called and the service is stopped by using CastRemoteDisplayLocalService.stopService.

For the mobile display, we display an image bitmap and forward the PApplet touch events to the existing surfaceTouchEvent method. When you run the Android app, you can use touch gestures on the display of the mobile device to control the interaction on the TV. Take a look at this video of some of the Processing apps running on a Chromecast.

Most of the new code is contained in the PresentationService and RemoteDisplayHelper classes. Your mobile device needs to have at least Android KitKat and Google Play services version 7.5.71.

You can too

Now you can try the Remote Display APIs in your Processing apps. Instead of changing the generated code every time you export your Android Mode project, we recommend that you use our project as a base and simply copy your generated Android code and libraries to our project. Then simply modify the project build file and update the manifest to start the app with your sketch’s main Activity.

To see a more detailed description on how to use the Remote Display APIs, read our developer documentation. We are eager to see what Processing artists can do with this code in their projects.