Android includes support for high performance 2D and 3D graphics with the Open Graphics Library (OpenGL), specifically, the OpenGL ES API. OpenGL is a cross-platform graphics API that specifies a standard software interface for 3D graphics processing hardware. OpenGL ES is a flavor of the OpenGL specification intended for embedded devices. The OpenGL ES 1.0 and 1.1 API specifications have been supported since Android 1.0. Beginning with Android 2.2 (API Level 8), the framework supports the OpenGL ES 2.0 API specification.
Note: The specific API provided by the Android framework is similar to the J2ME JSR239 OpenGL ES API, but is not identical. If you are familiar with J2ME JSR239 specification, be alert for variations.
Android supports OpenGL both through its framework API and the Native Development Kit (NDK). This topic focuses on the Android framework interfaces. For more information about the NDK, see the Android NDK.
There are two foundational classes in the Android framework that let you create and manipulate graphics with the OpenGL ES API: {@link android.opengl.GLSurfaceView} and {@link android.opengl.GLSurfaceView.Renderer}. If your goal is to use OpenGL in your Android application, understanding how to implement these classes in an activity should be your first objective.
The {@link android.opengl.GLSurfaceView.Renderer} interface requires that you implement the following methods:
Once you have established a container view for OpenGL using {@link android.opengl.GLSurfaceView} and {@link android.opengl.GLSurfaceView.Renderer}, you can begin calling OpenGL APIs using the following classes:
If you'd like to start building an app with OpenGL right away, follow the Displaying Graphics with OpenGL ES class.
If your application uses OpenGL features that are not available on all devices, you must include these requirements in your AndroidManifest.xml file. Here are the most common OpenGL manifest declarations:
<!-- Tell the system this app requires OpenGL ES 2.0. -->
<uses-feature android:glEsVersion="0x00020000" android:required="true" />
Adding this declaration causes Google Play to restrict your application from being installed on devices that do not support OpenGL ES 2.0.
Declaring texture compression requirements in your manifest hides your application from users with devices that do not support at least one of your declared compression types. For more information on how Google Play filtering works for texture compressions, see the Google Play and texture compression filtering section of the {@code <supports-gl-texture>} documentation.
One of the basic problems in displaying graphics on Android devices is that their screens can vary in size and shape. OpenGL assumes a square, uniform coordinate system and, by default, happily draws those coordinates onto your typically non-square screen as if it is perfectly square.
Figure 1. Default OpenGL coordinate system (left) mapped to a typical Android device screen (right).
The illustration above shows the uniform coordinate system assumed for an OpenGL frame on the left, and how these coordinates actually map to a typical device screen in landscape orientation on the right. To solve this problem, you can apply OpenGL projection modes and camera views to transform coordinates so your graphic objects have the correct proportions on any display.
In order to apply projection and camera views, you create a projection matrix and a camera view matrix and apply them to the OpenGL rendering pipeline. The projection matrix recalculates the coordinates of your graphics so that they map correctly to Android device screens. The camera view matrix creates a transformation that renders objects from a specific eye position.
In the ES 1.0 API, you apply projection and camera view by creating each matrix and then adding them to the OpenGL environment.
public void onSurfaceChanged(GL10 gl, int width, int height) {
gl.glViewport(0, 0, width, height);
// make adjustments for screen ratio
float ratio = (float) width / height;
gl.glMatrixMode(GL10.GL_PROJECTION); // set matrix to projection mode
gl.glLoadIdentity(); // reset the matrix to its default state
gl.glFrustumf(-ratio, ratio, -1, 1, 3, 7); // apply the projection matrix
}
public void onDrawFrame(GL10 gl) {
...
// Set GL_MODELVIEW transformation mode
gl.glMatrixMode(GL10.GL_MODELVIEW);
gl.glLoadIdentity(); // reset the matrix to its default state
// When using GL_MODELVIEW, you must set the camera view
GLU.gluLookAt(gl, 0, 0, -5, 0f, 0f, 0f, 0f, 1.0f, 0.0f);
...
}
In the ES 2.0 API, you apply projection and camera view by first adding a matrix member to the vertex shaders of your graphics objects. With this matrix member added, you can then generate and apply projection and camera viewing matrices to your objects.
private final String vertexShaderCode =
// This matrix member variable provides a hook to manipulate
// the coordinates of objects that use this vertex shader.
"uniform mat4 uMVPMatrix; \n" +
"attribute vec4 vPosition; \n" +
"void main(){ \n" +
// The matrix must be included as part of gl_Position
// Note that the uMVPMatrix factor *must be first* in order
// for the matrix multiplication product to be correct.
" gl_Position = uMVPMatrix * vPosition; \n" +
"} \n";
Note: The example above defines a single transformation matrix member in the vertex shader into which you apply a combined projection matrix and camera view matrix. Depending on your application requirements, you may want to define separate projection matrix and camera viewing matrix members in your vertex shaders so you can change them independently.
public void onSurfaceCreated(GL10 unused, EGLConfig config) {
...
muMVPMatrixHandle = GLES20.glGetUniformLocation(mProgram, "uMVPMatrix");
...
}
public void onSurfaceCreated(GL10 unused, EGLConfig config) {
...
// Create a camera view matrix
Matrix.setLookAtM(mVMatrix, 0, 0, 0, -3, 0f, 0f, 0f, 0f, 1.0f, 0.0f);
}
public void onSurfaceChanged(GL10 unused, int width, int height) {
GLES20.glViewport(0, 0, width, height);
float ratio = (float) width / height;
// create a projection matrix from device screen geometry
Matrix.frustumM(mProjMatrix, 0, -ratio, ratio, -1, 1, 3, 7);
}
public void onDrawFrame(GL10 unused) {
...
// Combine the projection and camera view matrices
Matrix.multiplyMM(mMVPMatrix, 0, mProjMatrix, 0, mVMatrix, 0);
// Apply the combined projection and camera view transformations
GLES20.glUniformMatrix4fv(muMVPMatrixHandle, 1, false, mMVPMatrix, 0);
// Draw objects
...
}
For a complete example of how to apply projection and camera view with OpenGL ES 2.0, see the Displaying Graphics with OpenGL ES class.
In OpenGL, the face of a shape is a surface defined by three or more points in three-dimensional space. A set of three or more three-dimensional points (called vertices in OpenGL) have a front face and a back face. How do you know which face is front and which is the back? Good question. The answer has to do with winding, or, the direction in which you define the points of a shape.
Figure 1. Illustration of a coordinate list which translates into a counterclockwise drawing order.
In this example, the points of the triangle are defined in an order such that they are drawn in a counterclockwise direction. The order in which these coordinates are drawn defines the winding direction for the shape. By default, in OpenGL, the face which is drawn counterclockwise is the front face. The triangle shown in Figure 1 is defined so that you are looking at the front face of the shape (as interpreted by OpenGL) and the other side is the back face.
Why is it important to know which face of a shape is the front face? The answer has to do with a commonly used feature of OpenGL, called face culling. Face culling is an option for the OpenGL environment which allows the rendering pipeline to ignore (not calculate or draw) the back face of a shape, saving time, memory and processing cycles:
// enable face culling feature gl.glEnable(GL10.GL_CULL_FACE); // specify which faces to not draw gl.glCullFace(GL10.GL_BACK);
If you try to use the face culling feature without knowing which sides of your shapes are the front and back, your OpenGL graphics are going to look a bit thin, or possibly not show up at all. So, always define the coordinates of your OpenGL shapes in a counterclockwise drawing order.
Note: It is possible to set an OpenGL environment to treat the clockwise face as the front face, but doing so requires more code and is likely to confuse experienced OpenGL developers when you ask them for help. So don’t do that.
The OpenGL ES 1.0 and 1.1 API specifications have been supported since Android 1.0. Beginning with Android 2.2 (API Level 8), the framework supports the OpenGL ES 2.0 API specification. OpenGL ES 2.0 is supported by most Android devices and is recommended for new applications being developed with OpenGL. For information about the relative number of Android-powered devices that support a given version of OpenGL ES, see the OpenGL ES Versions Dashboard.
Texture compression can significantly increase the performance of your OpenGL application by reducing memory requirements and making more efficient use of memory bandwidth. The Android framework provides support for the ETC1 compression format as a standard feature, including a {@link android.opengl.ETC1Util} utility class and the {@code etc1tool} compression tool (located in the Android SDK at {@code <sdk>/tools/}). For an example of an Android application that uses texture compression, see the CompressedTextureActivity code sample.
The ETC format is supported by most Android devices, but it not guarranteed to be available. To check if the ETC1 format is supported on a device, call the {@link android.opengl.ETC1Util#isETC1Supported() ETC1Util.isETC1Supported()} method.
Note: The ETC1 texture compression format does not support textures with an alpha channel. If your application requires textures with an alpha channel, you should investigate other texture compression formats available on your target devices.
Beyond the ETC1 format, Android devices have varied support for texture compression based on their GPU chipsets and OpenGL implementations. You should investigate texture compression support on the devices you are are targeting to determine what compression types your application should support. In order to determine what texture formats are supported on a given device, you must query the device and review the OpenGL extension names, which identify what texture compression formats (and other OpenGL features) are supported by the device. Some commonly supported texture compression formats are as follows:
Warning: These texture compression formats are not supported on all devices. Support for these formats can vary by manufacturer and device. For information on how to determine what texture compression formats are on a particular device, see the next section.
Note: Once you decide which texture compression formats your application will support, make sure you declare them in your manifest using <supports-gl-texture> . Using this declaration enables filtering by external services such as Google Play, so that your app is installed only on devices that support the formats your app requires. For details, see OpenGL manifest declarations.
Implementations of OpenGL vary by Android device in terms of the extensions to the OpenGL ES API that are supported. These extensions include texture compressions, but typically also include other extensions to the OpenGL feature set.
To determine what texture compression formats, and other OpenGL extensions, are supported on a particular device:
String extensions = javax.microedition.khronos.opengles.GL10.glGetString(GL10.GL_EXTENSIONS);
Warning: The results of this call vary by device! You must run this call on several target devices to determine what compression types are commonly supported.
OpenGL ES API version 1.0 (and the 1.1 extensions) and version 2.0 both provide high performance graphics interfaces for creating 3D games, visualizations and user interfaces. Graphics programming for the OpenGL ES 1.0/1.1 API versus ES 2.0 differs significantly, and so developers should carefully consider the following factors before starting development with either API:
While performance, compatibility, convenience, control and other factors may influence your decision, you should pick an OpenGL API version based on what you think provides the best experience for your users.