Adding Motion

Drawing objects on screen is a pretty basic feature of OpenGL, but you can do this with other Android graphics framwork classes, including Canvas and Drawable objects. OpenGL ES provides additional capabilities for moving and transforming drawn objects in three dimensions or in other unique ways to create compelling user experiences.

In this lesson, you take another step forward into using OpenGL ES by learning how to add motion to a shape with rotation.

Rotate a Shape

Rotating a drawing object with OpenGL ES 2.0 is relatively simple. You create another transformation matrix (a rotation matrix) and then combine it with your projection and camera view transformation matrices:

private float[] mRotationMatrix = new float[16];
public void onDrawFrame(GL10 gl) {
    ...
    float[] scratch = new float[16];

    // Create a rotation transformation for the triangle
    long time = SystemClock.uptimeMillis() % 4000L;
    float angle = 0.090f * ((int) time);
    Matrix.setRotateM(mRotationMatrix, 0, angle, 0, 0, -1.0f);

    // Combine the rotation matrix with the projection and camera view
    // Note that the mMVPMatrix factor *must be first* in order
    // for the matrix multiplication product to be correct.
    Matrix.multiplyMM(scratch, 0, mMVPMatrix, 0, mRotationMatrix, 0);

    // Draw triangle
    mTriangle.draw(scratch);
}

If your triangle does not rotate after making these changes, make sure you have commented out the GLSurfaceView.RENDERMODE_WHEN_DIRTY setting, as described in the next section.

Enable Continuous Rendering

If you have diligently followed along with the example code in this class to this point, make sure you comment out the line that sets the render mode only draw when dirty, otherwise OpenGL rotates the shape only one increment and then waits for a call to requestRender() from the GLSurfaceView container:

public MyGLSurfaceView(Context context) {
    ...
    // Render the view only when there is a change in the drawing data.
    // To allow the triangle to rotate automatically, this line is commented out:
    //setRenderMode(GLSurfaceView.RENDERMODE_WHEN_DIRTY);
}

Unless you have objects changing without any user interaction, it’s usually a good idea have this flag turned on. Be ready to uncomment this code, because the next lesson makes this call applicable once again.