OpenGL ES Android Matrix Transformations

Question

I have a renderer implementing GLSurfaceView.Renderer interface; a subclass of GLSurfaceView and some classes representing my objects I want to draw. I have the code from ht

Answer 1

I´ve been working with the example from the Android Training, the following approach finally works for me. (Based on Android Training > Displaying Graphics with OpenGL ES > Adding Motion)

1. Use the correct vertex shader:

   private final String vertexShaderCode =
   // This matrix member variable provides a hook to manipulate
   // the coordinates of the objects that use this vertex shader
   "uniform mat4 uMVPMatrix;" +
   "attribute vec4 vPosition;" +
   "void main() {" +
   // the matrix must be included as a modifier of gl_Position
   "  gl_Position = uMVPMatrix * vPosition;" +
   "}";
   

2. In the renderer class:

   public class MyGL20Renderer implements GLSurfaceView.Renderer {
   [...]
   // create a model matrix for the triangle
   private final float[] mModelMatrix = new float[16];
   // create a temporary matrix for calculation purposes,
   // to avoid the same matrix on the right and left side of multiplyMM later
   // see https://stackoverflow.com/questions/13480043/opengl-es-android-matrix-transformations#comment18443759_13480364
   private float[] mTempMatrix = new float[16];
   [...]
   

3. Apply transformations in onDrawFrame, start with translation:

   public void onDrawFrame(GL10 unused) {
   [...]
   Matrix.setIdentityM(mModelMatrix, 0); // initialize to identity matrix
   Matrix.translateM(mModelMatrix, 0, -0.5f, 0, 0); // translation to the left
   

4. Then rotation:

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

5. Combine rotation and translation, avoid using mModelMatrix

   "as the same matrix on the right and left side of multiplyMM" (see 2)

   // Combine Rotation and Translation matrices 
   mTempMatrix = mModelMatrix.clone();
   Matrix.multiplyMM(mModelMatrix, 0, mTempMatrix, 0, mRotationMatrix, 0);
   

6. Combine the model matrix with the projection and camera view; again avoid using mModelMatrix

   "as the same matrix on the right and left side of multiplyMM" (see 2)

   // Combine the model matrix with the projection and camera view
   mTempMatrix = mMVPMatrix.clone();
   Matrix.multiplyMM(mMVPMatrix, 0, mTempMatrix, 0, mModelMatrix, 0);
   

7. Draw the shape

   // Draw shape
   mTriangle.draw(mMVPMatrix);
   

Thank you all, for all the useful input I could draw from this thread.