Question

I want to create a camera moving above a tiled plane. The camera is supposed to move in the XY-plane only and to look straight down all the time. With an orthogonal projection I expect a pseudo-2D renderer.
My problem is, that I don’t know how to translate the camera. After some research it seems to me, that there is nothing like a “camera” in OpenGL and I have to translate the whole world. Changing the eye-position and view center coordinates in the Matrix.setLookAtM-function just leads to distorted results.
Translating the whole MVP-Matrix does not work either.

I’m running out of ideas now; do I have to translate every single vertex every frame directly in the vertex buffer? That does not seem plausible to me.

I derived GLSurfaceView and implemented the following functions to setup and update the scene:

public void onSurfaceChanged(GL10 unused, int width, int height) {

    GLES20.glViewport(0, 0, width, height);
    float ratio = (float) width / height;

    // Setup the projection Matrix for an orthogonal view
    Matrix.orthoM(mProjMatrix, 0, -ratio, ratio, -1, 1, 3, 7);

}

public void onDrawFrame(GL10 unused) {

    // Draw background color
    GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT);

    //Setup the camera
    float[] camPos = {  0.0f,  0.0f, -3.0f }; //no matter what else I put in here the camera seems to point
    float[] lookAt = {  0.0f,  0.0f,  0.0f }; //  to the coordinate center and distorts the square

    // Set the camera position (View matrix)
    Matrix.setLookAtM( vMatrix, 0, camPos[0], camPos[1], camPos[2], lookAt[0], lookAt[1], lookAt[2], 0f, 1f, 0f);

    // Calculate the projection and view transformation
    Matrix.multiplyMM( mMVPMatrix, 0, projMatrix, 0, vMatrix, 0);

    //rotate the viewport
    Matrix.setRotateM(mRotationMatrix, 0, getRotationAngle(), 0, 0, -1.0f);
    Matrix.multiplyMM(mMVPMatrix, 0, mRotationMatrix, 0, mMVPMatrix, 0);

    //I also tried to translate the viewport here
    //  (and several other places), but I could not find any solution

    //draw the plane (actually a simple square right now)
    mPlane.draw(mMVPMatrix);

}

Answer 1

Changing the eye-position and view center coordinates in the “LookAt”-function just leads to distorted results.

If you got this from the android tutorial, I think they have a bug in their code. (made a comment about it here)

Try the following fixes:

1. Use setLookatM to point to where you want the camera to be.

2. In the shader, change the gl_Position line

   from: " gl_Position = vPosition * uMVPMatrix;"
   to: " gl_Position = uMVPMatrix * vPosition;"

3. I’d think the //rotate the viewport section should be removed as well, as this is not rotating the camera properly. You can change the camera’s orientation in the setlookat function.