Question

I started drawing my Quad but when I started playing around with the Vertices I noticed that the X Coordinates are flipped. Heres a picture to show what I mean:

Here are my Vertices – Indices and Texture Coordinates which I don’t really have to show.

static final int COORDS_PER_VERTEX = 3;
static float positionCoords[] = { 
    -0.5f,  0.5f, 0.0f,   // top left
    -0.5f, -0.5f, 0.0f,   // bottom left
     0.5f, -0.5f, 0.0f,   // bottom right
     0.5f,  0.5f, 0.0f }; // top right

static final int COORDS_PER_TEXTURE = 2;
static float textureCoords[] = { 
    0.0f, 0.0f, 
    0.0f, 1.0f,
    1.0f, 1.0f,
    1.0f, 0.0f, };

private final short indices[] = { 0, 1, 2 };

And this is where I change the Projection and View Matrices.

public void onSurfaceChanged(GL10 naGl, int width, int height)
{
        Log.d(TAG, "GL Surface Changed - Setting Up View");

    GLES20.glViewport(0, 0, width, height);

    float ratio = (float) width / height;

    Matrix.frustumM(ProjectionMatrix, 0, -ratio, ratio, -1, 1, 3, 7);

    Matrix.setLookAtM(ViewMatrix, 0, 0, 0, -3, 0f, 0f, 0f, 0f, 1.0f, 0.0f);
}

Why would it be drawn ‘backwards’. I also thought that my Camera might be behind the object so that in 3 dimensional space left would be positive if I’m behind the object.

Answer 1

You are indeed looking at the object from the reverse side.

Your lookAt function is placing the eye at (0,0,-3), and the lookAt point at (0, 0, 0). By default the negative z axis points into the screen, but you’re looking at it from the reverse direction (towards the positive z axis).

You should have your eye at (0,0,3) looking toward (0,0,0) to get the view that you expect.

You may find this chapter of the Red Book informative.