OpenGL ES Vertex / Indicies

Question

I'm just starting to learn OpenGL ES but am having some trouble understanding how the vertex and indices work. My current understanding is that a Vertex is a point on the shape itself, and that the indices represent the 'triangles' within the vertex points. I'm following a tutorial that has me define the vertex and indices points as below...

Vertex data

-1.0f, -1.0f 1.0f, -1.0f -1.0f, 1.0f 1.0f, 1.0f

indices data

0,3,1, 0,2,3

I understand that defining indices should always start at one vertex but to me these numbers just dont add up. When I draw this on paper it looks like the actual image drawn should be two triangles together that create a 'crown' shape. Can someone explain why this is actually drawing a square instead of the 'crown' that I am expecting?

Source code for the Square class:

public class Square {

private FloatBuffer mFVertexBuffer;
private ByteBuffer mColorBuffer;
private ByteBuffer mIndexBuffer;

public Square() {

    // 2D Points
    float[] square = {

    -1.0f, -1.0f, 
    1.0f, -1.0f, 
    -1.0f, 1.0f, 
    1.0f, 1.0f,         

    };

    byte maxColor = (byte) 225;

    /**
     * Each line below represents RGB + Alpha transparency
     */
    byte colors[] = {

    0, maxColor, 0, maxColor,
    0, maxColor, maxColor, maxColor,
    0, 0, 0, maxColor, 
    maxColor, 0, maxColor, maxColor,

    };

    //triangles
    byte[] indicies = {

            0,3,1,
            0,2,3

    };

    /**
     * Make sure that bytes are in correct order, otherwise they might be
     * drawn backwards
     */
    ByteBuffer byteBuffer = ByteBuffer.allocateDirect(square.length * 4);
    byteBuffer.order(ByteOrder.nativeOrder());
    mFVertexBuffer = byteBuffer.order(ByteOrder.nativeOrder())
            .asFloatBuffer();
    mFVertexBuffer.put(square);
    mFVertexBuffer.position(0);

    mColorBuffer = ByteBuffer.allocateDirect(colors.length);
    mColorBuffer.put(colors);
    mColorBuffer.position(0);

    mIndexBuffer = ByteBuffer.allocateDirect(indicies.length);
    mIndexBuffer.put(indicies);
    mIndexBuffer.position(0);
}

public void draw(GL10 gl) {

    /**
     * Make open GL only draw the front of the triangle (GL_CW = Graphics
     * Library Clockwise)
     * 
     * Back of triangle will not be drawn
     */
    gl.glFrontFace(GL11.GL_CW);

    /**
     * specifies number of elements per vertex
     * 
     * specifies floating point type
     * 
     * Sets stride = 0 bytes* (Stride allows to use different types of data
     * interchangably with opengl )
     */
    gl.glVertexPointer(2, GL11.GL_FLOAT, 0, mFVertexBuffer);

    // 4 because we are using 4 colors in our color bufer array
    gl.glColorPointer(4, GL11.GL_UNSIGNED_BYTE, 0, mColorBuffer);

    /**
     * draws the image
     * 
     * first argument specifies geomety format
     */
    gl.glDrawElements(GL11.GL_TRIANGLES, 6, GL11.GL_UNSIGNED_BYTE,
            mIndexBuffer);

    // Reset to CounterClockwise
    gl.glFrontFace(GL11.GL_CCW);

}

}

Let me know if more info is needed...

Answer 1

I don't think your indexes are correct, try drawing the bottom line then moving to the top verts. If I am picturing your indexes correctly, they really are trying to draw a square.

Try:
0, 1, 3
0, 1, 2

Instead

Edit: Even I got the order mixed up, fixed for a mistake

Answer 2

You defined four vertices:

2    3

0    1

Your indices then defined two triangles, 0-3-1:

     .
   ...
  ....
 .....

and 0-2-3:

.....
....
...
.

put together they form a square.