Question

I have an orthogonal perspective which I initialize like so:

gl.glViewport(0, 0, Constants.SCREEN_WIDTH, Constants.SCREEN_HEIGHT);   
gl.glMatrixMode(GL10.GL_PROJECTION);    
gl.glLoadIdentity();                    
gl.glOrthof(0,Constants.GAME_AREA_WIDTH, Constants.GAME_AREA_HEIGHT, 0, 1, 10);
gl.glMatrixMode(GL10.GL_MODELVIEW);     
gl.glLoadIdentity();

What I want to do here is have a square start off the top of the screen (at like (x,-100,z) and the that square should descend (on y) while at the same time roate (on z).

The square’s upper-left is what I use as reference for the square’s position.

Ok, now, I think I get how to roate it around itself. I translate the thing to (-squareSize/2, -squareSize/2,z), rotate it along z, then translate back. And indeed, if I only test this rotation it works ok:

    gl.glLoadIdentity();
    angle = angle + 3;
    if(angle>360) {
        angle = angle - 360;
    }
    gl.glTranslatef(xCurrent+size/2, yCurrent+size/2,0);
    gl.glRotatef(angle, 0, 0, 1);
    gl.glTranslatef(-(xCurrent+size/2), -(yCurrent+size/2),0);
//omitted: enable client state, draw elements, disable client state.

With just this, no matter where I place my square (even small negative values for x and y which only make it partially show on the screen), it will rotate around its center.

However I can’t figure out how to add the downwards translation on y. If I do something like this:

    angle = angle + 3;
    if(angle>360) {
        angle = angle - 360;
    }
    gl.glTranslatef(xCurrent+size/2, yCurrent+size/2,0);
    gl.glRotatef(angle, 0, 0, 1);
    gl.glTranslatef(-(xCurrent+size/2), -(yCurrent+size/2),0);

    yCurrent = yCurrent + realSpeed;
    if(yCurrent>Constants.GAME_AREA_HEIGHT+size) {
        yCurrent=-size;
    }
    gl.glTranslatef(0f, yCurrent,0f);

it will only work ok if my square start at (0,0,z) – in which case it will move down and rotate around it’s center.

If however I start it at any positive or negative non 0 value for either x or y, it will still move down, but do a weird spiral motion instead of rotating agains its center.

Answer 1

The OpenGL matrix stack post multiplies. Which effectively means that you should do the most local transformation last.

So what you probably want to do is to perform a glTranslatef to the tile’s current position, then do the translate/rotate/untranslate sequence to effect your rotation.