Question

I have some 3D models that I render in OpenGL in a 3D space, and I’m experiencing some headaches in moving the ‘character’ (that is the camera) with rotations and translation inside this world.

I receive the input (ie the coordinates where to move/the dregrees to turn) from some extern event (image a user input or some data from a GPS+compass device) and the kind of event is rotation OR translation .

I’ve wrote this method to manage these events:

- (void)moveThePlayerPositionTranslatingLat:(double)translatedLat Long:(double)translatedLong andRotating:(double)degrees{

    [super startDrawingFrame];
    if (degrees != 0)
    {
        glRotatef(degrees, 0, 0, 1);
    }

    if (translatedLat != 0)
    {
        glTranslatef(translatedLat, -translatedLong, 0);
    }

   [self redrawView];
}

Then in redrawView I’m actualy drawing the scene and my models. It is something like:

glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

NSInteger nModels = [models count];

for (NSInteger i = 0; i < nModels; i++) 
{
    MD2Object * mdobj = [models objectAtIndex:i];
    glPushMatrix();
    double * deltas = calloc(sizeof(double),2);

    deltas[0] = currentCoords[0] - mdobj.modelPosition[0];
    deltas[1] = currentCoords[1] - mdobj.modelPosition[1];

    glTranslatef(deltas[0], -deltas[1], 0);

    free(deltas);
    [mdobj setupForRenderGL];
    [mdobj renderGL];   
    [mdobj cleanupAfterRenderGL];
    glPopMatrix();

}
[super drawView];

The problem is that when translation an rotation events are called one after the other: for example when I’m rotating incrementally for some iterations (still around the origin) then I translate and finally rotate again but it appears that the last rotation does not occur around the current (translated) position but around the old one (the old origin). I’m well aware that this happens when the order of transformations is inverted, but I believed that after a drawing the new center of the world was given by the translated system.

What am I missing? How can I fix this? (any reference to OpenGL will be appreciated too)

Answer 1

I would recommend not doing cummulative transformations in the event handler, but internally storing the current values for your transformation and then only transforming once, but I don’t know if this is the behaviour that you want.

Pseudocode:

someEvent(lat, long, deg)
{
  currentLat += lat;
  currentLong += long;
  currentDeg += deg;
}

redraw()
{
  glClear()
  glRotatef(currentDeg, 0, 0, 1);
  glTranslatef(currentLat, -currentLong, 0);
  ... // draw stuff
}