Question

I would like to move an object (I have a simple rectangle) and I would like to move it up to a specific position and back (maybe repeat forever).

I know I should have a variable in the matrix translation (gltranslate) but where/how do I make this variable and animate each frame.

Answer 1

It’s hard to be certain what stage you are at but assuming you already have a rectangle drawing on screen:

Wherever you have your ‘drawMyRectangle’ function create these variables:

//Your starting position
float startPositionX;
float startPositionY;
//Your ending position
float endPositionX;
float endPositionY;
//The value that describes where in between these positions you are

float translationAlpha;
float animationDirection;

So fill in those values with translationAlpha starting at 0 and animatingForward at 1.

I recommend doing this next part in a separate function but you will need code that steps your animation, simply

void step()
{
    translationAlpha += .01 * animationDirection;//.01 is the per
    if (animationDirection > 0.0 && translationAlpha >= 1.0 ||
        animationDirection < 0.0 && translationAlpha <= 0.0) {
        animationDirection *= -1.0;
    }
}

It sounds like you are keeping it simple so just call that every time you draw.

So make sure you have a glPushMatrix and a glPopMatrix surrounding your draw.
after you pushMatrix and before you call your draw add:

glTranslate2f(startPositionX + translationAlpha * (endPositionX - startPositionX),
              startPositionY + translationAlpha * (endPositionY - startPositionY));

That covers a really primitive way to do what you described, also consider this pseudocode because I don’t know what functions you are using and how you are handling your matrices.