Projectile Motion with C - XY conversion

[Spud1]

OK i'll do my best to explain this well

Part of my assignment this year is to make a game with a tank shooting at an alien - and it has to include some semi realistic physics.

this basically means that the projectile has to travel along a realistic path to hit the alien. The calculations easy enough, and it produces an X co-ordinate and a Y co-ordinate for a given time index

int getProjectileX(float timeIndex, float theta, float velocity)
{
	return ((velocity*cos(theta*_PI_/180))*timeIndex);
}

int getProjectileY(float timeIndex, float theta, float velocity)
{
	return (velocity*sin(theta*_PI_/180))*timeIndex-0.5*_GRAVITY_*(timeIndex*timeIndex);
}

nothing wrong with that, it produces accurate values as far as i can see.

the problem comes when drawing the projectile, as it draws it upside down cos of the computer having a retarded Y-axis

so my basic question would be how could I convert the co-ordinates so that they make the projectile display properly - ie the right way up. any ideas?

if you need stuff clarifying just reply here, i'm not the best at explaining my problems

ty

spud

[Big RICHARD]

new y = max y - old y

[Spud1]

thx for you help it set me off on the right track