Do you want this effect on a custom projectile, or with a custom weapon?
If it's the weapon part you want, you can override the Shoot function on the ModItem and give the position.Y (of the projectile you want to shoot) some negative values like 'position.Y -= 100;' and fiddle around a bit with the speedX and speedY values depending on which side the player is facing (if you want to take that into account). After that just return true in the shoot function and (with a bit of fiddling around with the values) you might just end up with the effect you want.
If you want this effect on a custom projectile, it's going to be a bit harder (still quite possible, though) you actually want to do the same as with the custom weapon (change the position etc.) in the AI function (which you'll have to override). Inside this AI, you have to do a check if the projectile hasn't updated yet so something like this will do (note, this can be better)
Code:
private bool hasUpdated = false;
public override void AI()
{
if(!hasUpdated)
{
hasUpdated = true;
projectile.position.Y -= 100;
// Change the velocity parameter of the projectile here.
}
}
As you can see, I've added a comment. Instead of changing speedX and speedY parameters like you'd do with the custom weapon, you have to change the projectile.velocity.X and projectile.velocity.Y parameters to get the effect you want. Again, fiddle around with this a bit untill you end up with the effect you want (since the effect you want to achieve might differ from the effect I'm thinking of, so I'm not giving you resolute values).
Hope this helps you on your way.
P.S. Sorry for the large amount of text ><
up my computer?
