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  Moving from point A to B - Shooting and movement  (Read 289 times)
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Offline SauronWatchesYou
« Posted 2014-07-06 09:57:54 »

Hi guys so i'm looking into making my enemy get the position of the player and then fire a bullet towards the previous location and was wondering what the best way to do this was. (My game is a 2D vertical scrolling game and the enemy shoots down towards the player)

I've searched online but i'm a little confused right now. People seem to suggest using trig and was wondering if anyone could point me in the right direction? A common piece of code I found was this:

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double dx = player.x - enemy.x, dy = player.y - enemy.y;
double distance = Math.sqrt(dx*dx + dy*dy);

double multiplier = moveSpeedOfEnemy / distance;

double velocityX = dx * multiplier, velocityY = dy * multiplier;


I'll likely have to brush up my maths as this is confusing me but if anyone can explain exactly what I should do and any useful resources then it would be great! Smiley

Sorry if the question is a little vague but just ask if you need more info etc Cheesy

Thank you
Offline PandaMoniumHUN
« Reply #1 - Posted 2014-07-06 11:24:59 »

That's actually a pretty good code snippet you found there. Cheesy

To be honest you don't even need trigonometry for this, just the very basics of linear algebra, mostly vectors.
You can create a vector that points from point A to point B by subtracting all of A's components from all of B's components.
That's what
dx
and
dy
are in your code.
The
distance
is just the distance formula to calculate the distance between the player and the enemy, and the
multiplier
is there to make the projectile go faster as it gets closer to it's target. However, it's a really bad way of achieving acceleration and I wouldn't use that.

To make your enemies fire a bullet at your player's current position you should save the player's actual position (by value and not by reference) and calculate against that like so:
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Vector2f direction = new Vector2f(target.x-position.x, target.y-position.y);
direction.normalise(direction);
position.x += direction.x*speed*delta;
position.y += direction.y*speed*delta;

position
is just a Vector2f that contains your projectile's current position and
target
is a Vector2f too which contains the projectile's target location.
speed
should be the speed of the projectile.

This is just the basics and the first thing that I would change on this code as soon as you understand what's going on is to add some kind of time based (and definitely not distance based) acceleration. Smiley

Offline SauronWatchesYou
« Reply #2 - Posted 2014-07-07 09:27:05 »

Thanks Panda! Smiley I really appreciate the detailed response about what each thing is doing. I'm going to go over it a few more times just to make sure I understand it and then will mess around with the code Tongue
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