Basic 2D Enemy AI

Question

This is the basic layout of my game: http://www.youtube.com/watch?v=qcd-CcKWjGU&feature=youtu.be

To make him move I just made it so you click left or right to move, just for a test.

Anyway, I just need an idea on what I need to do to the enemy follow the player. I'm not sure what variables I need and if I need some sort of angle variable so he can move in all possible direction. I probably just want to start out by making it follow the player on the x direction and then start making him move towards the player. This is my first time really having ago at proper AI as the last game I made invlolving AI what just making the enemy move right and turn left once reaching a certain point and vice versa.

This is all really have so far:

rectangle = new Rectangle(
                (int)position.X,
                (int)position.Y,
                texture.Width,
                texture.Height);

        float et = (float)gameTime.ElapsedGameTime.TotalMilliseconds;

        position += velocity;

        if (InputManager.Instance.KeyDown(Keys.Left))
            velocity.X = -3f;
        else if (InputManager.Instance.KeyDown(Keys.Right))
            velocity.X = 3f;

        if (velocity.X > 0)
            facingRight = true;
        else if(velocity.X < 0)
            facingRight = false;//This is just so that the enemy will look in the right direction of the way he's moving.

I also have an origin which is a Vector2 of the enemy's centre which I feel I may need.

Answer 1

When there are no obstacles, you just need to check if the enemy is left or right, above or below the player-character. You can do so by comparing their X- and Y- coordinates. Then move the enemy in that direction.

When there are obstacles and you want your enemies to find a path around them, you will need a route-finding algorithm like A*.

Answer 2

This should do the job, it will follow the player and face the player as well. Hope this helped.

using UnityEngine;
using System.Collections;

public class ChasePlayerAI : MonoBehaviour {

  public Transform target;//set target from inspector instead of looking in Update
  public float speed = 3f;


  void Update() {
    //rotate to look at the player
    transform.LookAt(target.position);
    //correcting the original rotation
    transform.Rotate(new Vector3(0,-90,0),Space.Self);

    //move towards the player if distance from target is greater than 1
    if (Vector3.Distance(transform.position,target.position)>1f) {
      transform.Translate(new Vector3(speed* Time.deltaTime,0,0) );
    }
  }
}

Answer 3

As Philipp stated, you just have to compute a vector from the enemy to the hero and move the enemy in that direction. That's how you do it when there are no obstacles:

// heroPos is the position of the hero.
Vector2 direction = position - heroPos; 

//assuming here that velocity is a length and not a vector.
position += direction*velocity; 

if (direction.X > 0)
    facingRight = true;
else if(direction.X < 0)
    facingRight = false;