2D Topdown Shooter - Player Movement Relative to Mouse

Question

I'm trying to make a topdown 2D space game for my school project. I'm almost done but I just want to add a few little things to make the game more fun to play.

if (keystate.IsKeyDown(Keys.W))
        {
            vPlayerPos += Vector2.Normalize(new Vector2(Mouse.GetState().X - vPlayerPos.X, Mouse.GetState().Y - vPlayerPos.Y)) * 3;

            rPlayer.X = (int)vPlayerPos.X;
            rPlayer.Y = (int)vPlayerPos.Y;
        }

        if (keystate.IsKeyDown(Keys.S))
        {
            vPlayerPos += Vector2.Normalize(new Vector2(Mouse.GetState().X - vPlayerPos.X, Mouse.GetState().Y - vPlayerPos.Y)) * -3;
            rPlayer.X = (int)vPlayerPos.X;
            rPlayer.Y = (int)vPlayerPos.Y;
        }

This is what i use to move towards and away from my mouse crossair. I tried to make a somewhat similar function to make it strafe with "A" and "D". But for some reason I just couldn't get it done.

Any thoughts?

Answer 1

I assume you mean that forwards and backwards are working fine (from what I can tell they should). In order to move left and right you merely need to make the Vector2 perpendicular and use almost the exact same logic:

public static class Vector2Extensions
{
  public static Vector2 Perpendicular(this Vector2 vector)
  {
    // Swap y and x, and negate y.
    return new Vector2(-vector.Y, vector.X);
  }
}

Now your logic would look something like (notice I have improved the layout and made it easier to understand):

var mouseState = Mouse.GetState();
var mouseOffset = new Vector2(mouseState.X - vPlayerPos.X, mouseState.Y - vPlayerPos.Y);
var movement = mouseOffset.Normalize() 
  // Your game is currently frame-rate dependant, you should be doing this.
  // See: http://gamedev.stackexchange.com/q/19529/1494
  * 3000 * (float)gameTime.ElapsedGameTime.TotalSeconds;
var perpendicular = movement.Perpendicular();

if (keystate.IsKeyDown(Keys.W))
  vPlayerPos += movement;
if (keystate.IsKeyDown(Keys.S))
  vPlayerPos -= movement;

// You might need change the +/- here around.
if (keystate.IsKeyDown(Keys.A))
  vPlayerPos += perpendicular;
if (keystate.IsKeyDown(Keys.D))
  vPlayerPos -= perpendicular;

// You shouldn't do this. If you need them as int cast them when you need them as such.
// rPlayer.X = (int)vPlayerPos.X;
// rPlayer.Y = (int)vPlayerPos.Y;

Side note: Good thinking with using a normalized vector to determine movement, this is probably the 'best' way to do something like this.