Amending mistakes – facing direction is vector, not angle

OK, so let’s be honest. Angle option from previous post is not the best idea. It’s not very bad I think, but there are few problems with it.

First one – angles are not very useful once you think about it. If you want to draw something – you have to calculate the points, which means sin and cos math all over the place. Will get complex soon and is easy to get wrong.

Talking about wrong. Dot product from previous post is great for calculating angles, it really is. It just has one tiny caveat in 2D – the values will always be between 0 and 180 degrees. Once I looked at math and thought for a minute or two this became painfully obvious:

var vector = { x: mouse.x - p.Position.X, y: mouse.y - p.Position.Y };
var vectorLength = vector.x * vector.x + vector.y * vector.y;
vectorLength = Math.sqrt(vectorLength);
vector.x = vector.x / vectorLength;
vector.y = vector.y / vectorLength;
var screenVector = { x: 1, y: 0};

var dotProduct = vector.x * screenVector.x + vector.y * screenVector.y;

Well, the last part can be simplified to:

var dotProduct = vector.x * 1 + vector.y * 0;

So basically var dotProduct = vector.x and this can only be between -1 and 1 (this is unit vector after all). D’uh! Basic math.

So what seems to be better aproach? Of course vector – the one that’s already built for calculating dot product. I can simply use it to indicate direction player is facing. Code gets simpler – gain. Code actually works – gain. Drawing direction in which player is lookign gets very simple – gain.


drawingContext.strokeStyle = "#0F0";

var x = gameObject.Position.X + gameObject.FacingDirection.X * gameObject.Size / 2;
var y = gameObject.Position.Y + gameObject.FacingDirection.Y * gameObject.Size / 2;
drawingContext.moveTo(x, y);

x = gameObject.Position.X + gameObject.FacingDirection.X * gameObject.Size;
y = gameObject.Position.Y + gameObject.FacingDirection.Y * gameObject.Size;
drawingContext.lineTo(x, y);

So much gained by making things easier.


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