Aiming Sample

This sample shows how to turn one object to face another. This can be useful for aiming, for example.

Sample Overview

This sample shows how to use trigonometry to make one 2D object aim towards another object as fast as it can. This can be useful when making spotlights, gun turrets, and more.

The function TurnToFace does the hard work in this sample. You can copy it and use it in your game.

Minimum Shader Profile

Vertex Shader Model 1.1
Pixel Shader Model 1.1

Sample Controls

This sample uses the following keyboard and gamepad controls.

Action	Keyboard Control	Gamepad Control
Move the cat.	UP ARROW, DOWN ARROW, LEFT ARROW, and RIGHT ARROW	Right thumb stick, D-Pad
Exit the sample.	ESC or ALT+F4	BACK

How the Sample Works

The core of this sample is in the function TurnToFace, which uses trigonometry to determine the "desired angle" that the spotlight should be facing, then turns towards that angle. How do you calculate the desired angle?

In this diagram, S is the position of the spot light, C is the position of the cat, and "o" is the angle that the spot light should be facing in order to point at the cat, our "desired angle."

We know x and y: those are the difference in position between the cat's position and the spotlight's position. Using trigonometry, we know that:

tan(θ) = opposite / adjacent

Substituting in our values, this equation becomes:

tan(o) = y / x

Next, we take the arc tangent of both sides, giving us:

o = arctan( y / x )

Once we have the desired angle, it is relatively simple to turn towards that angle. The code contains more details explaining this process.

Extending the Sample

Try using TurnToFace to make one object turn toward another as it moves: it can be used to make objects move toward or away from one another.

To see how this would work, try adding the following code to the sample's Update function, right after spotlightAngle is calculated.

C#
Vector2 spotlightVelocity = new Vector2(); spotlightVelocity.X = (float)Math.Cos(spotlightAngle); spotlightVelocity.Y = (float)Math.Sin(spotlightAngle); spotlightPosition += spotlightVelocity;

Vector2 spotlightVelocity = new Vector2();
spotlightVelocity.X = (float)Math.Cos(spotlightAngle);
spotlightVelocity.Y = (float)Math.Sin(spotlightAngle);
spotlightPosition += spotlightVelocity;

This will make the spotlight move towards the cat. This could be useful in a variety of different situations. For example, in a racing game, AI controlled cars could use TurnToFace to steer toward waypoints on a track.