3D camera position and LookAt?

MuntyScruntFundle · August 30, 2019, 7:43pm

Hi there.

Before I set off on another completely uncharted journey, I thought I’d ask…

If in a 3D game I’ve decided all the scenery is going to remain in the same position and I’m going to move the camera around, do I need to use the CreateLookAt and create an imaginary point on a sphere around ‘me’ in space to LookAt? If that’s the case can I simply use a projection or normalised copy of my rotation vector?

Many thanks.

willmotil · August 31, 2019, 4:45am

Create look at takes a Target Position in your world so yes you can just rotate a point around with some matrices then add that to the cameras position then pass that back to the CreateLookAt function as the Target Vector.

You can alternately just create a matrix with create world or any of the rotation methods and grab the forward from it to add to your cameras position then pass it to create look at.

 view = CreateLookAt
 (
 myCameraPosition,  
 myCameraPosition + Matrix.CreateRotationY(someAbitraryAngleInRadions).Forward ,
 myUp
 );

 //Or


 view = CreateLookAt
 (
 myCameraPosition,  
 myCameraPosition + SomeOtherObjectsPositionInWorld ,
 myUp
 );

 //Or

 camerasWorld = CreateWorld
 (
 camPosition,
 Vector3.Normalize(new Vector3(5,0, 2)),
 Vector3.Up
 );

 //then

 view = CreateLookAt
 (
 camerasWorld.Translation ,  
 camerasWorld.Translation + camerasWorld.Forward,
 camerasWorld.Up
 );

github.com

MonoGame/MonoGame/blob/develop/MonoGame.Framework/Matrix.cs#L754


/// <param name="cameraTarget">Lookup vector of the camera.</param>

/// <param name="cameraUpVector">The direction of the upper edge of the camera.</param>

/// <returns>The viewing <see cref="Matrix"/>.</returns>

public static Matrix CreateLookAt(Vector3 cameraPosition, Vector3 cameraTarget, Vector3 cameraUpVector)

{

    Matrix matrix;

    CreateLookAt(ref cameraPosition, ref cameraTarget, ref cameraUpVector, out matrix);

    return matrix;

}



/// <summary>

/// Creates a new viewing <see cref="Matrix"/>.

/// </summary>

/// <param name="cameraPosition">Position of the camera.</param>

/// <param name="cameraTarget">Lookup vector of the camera.</param>

/// <param name="cameraUpVector">The direction of the upper edge of the camera.</param>

/// <param name="result">The viewing <see cref="Matrix"/> as an output parameter.</param>

public static void CreateLookAt(ref Vector3 cameraPosition, ref Vector3 cameraTarget, ref Vector3 cameraUpVector, out Matrix result)

{

    var vector = Vector3.Normalize(cameraPosition - cameraTarget);

    var vector2 = Vector3.Normalize(Vector3.Cross(cameraUpVector, vector));

The two methods are a little different CreateWorld takes a Forward Vector as if its just a direction were CreateLookAt takes a actual position to look at in world space and figures out the forward direction by comparison to the previous position you just gave it.

Conceptually …

CreateWorld is for creating (world) objects orientation matrices in world space for all your game objects that are typically drawn, However this can include the camera as if it were a physical object in your world that moves and looks around though the camera is typically not drawn.

CreateLookAt is for creating the special requisite (view) matrice that typically endures for a single frame unchanged, from one of those objects that acts as your camera.