Question

Im doing some practices on XNA, and i created a class that represents a Camera.

My objective is that when the user press some keys make a translation of the camera (not the target) 90 degrees in the X axys (to see an object that i placed in the scene from different angles). By the moment i move the camera in X, Y, and Z without problems.

Actually to set up my camera i use the following lines of code:

public void SetUpCamera()
{
            #region ## SET DEFAULTS ##
            this.FieldOfViewAngle = 45.0f;
            this.AspectRatio =1f;
            this.NearPlane = 1.0f;
            this.FarPlane = 10000.0f;
            #endregion

            this.ProjectionMatrix = Matrix.CreatePerspectiveFieldOfView(MathHelper.ToRadians(this.FieldOfViewAngle), 16 / 9, this.NearPlane, this.FarPlane);
            this.ViewMatrix = Matrix.CreateLookAt(new Vector3(this.PositionX, this.PositionY, this.PositionZ), new Vector3(this.TargetX, this.TargetY, this.TargetZ), Vector3.Up);
}

I have this method to move the camera:

public void UpdateView()
{
     this.ViewMatrix = Matrix.CreateLookAt(new Vector3(this.PositionX, this.PositionY, this.PositionZ), new Vector3(this.TargetX, this.TargetY, this.TargetZ), Vector3.Up);
}

Then in the game (update event handler i have the following code)

if (keyboardstate.IsKeyDown(Keys.NumPad9))
{
   this.GameCamera.PositionZ -= 1.0f;
}
if (keyboardstate.IsKeyDown(Keys.NumPad3))
{
   this.GameCamera.PositionZ += 1.0f;
}
this.GameCamera.UpdateView();

I would like to know how to make this camera translation of 90 degrees to surround one object that i placed in the screen.

To explain my self better about the camera movement here is a video on youtube that uses the exact movement that im trying to describe (see from 14 second)
http://www.youtube.com/watch?v=19mbKZ0I5u4

Answer 1

Assuming the camera in the video is orbiting the car, here is how you would accomplish that in XNA.

For the sake of readability, we’ll just use vectors instead of their individual components. So ‘target’ means it’s a Vector3 that includes TargetX, TargetY, & TargetZ. Same with the camera’s position. You can break X, Y, Z values out into fields and make Vector3s out of them to plug into this code if you want to later, but really it would be best for you to work at vector level instead of component level.

//To orbit the car (target)
cameraPosition = Vector3.Transform(cameraPosition – target, Matrix.CreateRotationY(0.01f)) + target;
this.ViewMatrix = Matrix.CreateLookAt(cameraPosition, target, Vector3.Up);

Since all matrix rotations act about an axis that intersects the world origin, to use a rotation matrix to rotate the camera around the car, the object of rotation has to be shifted such that the target (and thus the rotation axis) is located at the world origin. CameraPosition – target accomplishes that. Now cameraPosition can be rotated a little bit. Once cameraPosition is rotated a little bit, it needs to be sent back to the scene at hand, that’s what the ‘+ target’ at the end of the line is for.

The 0.01f can be adjusted to whatever rotation rate suits you.