How To: Rotate and Move a Camera

# How To: Rotate and Move a Camera

XNA Game Studio 3.0

Demonstrates how to rotate and move a first-person camera in a 3D environment. You can rotate the camera about its y-axis, and move it forward and backward. You control the camera's position and orientation by using the directional keys on your keyboard or by using the D-pad of your Xbox 360 gamepad.

Note

# The Complete Sample

The code in the topic shows you the technique. You can download a complete code sample for this topic, including full source code and any additional supporting files required by the sample.

# Overview

This sample is based on several assumptions.

• The camera will move frequently, so the camera view Matrix is created and set every time Game.Update is called.
• The projection Matrix may also change frequently for effects such as zooming.
• You have added a model to the project as described in How To: Render a Model.

For the sake of simplicity, the sample limits the camera object to an axis of rotation (y-axis) and forward and backward movement. This simplifies the process (and resultant code) for rendering the sample 3D scene. The following steps show you how to render the sample scene.

### To render the sample scene

1. Determine the location and orientation of the camera object.

2. To create a view matrix as part of the standard world-view-projection matrix trilogy of 3D rendering using the camera position, orientation (also referred to as the "look at point"), and the world space's up vector, call CreateLookAt.

3. Create a perspective matrix that determines the near and far clipping planes and the aspect of the projection with a call to CreatePerspectiveFieldOfView.

4. In the Draw method of your game, initialize a BasicEffect object with the transformational matrices created earlier (world, view projection), and then render all existing 3D models.

# Rotating and Moving a Camera

### To rotate and move the camera

1. Determine the camera's position in world coordinates.
2. Determine the reference Vector3 to which the rotation of the camera is relative.

The direction should not change during the game, and usually it will be (0, 0, 1) or (0, 0, −1).

```// Set the direction the camera points without rotation.
Vector3 cameraReference = new Vector3(0, 0, 1);
```
3. Create a rotation Matrix determining the current amount of rotation for the camera object.

Because the camera is limited to one axis of rotation, this matrix represents the rotation of the camera around its own y-axis. Use CreateRotationY to create a rotation Matrix representing the rotation around the y-axis.

```Matrix rotationMatrix = Matrix.CreateRotationY(avatarYaw);
```
4. Use Transform and the rotation Matrix to transform a copy of the reference vector.

This represents the direction the camera is pointing in transformed (or view) space.

```// Create a vector pointing the direction the camera is facing.
Vector3 transformedReference = Vector3.Transform(cameraReference, rotationMatrix);
```
5. Add the camera's current position to the transformed direction vector.

The result is the position to which the camera is pointing.

```// Calculate the position the camera is looking at.
Vector3 cameraLookat = cameraPosition + transformedReference;
```
6. Create a new view Matrix using CreateLookAt.

7. Use CreateLookAt to pass the camera's current position and the transformed direction vector.

The third parameter of CreateLookAt is the up direction of the camera. Typically, it is Vector3.Up (0, 1, 0). This matrix Matrix controls how world coordinates are transformed to camera coordinates.

```// Set up the view matrix and projection matrix.
view = Matrix.CreateLookAt(cameraPosition, cameraLookat, new Vector3(0.0f, 1.0f, 0.0f));
```
8. Use CreatePerspectiveFieldOfView to create a new projection Matrix.

This Matrix controls how camera coordinate values are transformed to screen coordinates.

The first parameter is the field of view of the projection Matrix expressed in radians. A typical field of view of 45 degrees would be expressed as π/4 radians. The second parameter is the aspect ratio of the projection Matrix; it corrects for the difference in width and height of a viewspace. The third and fourth parameters specify the near and far distances at which the objects will be visible.

```// Set distance from the camera of the near and far clipping planes.
static float nearClip = 1.0f;
static float farClip = 2000.0f;
Viewport viewport = graphics.GraphicsDevice.Viewport;
float aspectRatio = (float)viewport.Width / (float)viewport.Height;

proj = Matrix.CreatePerspectiveFieldOfView(viewAngle, aspectRatio, nearClip, farClip);
```
9. Loop through each 3D model to be rendered as described in How To: Render a Model using the projection matrix and view matrix created above.

An identity matrix simplifies the code for the world matrix.

```void DrawModel(Model model, Matrix world, Texture2D texture)
{
foreach (ModelMesh mesh in model.Meshes)
{
foreach (BasicEffect be in mesh.Effects)
{
be.Projection = proj;
be.View = view;
be.World = world;
be.Texture = texture;
be.TextureEnabled = true;
}
mesh.Draw();
}
}
```