Moving the camera around objects - silverlight

Question

I'm trying to set the camera to move around the rendered object and I'm following this guide:
MSDN Guide on Rotating and Moving The Camera
here's the code I got:
public partial class MainPage : UserControl
{
float _cameraRotationRadians = 0.0f;
Vector3 _cameraPosition = new Vector3(5.0f, 5.0f, 5.0f);
List<TexturedMesh> _meshes;
BasicEffect _effect;
public MainPage()
{
InitializeComponent();
}
private void DrawingSurface_Loaded(object sender, RoutedEventArgs e)
{
GraphicsDevice device = GraphicsDeviceManager.Current.GraphicsDevice;
RenderModeReason reason = GraphicsDeviceManager.Current.RenderModeReason;
_meshes = StreamHelper.ToMesh(device, "capsule.obj");
_effect = new BasicEffect(GraphicsDeviceManager.Current.GraphicsDevice);
_effect.TextureEnabled = false;
_effect.World = Matrix.Identity;
_effect.View = Matrix.CreateLookAt(_cameraPosition, new Vector3(0.0f, 0.0f, 0.0f), Vector3.Up);
_effect.Projection = Matrix.CreatePerspectiveFieldOfView(MathHelper.PiOver4, 1.667f, 1.0f, 100.0f);
}
private void DrawingSurface_Draw(object sender, DrawEventArgs e)
{
GraphicsDevice device = GraphicsDeviceManager.Current.GraphicsDevice;
device.Clear(ClearOptions.Target | ClearOptions.DepthBuffer, new Microsoft.Xna.Framework.Color(0, 0, 0, 0), 10.0f, 0);
device.RasterizerState = new RasterizerState()
{
CullMode = CullMode.None
};
foreach (TexturedMesh mesh in _meshes)
{
// Load current vertex buffer
device.SetVertexBuffer(mesh.VertexBuffer);
// Apply texture
if (mesh.Texture != null)
{
_effect.Texture = mesh.Texture;
_effect.TextureEnabled = true;
}
else
_effect.TextureEnabled = false;
// Draw the mesh
foreach (EffectPass pass in _effect.CurrentTechnique.Passes)
{
pass.Apply();
device.SamplerStates[0] = SamplerState.LinearClamp;
device.DrawPrimitives(PrimitiveType.TriangleList, 0, mesh.VertexBuffer.VertexCount / 3);
}
// updates camera position
UpdateCameraPosition();
}
e.InvalidateSurface();
}
private void UpdateCameraPosition()
{
float nearClip = 1.0f;
float farClip = 2000.0f;
// set object origin
Vector3 objectPosition = new Vector3(0, 0, 0);
// set angle to rotate to
float cameraDegree = _cameraRotationRadians;
Matrix rotationMatrix = Matrix.CreateRotationX(MathHelper.ToRadians(cameraDegree));
// set where camera is looking at
Vector3 transformedReference = Vector3.Transform(objectPosition, rotationMatrix);
Vector3 cameraLookAt = _cameraPosition + transformedReference;
// set view matrix and projection matrix
_effect.View = Matrix.CreateLookAt(_cameraPosition, objectPosition, Vector3.Up);
_effect.Projection = Matrix.CreatePerspectiveFieldOfView(MathHelper.PiOver4, 1.667f, nearClip, farClip);
}
private void slider_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
{
// slider.Value;
_cameraRotationRadians += (float)slider.Value;
}
}
I have a slider control that gives the radians value. When the slider value changes, I then set a new angle for the camera. The UpdateCameraPosition() is called every time the DrawingSurface refreshes after the objects are loaded.
Now when I try to move the slider, the position of the camera never changes. Can someone help and tell me what to fix to make it revolve around the whole group of objects?

Answer 1

You are basically trying to rotate the vector (0,0,0). Rotating a vector in the origo will have no result.
You should instead rotate the camera-position.

Answer 2

That guide is how to rotate the camera in place, but it sounds like you are trying to have your camera orbit your object. In that case, both transformedReference and cameraLookAt are not needed.
Simply remove those two lines and rotate the _cameraPosition around the objectPosition to have the camera orbit the object.
//add this line in place of those two lines:
_cameraPosition = Vector3.Transform(_cameraPosition, rotationMatrix);
//now when you plug _cameraPosition into Matrix.CreateLookAt(), it will offer it a new camera position
Technically my snippet causes _cameraPosition to orbit the world origin 0,0,0. It works because your objectPosition happens to be at the world origin so the camera is orbiting objectPosition too.
But if objectPosition ever wanders away from the origin, you can still cause the camera to orbit the object by translating the camera/object system to the origin, perform the rotation, the translate them back. it is easier that it sounds:
_cameraPosition = Vector3.Transform(_cameraPosition - objectPosition, rotationMatrix) + objectPosition;
//now the camera will orbit the object no matter where in the world the object is.