I am trying to understand what the value range of projected vectors is. It seems at odds with what MSDN says.
The code below outputs a colored triangle. The triangle is fully visible. I would expect that after the verticies have been projected, the following would be true:
X and Y are in the range of -1 to 1. Z is in the range of 0 to 1.
I got this information from here: http://msdn.microsoft.com/en-us/library/bb195665.aspx
However, when I manually transform the verticies of the triangle using the product of view and projection matrix, they have the following values:
{X:1,8 Y:0 Z:3,991996} {X:0 Y:3 Z:3,991996} {X:-1,8 Y:0 Z:3,991996}
Why do the values fall out of the visible range, yet the trinangle is fully visible?
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using System;
using Microsoft.Xna.Framework.Input;
namespace WasMachtDieProjektion
{
public class Game1 : Microsoft.Xna.Framework.Game
{
GraphicsDeviceManager graphics;
Matrix _view;
Matrix _projection;
VertexPositionColor[] _verticies;
Vector3[] _vectors;
BasicEffect _effect;
public Game1()
{
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = "Content";
}
protected override void LoadContent()
{
_vectors = new Vector3[] {
new Vector3(-3,0,0),
new Vector3(0,3,0),
new Vector3(3,0,0)
};
_verticies = new VertexPositionColor[] {
new VertexPositionColor(_vectors[0], Color.AliceBlue),
new VertexPositionColor(_vectors[1], Color.Yellow),
new VertexPositionColor(_vectors[2], Color.Red)
};
_effect = new BasicEffect(graphics.GraphicsDevice);
_effect.VertexColorEnabled = true;
GraphicsDevice.RasterizerState = new RasterizerState() { CullMode = CullMode.None };
}
protected override void Update(GameTime gameTime)
{
_view = Matrix.CreateLookAt(new Vector3(0, 0, -4f), new Vector3(0, 0, 0), Vector3.Up);
_projection = Matrix.CreatePerspectiveFieldOfView(MathHelper.PiOver2, GraphicsDevice.Viewport.AspectRatio, 0.01f, 20f);
Vector3[] transformed = new Vector3[3];
Matrix trans = _view * _projection;
Vector3.Transform(_vectors, ref trans, transformed);
foreach (var v in transformed)
{
Console.WriteLine(v);
}
Console.WriteLine("---");
if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
this.Exit();
base.Update(gameTime);
}
protected override void Draw(GameTime gameTime)
{
GraphicsDevice.Clear(Color.CornflowerBlue);
_effect.View = _view;
_effect.Projection = _projection;
foreach (var pass in _effect.CurrentTechnique.Passes)
{
pass.Apply();
GraphicsDevice.DrawUserPrimitives(PrimitiveType.TriangleList, _verticies, 0, 1);
}
base.Draw(gameTime);
}
}
}