# How does XNA/MonoGame project 3d to 2d?

I am trying to figure out how XNA is actually projecting a 3d world space coordinate to screen space.

I decompiled the MonoGame framework and copied the CreateLookAt und CreatePerspectiveFieldOfView methods of Matrix and multiplied thereafter 3 vectors like XNA does:

First multiply each position with world, then multiply resultig new vector4 with view and then the new resulting vector4 finally by projection.

      public static Vector2<T> Project(Vector3<T> source, Matrix4x4<T> world, Matrix4x4<T> view, Matrix4x4<T> projection, int width, int height)
{
Vector4<T> worldPosition = world * new Vector4<T>(source.X.ParseToNumeric<T>(), source.Y.ParseToNumeric<T>(), source.Z.ParseToNumeric<T>(), 1);
Vector4<T> viewPosition = view * worldPosition;
Vector4<T> position = projection * viewPosition;

var X = position.X.ParseToNumeric<T>();
var Y = position.Y.ParseToNumeric<T>();
var Z = position.Z.ParseToNumeric<T>();

return new Vector2<T>((X / Z) + width / 2, (Y / Z) + height / 2);

}


Because (0,0,0) would be the center of the screen I added to the corresponding X and Y coordinates width and height divided by 2. Just because it made sense I divided finally the xy components by z.

The result is sobering:  I used following matrices:

      basicEffect.World = Matrix.Identity;
basicEffect.View = Matrix.CreateLookAt(new Vector3(0, 0, 0), new Vector3(0, 0, 1), new Vector3(0, 1, 0));
basicEffect.Projection = Matrix.CreatePerspectiveFieldOfView(MathHelper.PiOver4, GraphicsDevice.Viewport.AspectRatio, 0.1f, 1000.0f);


And these points:

            vertices = new VertexPositionColor(new Vector3(0, 0, 2), Color.Red);
vertices = new VertexPositionColor(new Vector3(1, 0, 2), Color.Red);
vertices = new VertexPositionColor(new Vector3(0, 1, 2), Color.Red);


Do you know why this happens?

• I'm pretty sure your triangle is too small. – Bálint Apr 11 '17 at 6:05
• That's the point.. – φ Const. NET Apr 11 '17 at 17:47
• Why decompile MonoGame? It is open source. – lozzajp Jun 13 '17 at 12:30