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World Space coordinates and camera view

Last chapter we drew a triangle, using 'transformed' coordinates. These coordinates are already 'transformed' so you can directly specify their position on the screen. However, you will usually use the untransformed coordinates, the so called World space coordinates. These allow you to create a whole scene using simple 3D coordinates, and, most important, to position a camera and set its viewing point.

So we'll start with redefining our triangle coordinates in world space. Replace the code in your OnPaint method with this code:

 CustomVertex.PositionColored[] vertices = new CustomVertex.PositionColored[3];
 vertices[0].Position = new Vector3(0f, 0f, 0f);
 vertices[0].Color = Color.Red.ToArgb();
 vertices[1].Position = new Vector3(10f, 0f, 0f);
 vertices[1].Color = Color.Green.ToArgb();
 vertices[2].Position = new Vector3(5f, 10f, 0f);
 vertices[2].Color = Color.Yellow.ToArgb();

All you've done here is changed the format from pre-transformed coordinates to 'normal' coordinates. Also let the device know your format has changed :

 device.VertexFormat = CustomVertex.PositionColored.Format;

Let's run this code.

Very nice, your triangle has disappeared again. Why's that ? Easy, because you haven't told DirectX yet where to position the camera and where to look at! To position your camera, simply add the following code as the first lines in your OnPaint method :

 device.Transform.Projection = Matrix.PerspectiveFovLH((float)Math.PI/4, this.Width/this.Height, 1f, 50f);
 device.Transform.View = Matrix.LookAtLH(new Vector3(0,0,30), new Vector3(0,0,0), new Vector3(0,1,0));

The first line tells the device what and how the camera should look at the scene. The first parameter sets the view angle, 90 in our case. The we set the view aspect ratio, which is 1 in our case, but will be different if our window is a rectangle instead of a square. The last parameters define the view range. Any objects closer to the camera than 1f will not be shown. Any object farther than 50f won't be shown either. These distances are called the near and the far clipping planes, since all objects between these planes will be clipped (=not drawn).

The second line actually positions the camera. The first parameter defines the position. We position it 30 units above our (0,0,0) point, the origin. The next parameter sets the target point the camera is looking at. We will be looking at our origin. At this point, we have defined the viewing axis of our camera, but we can still rotate our camera around this axe. So we still need to define which vector will be considered as 'up'. Now run the code again.

This time, we see a triangle, but it's all black! This is because world space is a little more advanced than our previous chapter. Here we are also required to place some lights. However, these will be handled in a following chapter, thus here we will simply tell our device not to expect any lights. Add the following line underneath your camera definition and you'll see our colored triangle again:

 device.RenderState.Lighting = false;

Now everything has been set to use world space coordinates. One thing you should notice: you'll see the green corner of the triangle on the LEFT side of the window, while you defined it on the POSITIVE x-axis. This is because DirectX uses left-handed coordinates!! So, if you would position your camera on the negative z-axis:

 device.Transform.View = Matrix.LookAtLH(new Vector3(0,0,-30), new Vector3(0,0,0), new Vector3(0,1,0));

you would expect to see the green point in the right half of the window. Try to run this now.

This might again not be exactly what you expected. Something very important has happened. DirectX only draws triangles that are facing the camera. DirectX defines that triangles facing the camera should be drawn clockwise relative to the camera. If you position the camera on the negative z-axis, the triangle will be defined counter-clockwise relative to the camera, and thus will not be drawn! One way to remove this problem is simply redefining the vertices clockwise (this time clockwise relative to our camera on the negative part of the Z axis) :

 vertices[2].Position = new Vector3(0f, 0f, 0f);
 vertices[2].Color = Color.Red.ToArgb();
 vertices[0].Position = new Vector3(5f, 10f, 0f);
 vertices[0].Color = Color.Yellow.ToArgb();
 vertices[1].Position = new Vector3(10f, 0f, 0f);
 vertices[1].Color = Color.Green.ToArgb();

This will indeed draw the triangle with the green point to the right. The other way is to add the following line after you camera definition :

 device.RenderState.CullMode = Cull.None;

This will simply draw all triangles, even those not facing the camera. You should note that this should never be done in a final product, because it slows down the drawing process, as all triangles will be drawn, even those not facing the camera! However, while designing, it is wise to turn off culling, so you'll always see everything you draw. Also, I chose the background color to be non-black, again because if your triangle might be wrongly defined and drawn black, you'ld still see it. So, while designing, you should turn culling off and set a non-black background color.




DirectX Tutorial 4 - Camera

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  • I have a problem(with camera i think)
          Hello, great tutorial. But i have a problem, I've...
  • Camera Movement with Mouse
          I have come up with some code to rotate the camera...
  • mistake found
           hi, first i wanna say that i like you...
  • zbuffer needed?
          I have the same problem as in the other post - the...
  • Error in lesson
          I don't know if anyone has spotted it but there i...
  • worldspace coordinates
          I'm confused about the world space coordinates. I...
  • FOV Angle
           device.Transform.Projection = Matrix.Perspe...



    You can find some more information on culling in the Culling chapter of the C++ part of this tutorial, where I have made a small animation to visualize the culling process.

    Here's the complete code again :

     using System;
     using System.Drawing;
     using System.Collections;
     using System.ComponentModel;
     using System.Windows.Forms;
     using System.Data;
     using Microsoft.DirectX;
     using Microsoft.DirectX.Direct3D;
     
     namespace DirectX_Tutorial
     {
     
         public class WinForm : System.Windows.Forms.Form
         {
             private Device device;
             private System.ComponentModel.Container components = null;
     
             public WinForm()
             {
                 InitializeComponent();
                 this.SetStyle(ControlStyles.AllPaintingInWmPaint | ControlStyles.Opaque, true);
             }
     
             public void InitializeDevice()
             {
                 PresentParameters presentParams = new PresentParameters();
                 presentParams.Windowed = true;
                 presentParams.SwapEffect = SwapEffect.Discard;
                 device = new Device(0, DeviceType.Hardware, this, CreateFlags.SoftwareVertexProcessing, presentParams);
             }
     
             protected override void OnPaint(System.Windows.Forms.PaintEventArgs e)
             {
                 device.Transform.Projection = Matrix.PerspectiveFovLH((float)Math.PI/4, this.Width/this.Height, 1f, 50f);
                 device.Transform.View = Matrix.LookAtLH(new Vector3(0,0,-30), new Vector3(0,0,0), new Vector3(0,1,0));
                 device.RenderState.Lighting = false;
                 device.RenderState.CullMode = Cull.None;
     
                 CustomVertex.PositionColored[] vertices = new CustomVertex.PositionColored[3];
                 vertices[0].Position = new Vector3(0f, 0f, 0f);
                 vertices[0].Color = Color.Red.ToArgb();
                 vertices[1].Position = new Vector3(10f, 0f, 0f);
                 vertices[1].Color = Color.Green.ToArgb();
                 vertices[2].Position = new Vector3(5f, 10f, 0f);
                 vertices[2].Color = Color.Yellow.ToArgb();
     
     
                 device.Clear(ClearFlags.Target, Color.DarkSlateBlue , 1.0f, 0);
     
                 device.BeginScene();
                 device.VertexFormat = CustomVertex.PositionColored.Format;
                 device.DrawUserPrimitives(PrimitiveType.TriangleList, 1, vertices);
                 device.EndScene();
     
                 device.Present();
     
                 this.Invalidate();
             }
     
             protected override void Dispose (bool disposing)
             {
                 if (disposing)
                 {
                     if (components != null)
                     {
                         components.Dispose();
                     }
                 }
                 base.Dispose(disposing);
             }
     
             private void InitializeComponent()
             {
                 this.components = new System.ComponentModel.Container();
                 this.Size = new System.Drawing.Size(500,500);
                 this.Text = "DirectX Tutorial";
             }
     
             static void Main()
             {
                 using (WinForm our_directx_form = new WinForm())
                 {
                     our_directx_form.InitializeDevice();
                     Application.Run(our_directx_form);
                 }
             }
         }
     }
     


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    Contents

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    Matrices: geometrical
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    XNA 4.0 using C# (89)
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    Series 1:Terrain (14)
    Opening a window
    Linking to the Device
    Drawing a triangle
    Camera
    Rotation - Translation
    Indices
    Terrain creation
    Terrain from file
    DirectInput
    Importing bmp files
    Colored vertices
    DirectX Light basics
    Mesh creation
    Mesh lighting
    Series 2: Flightsim (19)
    Series 3: HLSL (19)
    Short Tuts (2)
    DirectX using C++ (15)
    DirectX using VB (2)
    -- Expand all --


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