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How can I draw a ray? (ie. why is my simple code not working?)

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I'm following the mouse picking tutorial but instead of trying to intersect objects with the ray I'm just trying to draw the ray itself. (I know that a ray is infinite but I'm trying to draw from it's starting position to 1000 units away from it) Here is how I'm creating my ray:
Vector3 nearSource = new Vector3(Mouse.GetState().X, Mouse.GetState().Y, camera.nearPlane); 
Vector3 farSource = new Vector3(Mouse.GetState().X, Mouse.GetState().Y, camera.farPlane); 
Vector3 nearPoint = Game.GraphicsDevice.Viewport.Unproject(nearSource, camera.projection, camera.view, Matrix.Identity); 
Vector3 farPoint = Game.GraphicsDevice.Viewport.Unproject(farSource, camera.projection, camera.view, Matrix.Identity); 
// find the direction vector that goes from the nearPoint to the farPoint 
// and normalize it.... 
Vector3 direction = farPoint - nearPoint; 
// and then create a new ray using nearPoint as the source. 
lastRay = new Ray(nearPoint, farPoint); 
VertexPositionColor[] v = new VertexPositionColor[2]; 
v[0] = new VertexPositionColor(lastRay.Position, Color.Blue); 
v[1] = new VertexPositionColor(lastRay.Position * (lastRay.Direction * 1000), Color.Red); 
vb = new VertexBuffer(Game.GraphicsDevice, VertexPositionColor.SizeInBytes * 2, BufferUsage.WriteOnly); 
...and here is how I'm drawing it:
if (vb != null) 
   GraphicsDevice.RenderState.DepthBufferEnable = true; 
   GraphicsDevice.RenderState.AlphaBlendEnable = false; 
   GraphicsDevice.VertexDeclaration = new VertexDeclaration(Game.GraphicsDevice, VertexPositionColor.VertexElements); 
   GraphicsDevice.Vertices[0].SetSource(vb, 0, VertexPositionColor.SizeInBytes); 
   effect.World = Matrix.Identity; 
   effect.View = camera.view; 
   effect.Projection = camera.projection; 
   foreach (EffectPass pass in effect.CurrentTechnique.Passes) 
      GraphicsDevice.DrawPrimitives(PrimitiveType.LineList, 0, 1); 
Am I doing something wrong? I'm just trying to see the mouse-picking line before I use it...

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Did you try rendering a line with known values for the two points to make sure that works first?

I've found that trying to visualize a picking ray every frame is hard sometimes, and it doesn't really show up - you can only really see it if the camera is moving perpendicular to the ray (strafing). Though I was always casting from the center of the screen, I recall it hardly showing up even if I was using an arbitrary screen position.

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I guess you know that drawing a ray from the camera directly into the scene won't be visible?
You are looking at the end of it which would be 1pixel if you are lucky. Try setting the root of the ray somewhere else - Increase every component (X,Y,Z) by a few units for example.

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sometimes if you attached to texture while rendering without texture coordinates it fail to render correctly, try settexture to 0.

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Thanks for all of the replies.

I've found half of my problem: The nearSource Z coordinate must be 0 and the farSource Z coordinate must be 1. I was using my actual near and far plane values (1, and 1000).

Now my line shows up!

However, it seems to be drawn a little off-center from where I click.

Are my source and destination vertices correct for the line?

ray.Position (for the starting point), and ray.Position * (ray.Direction * 100) for the destination? (eg. I just want to draw the actual ray for about 100 units of length)

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Ok, I've found my problem but I'm really confused what is causing it.

After checking MouseState.MiddleButton (or any button) the X, Y mouse coordinates change!

For example:

MouseState ms = Mouse.MouseState()

if (ms.MiddleButton == ButtonState.Pressed)
// Do something

// ms.X and ms.Y might be (0, 0) at this point, but if you check Mouse.GetState().X/Y you might see (203, 481) or something extremely different.

This was causing my mouse pick ray to have incorrect coordinates...

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