Question about Getting 2D screen coordinates

[DaphydTheBard]

I've read through the "picking" tutorial and that's all pretty cool, however, I need to do the opposite, effectively.

I'm writing a space-flightsim and I want to be able to display a targetting bitmap around whichever ship is currently targeted (using conventional 2D sprite methods).

Is there a straightforward way of getting a 3D Objects' actual 2D screen co-ordinates?

Any help greatly appreciated.

[mikeschuld]

multiply its position by the world view projection matrix

[DaphydTheBard]

can you give an example please mike?

I.e. store the result in what??

Thanks.

[mikeschuld]

Vector3 screenCoords = Vector3.Transform(obj.Position, obj.World * camera.View * camera.Projection);

[DaphydTheBard]

Mike

I couldn't get your method to work properly, however I've found another way which works:

Here's how to do it, for anyone interested.  Obviously the function needs tweaking for your own engine to pass in matrixes and viewport sizes.  However, this code works a treat:

public static Point Convert3DPointTo2D(Vector3 point, XNAViewPort objVP)
        {           
            Vector4 result4 = Vector4.Transform(point,
                objVP.MatrixView * objVP.MatrixProjection);
           
            if (result4.W == 0)
                result4.W = 0.000001f;
            Vector3 result = new Vector3(
                result4.X / result4.W,
                result4.Y / result4.W,
                result4.Z / result4.W); 
           
            return new Point(
                (int)Math.Round(+result.X * (800 / 2)) + (800 / 2)+100,
                (int)Math.Round(-result.Y * (600 / 2)) + (600 / 2)+100);
        }

I can't take credit for this code as it was modified from the rocket commander source code 

[Nemo Krad]

I have had a pop at this too, and thanks to DaphydTheBard's example I came up with this:

Code:

        public Point Get2DCoords(GameWindow window)
        {
            HMCameras.HMCamera camera = HMCameras.HMCameraManager.ActiveCamera;
            Matrix ViewProjectionMatrix = camera.View * camera.Projection;

            Vector4 result4 = Vector4.Transform(myPosition, ViewProjectionMatrix);
            if (result4.W == 0)
                result4.W = HMHelper.Epsilon;

            Vector3 result = new Vector3(result4.X / result4.W, result4.Y / result4.W, result4.Z / result4.W);

            return new Point((int)Math.Round(+result.X * (window.ClientBounds.Width / 2)) + (window.ClientBounds.Width / 2), (int)Math.Round(-result.Y * (window.ClientBounds.Height / 2)) + (window.ClientBounds.Height / 2));
        }

I placed the method in the HMObject class so that I can get the 2D coords of all my objects. I am starting to feel like a code troll! You guys pop the ideas up there and I nick em  Maybe I should start having ideas of my own.....

[EDIT]
Just goes to show how important QA is, testing this on my poop laptop seemed to be OK, but once I got home and gave it a go I altered it to the one you now see here in this post....  It's a good job I don't have ideas of my own, I might do some dammage....
[/EDIT]

[DaphydTheBard]

Nothing wrong with using other peoples' code - we all do it! 

I think the important thing is to understand the code if it's "borrowed".