XNA Framework

Re: Viewport.Unproject inconsistency in XNA 2.0 Beta

Nightin — Fri, 14 Dec 2007 09:11:43 GMT

Pushing the near clip plane isn't quite a good solution to this, here's the method I worked out for generating pick ray:

void UpdatePickRay()
{
    MouseState mouseState = Mouse.GetState();

    Vector3 v;
    v.X = (((2.0f * mouseState.X) / screenWidth) - 1);
    v.Y = -(((2.0f * mouseState.Y) / screenHeight) - 1);
    v.Z = 0.0f;

    pickRay.Position.X = viewInverse.M41;
    pickRay.Position.Y = viewInverse.M42;
    pickRay.Position.Z = viewInverse.M43;
    pickRay.Direction = Vector3.Normalize(
        Vector3.Transform(v, viewProjectionInverse) - pickRay.Position);
}

Where viewInverse, projectionInverse and viewProjectionInverse are computed every frame using

projection = camera.Projection;
viewProjection = view * projection;
viewInverse = Matrix.Invert(view);
projectionInverse = Matrix.Invert(projection);
viewProjectionInverse = projectionInverse * ViewInverse;

This method works well for picking.

The only difference is that the pick ray position is exacly the eye position using this method,

by using the method refered in the XNA documentation, pick ray position is on the near clip plane.

But in most cases this difference is trivial.

Re: Viewport.Unproject inconsistency in XNA 2.0 Beta

ajmiles — Fri, 14 Dec 2007 07:39:19 GMT

I've been having a look at that sample, and while I can't really see that you're doing anything wrong per se, the problem is certainly lessened substantially by pushing your near clip plane out to 10 or 100, which suggests to me there might be some floating point inaccuracy going on, perhaps when using/normalising vectors somewhere, but as yet I can't fix it for sure.

Re: Viewport.Unproject inconsistency in XNA 2.0 Beta

Nightin — Fri, 14 Dec 2007 06:39:52 GMT

This is the sample program modified from XNA tutorial, hope it demonstrates something.

I'm using Visual Studio 2005 Team + XNA 2.0 Build 2.0.11022.3.

http://files-upload.com/files/677911/Unproject.zip

The link will be expired in 14 days :(

Re: Viewport.Unproject inconsistency in XNA 2.0 Beta

ajmiles — Fri, 14 Dec 2007 05:00:56 GMT

I just converted the Triangle Picking with Triangle Accuracy sample from 1.0 to 2.0 to see if I could see any issues with the way Unproject worked and I could see none. No jittering, no inverse rays, it just worked. It does appear that Unproject (and possibly Project, I didn't check) are now implemented in C# rather than making a native call as in 1.0, but as I say, I can't see anything wrong with it. I don't pretend to be able to quite understand the math behind the Unproject method, but jittering isn't likely to occur unless you're feeding it different arguments each time you call it, it could conceivably be buggy, but its pretty unlikely Unproject is non-deterministic.

Re: Viewport.Unproject inconsistency in XNA 2.0 Beta

lidgren — Fri, 14 Dec 2007 03:08:52 GMT

We've also had problems with Viewport.Unproject when creating a ray for picking. When using x,y,1 - x,y,0 it sometimes created an inversed ray. We solved the problem by using x,y,0.75f - x,y,0.25f instead; since we normalize the result anyway.

Re: Viewport.Unproject inconsistency in XNA 2.0 Beta

The ZMan — Thu, 13 Dec 2007 10:47:52 GMT

Can you post the solution somewhere so we can try a repro (comment out one of the versions so we can switch between it).

Viewport.Unproject inconsistency in XNA 2.0 Beta

Nightin — Thu, 13 Dec 2007 04:58:20 GMT

When I ported my game from XNA 1.0 Refresh to XNA 2.0 (Beta), the picking isn't working properly.
The pick ray jitters a lot and I guess it's the problem with Viewport.Unproject.
Whenever my view matrix changed a little bit, the direction of the pick ray changed back and forth.
I'm using the approach refered in the XNA documentation to compute the pick ray.

After I implemented my own version of pick ray generation, the jitter problem went away and everything went back to normal :)

This isn't happening in XNA 1.0, so I guess it's probably a potential bug in XNA 2.0.

Is there anyone experienced the same problem? Tell me why this is happening.