Hi,

i have been spending a lot of time to track a bug in my projection pipeline. I am implementing ray picking for my diploma thesis - actually just a UI thing
I did not plan to waste so much time on it.

I used two different methods to unproject:
1) Place an imageplane in front of the camera and shoot a ray
2) Invert the projection matrix multiply and homogenize

In both cases i observed that the generated rays were a little of and missed the scene parts they were suppossed to hit slightly. Just because I ran out of ideas
I translated the ray origin to (0,0, -1) and suddenly it worked perfectly.

I could ignore that little glitch, but it leaves me with a bad feeling as I do not understand what happens behind the curtains. The ray origin should be (0,0,0) - right ?

As I ran into the problem twice, I suppose I did fail to understand all aspects of homogenous clip coordinates. I will post my projection matrices and maybe you guys could
give me some advice what I am doing wrong - This is really starting to frustrate me.


My viewport to NDC matrix (row vectors):

x: (1.0f / (width / 2.0f)), 0, 0 ,0)
y: (0, (1.0f / (height / 2.0f)), 0 ,0)
z: (0, 0, 1 ,0)
w:(-1.0f 1.0f 0 ,1)

My projection matrix: (symmetric frustum)

x: (near / right, 0 0 0)
y: (0 near/top 0 0)
z:(0, 0 (far + near)) / (far - near) 1) //Camera is looking in positive z, opposed to vanilla OpenGL
w(0, 0 -(2 * far * near) / (far - near) 1)


The inverse projection matrix (after Mouse Ray Picking Explained):

x: (right / near, 0 0 0)
y: (0 top / near 0 0)
z: (0 0 0 (far - near) / (-(2*far*near))
w:(0 0 1 (far + near) / ( 2*far*near))

I am not sure the matrices will help, but maybe you got an idea what i might have forgot ?

The actual method (java) looks like this:

/*
* Returns a ray in eye space
*/
public Ray raycast(Point2 position) {
//Viewport to NDC
Point3 pickPoint = paintingSurface.normalise(new Point3(position, 0));

//NDC to eye coordinates
Point3 reprojectedPoint = inverseProjection.multiply(pickPoint);
reprojectedPoint.homogenousDivide();

Vector3 rayDir = reprojectedPoint.subtract(Point3.ZERO);
rayDir.normalize();

return new Ray(new Point3(0,0,-1), rayDir); /// :-(((((( Why???
}


I would be glad if anybody could point out my error.

Thanks a lot,
Frederick

Hi Geometrian,

thank you so much for pointing that out. Of course it was meant as a projection matrix - and it did the visual - kind of.

I took the matrix from here
But I didn´t notice that the w coordinate of the translational part of this matrix is set to zero. I thought that w of the translational part must always be one. The w value
introduced the weird translation I had to fight with.

Maybe I have to rethink my whole approach concerning matrices. I learned from David Eberlys fine "Geometric Tools" that it is a good idea to distinct point and vectors.
And I distinct them per type. So I have a point and a vector class. Points w is always 1, Vector w is always 0. At least I thought so.
I had to relax this lately because the homogenous approach to perspective division demands w values different from zero or one.

My whole approach to matrices is somewhat broken now. Maybe I will rewrite them. My current approach looks like this:

public class Matrix3 {

private Vector3 x, y, z;
private Point3 w;

[...]
}[/quote]

Up to now I liked it and thought it was elegant, but there are cases like projection when w actually is not a point.

Now i do something like:

matrix3.w().setW(0.0f) [/quote] which contradicts the statement that w is of type point3.

Maybe I have to abandon this approach. I also have never seen anybody do it like this - there might be a reason....

Thank you so much and extra pair of eyes can really do wonders sometimes - I was so stuck in my approach - I would have never noticed that glitch.

Frederick