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Eye ray!

Started by edwinnie May 31, 2004 at 11:59 PM 1 reply 550+ views

Original Post

May 31, 2004 at 11:59 PM

hihi! i was trying to compute the eye ray direction. I was having some difficulty computing the right intersection coordinates with the scene bounding box, so i decided to fall back on the eye ray direction... i was using the forumla given somewhere in this forum: //x = xleft + (0.5 + i)dx //y = ytop + (0.5 + j)dy i suppose u all used the near view plane as a start, compute its world coords for its corners, and use the above forumla to determine the ray directions znear is 1.0f origin is "camera eye position" at (0,0,20) IN.position(WPOS) gives the window screen coords near viewplane world coord corners: //topleft(-1.33,1.56,19.99) //topright(2.08,1.56,19.99) //bottomleft(-1.33,-1.56,19.99) //bottomright(2.08,-1.56,19.99)

float3 ComputeRayDirection(Fragment IN, Viewplane viewplane, float2 viewport)
{	
  float3 dir = float3(0,0,0);
  float dx = (viewplane.topright.x - viewplane.topleft.x) / viewport.x;
  float dy = (viewplane.topright.y - viewplane.bottomright.y) / viewport.y;
  dir.x = viewplane.bottomleft.x + (0.5 + IN.position.x) * dx;
  dir.y = viewplane.bottomleft.y + (0.5 + IN.position.y) * dy;
  dir.z = 0;
  dir = normalize(dir - origin);
  return dir;
}

the resulting direction happened to be quite small numbers eg (0.014,0.008,-0.99) does that make sense?? thx! Edwinz

Nik02

276

Nik02

Jun 1, 2004 at 2:16 AM

Your example (0.014,0.008,-0.99) is very close to an unit vector, that is, signifying only direction and not magnitude.
For that part, it makes perfect sense.

-Nik

EDIT: With "very close", I meant "exact, in practice".

[edited by - Nik02 on June 1, 2004 3:18:50 AM]

edwinnie

Author

edwinnie

Jun 1, 2004 at 4:32 AM

hi there!
thx fer replying!

assuming the above is correct, i was trying to use the ray box intersection algo to test fot hits and calculate the actual intersection point.

unfortunately, i could only sense out of 1 coord, the other 2 were gibberish i think.

i am using a uniform cube, with min (-1,-1,-1) and max (1,1,1)
as the bounding box (the cube itself is the bounding box basically)

the camera is at (0,0,5) looking straight at (0,0,-1)
however, i could only get the Z intersection coord correct.

i am using this article as a reference:
RayBox Intersect article</html>

this is the start of the intersection calling function,
which make uses of a helper function to simplifiy code...


int IntersectBoxCoord(CGRay ray, float3 ray_dir, out float3 hit_coord)
{
   int result = 1;
   float Tnear = -999999.0f;
   float Tfar = 999999.0f;
		
   //X plane

   result = IntersectHelper(Tnear,Tfar,ray_dir.x,ray.origin.x,float2(box_min.x,box_max.x));

   //Y plane

   if(result == 1)
	result = IntersectHelper(Tnear,Tfar,ray_dir.y,ray.origin.y,float2(box_min.y,box_max.y));
	
   //Z plane

   if(result == 1)
	result = IntersectHelper(Tnear,Tfar,ray_dir.z,ray.origin.z,float2(box_min.z,box_max.z));
		
   if(result == 1)
      hit_coord = ray.origin + Tnear * ray_dir;
   else
      hit_coord = float3(0,0,0);
		
return result;
}

the helper fn:


int IntersectHelper(out float Tnear, out float Tfar, float ray_dir, float origin,float2 planeMinMax)
{
   int result = 1;

   //if ray dir is parallel to plane

   if(ray_dir == 0.0f)
   {
        //planeMinMax could be float2(box_min.x,box_max.x)

        //                     float2(box_min.y,box_max.y)

        //                     float2(box_min.z,box_max.z)

       	if(origin < planeMinMax.x || origin > planeMinMax.y)
	   result = 0;
   }
   else
   //ray dir is not parallel to plane

   {
	//ray: P = O + D.t

	//plane: P.N = d

	//--> O.N + t * (D.N) = d

	//--> t = (d - O.N) / (D.N)

	//X plane so N = (-1,0,0) & (1,0,0)

			
	//for each min/max plane

	float t1 = (planeMinMax.x - origin) / ray_dir;
	float t2 = (planeMinMax.y - origin) / ray_dir;

	//swap, always make t1 < t2

	if(t1 > t2)
	{
		float temp = t1;
		t1 = t2;
		t2 = temp;
	}
			
	//set the Tnear & Tfar points

	if(t1 > Tnear)
	    Tnear = t1;
	if(t2 < Tfar)
	    Tfar = t2;
			
	//missed

	if(Tnear > Tfar)
	    result = 0;
	else
	//behind

	if(Tfar < 0)
	    result = 0;
	}
return result;
}

hopefully someone can point out some silly mistakes...
thx!
Edwinz

[edited by - edwinnie on June 1, 2004 5:39:16 AM]

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