# OpenGL frustrum culling.. what in the world am I doing wrong??

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All right, I have wasted sooo much time trying to figure out what in the world I do wrong: I jsut wat to to simple frustum culling, using Oriented boxes: with the notation of openGL in mind: I first define an eye_frustrum with the same parameters that one passes to glFrustum, i.e. left,right,top,bottom,near and far. because I have been getting depserate I have computed the corners of the frustum as follows:
  //near
m_left_top_near=vec3(proj.m_left, proj.m_top, proj.m_near);
m_left_bottom_near=vec3(proj.m_left, proj.m_bottom, proj.m_near);
m_right_top_near=vec3(proj.m_right, proj.m_top, proj.m_near);
m_right_bottom_near=vec3(proj.m_right, proj.m_bottom, proj.m_near);

//far
ratio=proj.m_far/proj.m_near;
m_left_top_far=ratio*m_left_top_near;
m_left_bottom_far=ratio*m_left_bottom_near;
m_right_top_far=ratio*m_right_top_near;
m_right_bottom_far=ratio*m_right_bottom_near;

m_middle=0.5f*vec3(proj.m_left+proj.m_right, proj.m_top+proj.m_bottom, proj.m_near+proj.m_far)


I know that for Frustum culling you really do not need the corners, but I have them so I can draw the frustum more easily, I then get the planes with:
//set the left plane:
normal=VectorCrossProduct(m_left_top_far - m_left_top_near, m_left_bottom_far- m_left_bottom_near);
m_planes[left_plane].Set(normal, m_left_top_far);

//set the right plane:
normal=VectorCrossProduct(m_right_bottom_far - m_right_bottom_near, m_right_top_far - m_right_top_near);
m_planes[right_plane].Set(normal, m_right_bottom_far);

//set the top plane:
normal=VectorCrossProduct(m_right_top_far - m_right_top_near, m_left_top_far - m_left_top_near);
m_planes[top_plane].Set(normal, m_right_top_far);

//set the bottom plane:
normal=VectorCrossProduct(m_left_bottom_far - m_left_bottom_near, m_right_bottom_far - m_right_bottom_near);
m_planes[bottom_plane].Set(normal, m_left_bottom_far);

//set the near plane:
normal=VectorCrossProduct(m_right_bottom_near - m_left_bottom_near, m_left_top_near-m_left_bottom_near);
m_planes[near_plane].Set(normal, m_right_bottom_near);

//set the far plane:
normal=VectorCrossProduct(m_left_top_far-m_left_bottom_far, m_right_bottom_far - m_left_bottom_far);
m_planes[far_plane].Set(normal, m_right_bottom_far);


and then I get my frustum in world co-ordinates with:
#define lazy(x) do { x=GenericCameraClass::world_transformPt_eye(eye_matrix_world, eye.x); } while(0)

lazy(m_left_top_near);
lazy(m_left_bottom_near);
lazy(m_right_top_near);
lazy(m_right_bottom_near);

lazy(m_left_top_far);
lazy(m_left_bottom_far);
lazy(m_right_top_far);
lazy(m_right_bottom_far);

lazy(m_middle)


the relavent goo for the Plane code is:
class Plane
{
//stuff....

public:
void Set(const vec3 &normal, const vec3 pt)
{
m_normal=normal/normal.magnitude();
m_value=dot(pt,m_normal);
SetAABHelper();
}

void Set(const vec3 &normal, float v)
{
float mag;

mag=normal.magnitude();

m_normal=normal/mag;
m_value=v/mag;
SetAABHelper();
}

bool Intersects(const GeneralCube &cube) const;
//stuff...

};

//(in another file)

bool Plane::Intersects(const GeneralCube &cube) const
{
float p,v;
int i;
bool retval,retval2;

v=dot(cube.center(), m_normal);

LogStream(FrustumLog,"p=" << p << " v=" << v << " m_value=" << m_value << "\n");

retval=(p+v>=m_value);

//paranoia: the above test should be enough, but we'll go
//ahead and check all vertices anyways.
for(retval2=false, i=0;i<8;++i)
{
retval2=retval2 || (dot(m_normal, cube.Vertex(i))>=m_value);
}

ASSERT(retval==retval2);
return retval;
}


and I am using <cmath> so abs does do the right thing here... now at this point, if I just make a frustum, it works correctly, i.e. I correctly detect when a frustrum and cube intersect (actually when no plane of the frustum is entrirly on one side)... my cubes are in world coordinates always, and the code for GenericCameraClass::world_transformPt_eye is:
static vec3 world_transformPt_eye(const float4x4 &matrix, vec3 in)
{
in-=vec3(matrix.GLElement(0,3), matrix.GLElement(1,3), matrix.GLElement(2,3));
return in*matrix;
}


the relavent code for the matrix stuff is:
class float4x4
{
public:
GLfloat data[16];
GLfloat *operator[](int i)
{
ASSERT(0<=i && i<4);
return data + 4*i;
}

const GLfloat *operator[](int i) const
{
ASSERT(0<=i && i<4);
return data + 4*i;
}

GLfloat &GLElement(int i, int j)
{
ASSERT(0<=i && i<4);
ASSERT(0<=j && j<4);

return data[4*j+i];
}

//more stuff....

};

inline vec3 operator*(const float4x4 &matrix, const vec3 &in)
{
vec3 out;
int i,j;

for(i=0;i<3;++i)
{
out[i]=0.0f;
for(j=0;j<3;++j)
out[i]+=matrix[j][i]*in[j];
}

return out;
}


and I pass the matrix to openGL for modelview stuff with (here camera is a const float4x4*) and state is state stuff for a mesh where m_matrixState is the matrix that goes from model co-ordinatres to eye-cooridnates.
glMatrixMode(GL_MODELVIEW);
glMultMatrixf(state->m_matrixState.data);


so what in the world am I doing wrong in my Frustum code? as of now meshes get culled if the viewer is not "inside" the meshes bounding box... [Edited by - kRogue on December 12, 2006 2:09:54 PM]

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just one more note, reading the internet and man pages gives different anwers to what glFrustum does:

accorind to openGL specs from opengl.org:

M=| 2n/(r-l) 0 (r+l)/(r-l) 0 |
| 0 2n/(t-b) (t+b)/(t-b) 0 |
| 0 0 -(f+n)/(f-n) -2fn/(f-n) |
| 0 0 -1 0 |

where as apples website (http://developer.apple.com/documentation/Darwin/Reference/ManPages/man3/glFrustum.3.html) gives

M=| 2n/(r-l) 0 (r+l)/(r-l) 0 |
| 0 2n/(t-b) (t+b)/(t-b) 0 |
| 0 0 +(f+n)/(f-n) -2fn/(f-n) |
| 0 0 -1 0 |

another place (http://www.mevis.de/opengl/glFrustum.html) gives

M=| 2n/(r-l) 0 (r+l)/(r-l) 0 |
| 0 2n/(t-b) (t+b)/(t-b) 0 |
| 0 0 +(f+n)/(f-n) +2fn/(f-n) |
| 0 0 -1 0 |

and I would not be surprised if the 3rd row has 4 different answers depending on which man page one reads! Note: on my hardware (nVidia GeForce6600GT, Linux OS) I get the answer from openGL's specification.

note, this means something interesting about the planes (which I should ahve realized):
1) z_eye should be negative in order to see it with -far<=z_eye<=-near

at any rate, this made me realize that I was creating the eye_frustum incorrectly, the corner points should be:
/////////////////////////////////////////  //set the corners  //  // some horible truth, the near and far need  // to be negated to get an accurate picture of  // the frustum since the frustum is  // -far <= z_eye <= -near  // thus we use -near instead of near, and -far instead of far.  // note that to do this is the same as just changing   // m_*_near.z() to -near, and letting m_*_far=ratio*m_*_near;  //near  m_left_top_near=vec3(proj.m_left, proj.m_top, -proj.m_near);  m_left_bottom_near=vec3(proj.m_left, proj.m_bottom, -proj.m_near);  m_right_top_near=vec3(proj.m_right, proj.m_top, -proj.m_near);  m_right_bottom_near=vec3(proj.m_right, proj.m_bottom, -proj.m_near);  //far  ratio=proj.m_far/proj.m_near;  m_left_top_far=ratio*m_left_top_near;  m_left_bottom_far=ratio*m_left_bottom_near;  m_right_top_far=ratio*m_right_top_near;  m_right_bottom_far=ratio*m_right_bottom_near;  m_middle=0.5f*vec3(proj.m_left+proj.m_right, proj.m_top+proj.m_bottom, -proj.m_near-proj.m_far);

and there all the VectorCrossProducts in figuring out the normals to the plains need to be negated (or just change VectorCrossProduct(a,b) to VectorCrossProduct(b,a) )....

I put these posts up, because not only is some of the online docs wrong for glFrustum, but I wasted more than a day realzing this "negation" issue...

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