# Frustum - AABB intersection

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Hello. I can't quite get my intersection test beween a frustum and aabb box correct. I can test if the box is completely contained or reverse, if the frustum is completely contained. But i cant figure out how to do it if the box is intersecting with the frustum. I came up with a test that had like 50 if() cases, and it worked in most cases but not all. Is there some simpler elegant way to do this, like with the other test? This is what my intersection test between a box and a frustum looks like. It makes use of a function that tests intersection between a plane and a box. That function can return PLANE_INTERSECTION_BACK, PLANE_INTERSECTION_FRONT or PLANE_INTERSECTION_INTERSECT.
inline volume_intersection_t intersect(const box_t* a, const frustum_t* b)
{
plane_intersection_t left = intersect(&b->m_left_plane, a);
plane_intersection_t right = intersect(&b->m_right_plane, a);
plane_intersection_t top = intersect(&b->m_top_plane, a);
plane_intersection_t bottom = intersect(&b->m_bottom_plane, a);
plane_intersection_t near = intersect(&b->m_near_plane, a);
plane_intersection_t far = intersect(&b->m_far_plane, a);

// box contains the frustum
if( (left		== PLANE_INTERSECTION_FRONT)
&& (right	== PLANE_INTERSECTION_FRONT)
&& (top		== PLANE_INTERSECTION_FRONT)
&& (bottom	== PLANE_INTERSECTION_FRONT)
&& (near	== PLANE_INTERSECTION_FRONT)
&& (far		== PLANE_INTERSECTION_FRONT))
{
return VOLUME_INTERSECTION_A_CONTAINS_B;
}
// the frustum contains the box
else if( (left		== PLANE_INTERSECTION_BACK)
&& (right	== PLANE_INTERSECTION_BACK)
&& (top		== PLANE_INTERSECTION_BACK)
&& (bottom	== PLANE_INTERSECTION_BACK)
&& (near	== PLANE_INTERSECTION_BACK)
&& (far		== PLANE_INTERSECTION_BACK))
{
return VOLUME_INTERSECTION_B_CONTAINS_A;
}
// the box and the frustum is intersecting
else if(???)
{
return VOLUME_INTERSECTION_INTERSECTING;
}

return VOLUME_INTERSECTION_NO_RELATION;
}


Extra question: I find it hard to debug the frustum. Boxes and spheres i can create meshes of and render to make sure they are performing correctly. But i cant really render a frustum, as it is now i just stear the camera and look at things, and guess that what is outside the view frustum isnt renderd. Or look at what is beeing renderd from another camera that the frustum isnt attached to. But does anyone have any tip for a simple way to visualize the frustum? Thanks in advance! :-)

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I am at work, so this will be abreviated. It looks like you may be misunderstanding the AABB/frustum test. Let's say that your box/plane test is correct (you might post the code to be sure). Then the standard box/frustum test is:

for (int i = 0; i < 6; ++i)    if (TestBoxPlane(box, frustum.planes) == OUTSIDE)        return false;return true;

Whether OUTSIDE is FRONT or BACK depends on which way your plane normals point.

Note that this is not a perfect test - it can return false positives. But for frustum culling it is generally considered to be accurate enough.

For drawing your frustum, you can find the corners of the truncated pyramid that represents it by intersecting the frustum planes in sets of three. Look up 'plane intersection' for how to do this.

Gotta run, but 'hope that was of some help.

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Yes i found the approach you mention to be the standard. But the thing is that i want to be able to tell if a box is inside the frustum, or completly containing the frustum, or is intersecting with the frustum.

The above approach dosnt work if a box is completely containing the frustum, or atleast i think so.. my brain is boiling from trying to visualisze all this. :-)

The two first tests are easy, if a box is completely containing the frustum, all planes of the frustum should repoart that the they are intersecting with the box. (this is wrong in my code).

The second test, if the frustum is completely containg the box, all planes should repoart that the box is infront of them(normals pointing inwards).

But the third test.. if the box is intersecting with the frustum..
Hmm, maybe i can get that result if i use your test after i have tested if the frustum is completely containg the box, than that option isnt avalible anymore.. and if there are no planes that report behind.. than it should be a collision right? hmm, i will try that! :-P

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