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Nextgenneo

Edge cases of a bounding box vs the view frustum

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Nextgenneo    122
So I've implemented the basic view frustum culling algorithm but in my case the edge case of counting bounding boxes completely out of the frustum in is really effecting performance. Here's an image of what I am referring to: With the basic test the orange box is recorded as intersecting. Whats the fastest way to test for this case? Thanks

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Codeka    1239
I would imagine the simplest way would be to calculate a plane which is perpendicular to the far plane and where the far plane intersects with the frustum, and if the box is on the other side of that plane, cull it.

I hope you can understand what I wrote in that last paragraph without a diagram :-)

But I can't imagine there'd be all that many cases where this would actually have any effect... unless your bounding boxes are really huge, which kind of defeats the purpose (use an octree in that case).

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Nextgenneo    122
in more specific terms I am looking for exact frustum culling of an AABB.

and yeah, the above does make sense and I will implement this but is there a faster way?

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Nextgenneo    122
well I've researched it and I need to do a Separating Axis Test. I've figured out how to do this for an OBB and OBB but am not sure about a Frustum and an OBB.

Thanks

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Nextgenneo    122
Hi. Thanks for the replies and these are what I am currently implementing for my basic VF culling.

However let me show you the bug with some screenshots. I've captured what is drawn and the corresponding view frustum. The first screen shot is with the camera directly above the terrain and not rotated. The terrain is made up of concentric rings of increasing size (each ring made of 4 parts). You can see it correctly culls everything. I've also drawn the VF intercepts with the XZ Plane.



However when I rotate the camera as shown here the culling gets all messed up. I am fairly sure this is to do with the corner of the VF case and because my bounding box double in size every ring it causes lots of problems.



Any ideas?

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Codeka    1239
I think it might help if you also draw the bounding boxes. From what I can tell, the bounding box goes around a "ring" of the terrain, so the larger rings' bounding boxes actually go around the entire frustum, hence they're not being culled. That's what you would expect.

I think you need to break the terrain up differently -- i.e. not concentric rings, but instead into blocks. You could still use your current layout, but instead of one bounding box for the whole ring, you have 4 - one for each "side".

At least, that's assuming I'm understanding the situation here...

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Nextgenneo    122
Hi,

Sorry for not being clear enough. I've updated one of the screenshots to include some quickly drawn bounding boxes. So yeah, I split the rings into 4 rectangles.

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Nextgenneo    122
Here's a perfect example of this edge case I am referring to. The pink rectangle is the shape bounding box being culled and the black 3d trap is the view frustum. This VF returns intersecting with that bounding box when the bounding box is clearly outside.

If you extend the VF planes in your head you can see its the edge case (orange rectangle) in the originally post.

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Rockoon1    104
Quote:
Original post by Dave
It's outside the frustum when all of the points of the AABB are outside of the same side.


While this is true, it doesnt cover all outside cases.

No false negatives, but some false positives.

For example in the first post the orange rectangle is not in the frustum but is also is not classified correctly by this rule.

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dave    2187
Quote:
Original post by LogicalError
Quote:
Original post by Dave
It's outside the frustum when all of the points of the AABB are outside of the same side.


Unless the frustum is inside the AABB...


No. If the frustum is inside the AABB then how can all of the points of the AABB be on the same side of any of the frustum sides.

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Nextgenneo    122
you're rule is fine except it does create false positives like the orange box in the first example or the pink rectangle above. this is why I need EXACT View Frustum culling.

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Rockoon1    104
A little epiphany here

While the rule "It's outside the frustum when all of the points of the AABB are outside of the same side of the frustum`" isnt sufficient..

...you can also use the rule "It's outside the AABB when all points of the Frustum are outside the same side of the AABB"

These rules (combined) at least covers the specific false positive detailed in the original post, and may possible handle most/all false positives.

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Nextgenneo    122
Quote:
Original post by Rockoon1
A little epiphany here

While the rule "It's outside the frustum when all of the points of the AABB are outside of the same side of the frustum`" isnt sufficient..

...you can also use the rule "It's outside the AABB when all points of the Frustum are outside the same side of the AABB"

These rules (combined) at least covers the specific false positive detailed in the original post, and may possible handle most/all false positives.


oooooo. this makes total sense in my mind. will implement this tomorrow and I expect it to work. awesome.

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Rockoon1    104
Quote:
Original post by Nextgenneo
oooooo. this makes total sense in my mind. will implement this tomorrow and I expect it to work. awesome.


Indeed, I now think that it will work flawlessly for all convex geometry..

I'm wondering if as combined they are equivilent to a seperating axis test tho, expressed slightly differently but essentialy implemented the same..

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