307 lines
13 KiB
Text
307 lines
13 KiB
Text
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From fatcity!root@news.cts.com Wed Aug 20 16:34:24 1997
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X-VM-v5-Data: ([nil nil nil nil nil nil nil nil nil]
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["3412" "Wed" "20" "August" "1997" "14:14:39" "-0800" "Steve Baker" "steve@mred.bgm.link.com" nil "83" "Re: culling with view frustrum before view transformation" "^From:" nil nil "8" nil nil nil nil nil]
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Received: by fatcity.com (06-Aug-97/v1.0d-b55/bab) via UUCP id 0C90BB6D; Wed, 20 Aug 1997 14:14:39 -0800
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Message-ID: <F001.0C90BB6D.19970820141439@fatcity.com>
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X-Comment: OpenGL Game Developers Mailing List
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Organization: Fat City Network Services, San Diego, California
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X-ListServer: v1.0d, build 55; ListGuru (c) 1996-1997 Bruce A. Bergman
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Precedence: bulk
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From: steve@mred.bgm.link.com (Steve Baker)
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Sender: root@fatcity.com
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To: Multiple recipients of list OPENGL-GAMEDEV-L <OPENGL-GAMEDEV-L@fatcity.com>
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Subject: Re: culling with view frustrum before view transformation
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Date: Wed, 20 Aug 1997 14:14:39 -0800
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> >Brian Hook wrote:
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> >> You can do bounding spheres,
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> >> bounding boxes, or a combination of both (sphere first, then box).
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> >
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> >Which is faster to test for being completely inside, outside, or
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> >partial, of the view frustrum?
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>
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> Obviously bounding sphere will be the fastest, but since it's, well, a
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> sphere, it's not going to be particularly accurate (since it's rotationally
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> invariant).
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>
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> if ( distance( obj.position, plane ) >= obj_sphere.radius )
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> trivial_reject = true;
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>
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> (I assume you also check the sign of the distance)
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I echo your sentiments that bounding spheres are the fastest approach to
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culling to FOV frustum - also for some kinds of collision detection.
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The most important thing to bear in mind about culling is that the first
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trivial-reject test you apply is by far the most time-critical. This test
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is always applied to more nodes than any of the subsequent tests.
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So, do the cheapest test first.
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This is typically the NEAR plane test. Everything behind the viewers head
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gets chopped out - and it's an especially cheap test.
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if ( obj_sphere.center.z < near_plane - obj_sphere.radius )
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REJECT!!
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...next do the second cheapest test (assuming you know that your database
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could possibly beyond the far clip plane)...
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if ( obj_sphere.center.z - obj_sphere.radius > far_plane )
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REJECT!!
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...and *then* do...
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if ( distance( obj.position, plane ) >= obj_sphere.radius )
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REJECT!!
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It's also useful to know that in many applications, you cull more objects from
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the left and right faces of the frustum than you do from the top and bottom - so
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test left, then right, then bottom then top.
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Also, with bounding sphere tests, you shouldn't forget to do total-accept
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as well as total-reject tests. Once you know that an object's sphere is
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TOTALLY on screen, you don't have to descend into the daughter objects to
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cull-test them...you *know* they are all on-screen.
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Another way to look at that it to remember which of the six possible plane tests
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didn't even touch the sphere - as you work your way down the object hierarchy,
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you can accumulate those flags and avoid even testing those planes that a parent
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sphere has already cleanly passed. If you do this then a vast percentage of
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your spheres will only need to be tested against one plane.
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Sphere-based culling can be extremely cost-effective. It's so cheap that even if
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you feel the need to use a bounding cubeoid (or even a yet more complex shape),
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it's still worth doing a sphere-based cull first just to get rid of the trivial
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accept and reject cases.
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Steve Baker 817-619-1361 (Vox-Lab)
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Hughes Training Inc. 817-619-8776 (Vox-Office/Vox-Mail)
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2200 Arlington Downs Road 817-619-4028 (Fax)
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Arlington, Texas. TX 76005-6171 Steve@MrEd.bgm.link.com (eMail)
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http://www.hti.com http://web2.airmail.net/sjbaker1 (personal)
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** Beware of Geeks bearing GIF's. **
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--
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Author: Steve Baker
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INET: steve@mred.bgm.link.com
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Fat City Network Services -- (619) 538-5030
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San Diego, California -- Public Internet Access
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-------------------------------------------------------------------
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To REMOVE yourself from this mailing list, send an E-Mail message
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to: ListGuru@fatcity.com (note EXACT spelling of 'ListGuru') and in
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the message BODY, include a line containing: UNSUB OPENGL-GAMEDEV-L
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(or the name of mailing list you want to be removed from).
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From fatcity!root@news.cts.com Thu Aug 21 09:19:20 1997
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X-VM-v5-Data: ([nil nil nil nil nil nil nil nil nil]
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["2834" "Thu" "21" "August" "1997" "07:00:54" "-0800" "Steve Baker" "steve@mred.bgm.link.com" nil "77" "Re: culling with view FRUSTUM before view transformation" "^From:" nil nil "8" nil nil nil nil nil]
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nil)
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Received: from mailhub.cts.com (mailhub.cts.com [204.216.216.130])
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by meserv.me.umn.edu (8.8.6/8.8.6) with SMTP id JAA02567
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for <curt@me.umn.edu>; Thu, 21 Aug 1997 09:19:20 -0500 (CDT)
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for <curt@me.umn.edu>; Thu, 21 Aug 97 07:16:00 -0700 (PDT)
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(Smail-3.1.92 1996-Mar-19 #3 built 1996-Apr-21)
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(Smail3.1.29.1 #5) id m0x1Y1f-0000Qaa; Thu, 21 Aug 97 07:15 PDT
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Received: by fatcity.com (06-Aug-97/v1.0d-b55/bab) via UUCP id 0C90BDD1; Thu, 21 Aug 1997 07:00:54 -0800
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Message-ID: <F001.0C90BDD1.19970821070054@fatcity.com>
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X-Comment: OpenGL Game Developers Mailing List
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X-Sender: steve@mred.bgm.link.com (Steve Baker)
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Organization: Fat City Network Services, San Diego, California
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X-ListServer: v1.0d, build 55; ListGuru (c) 1996-1997 Bruce A. Bergman
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Precedence: bulk
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From: steve@mred.bgm.link.com (Steve Baker)
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Sender: root@fatcity.com
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To: Multiple recipients of list OPENGL-GAMEDEV-L <OPENGL-GAMEDEV-L@fatcity.com>
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Subject: Re: culling with view FRUSTUM before view transformation
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Date: Thu, 21 Aug 1997 07:00:54 -0800
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> OK, this is probably explained in books etc., but what is the fastest way
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> to find the distance between a point and a plane?
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A plane can be represented by the equation
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Ax + By + Cz + D = 0 ;
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A,B,C is just the surface normal of the plane and D is the shortest
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distance from the origin to the plane.
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So, if you need to find the distance of a point from the plane, just
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imagine a new plane that goes through your test point and is parallel
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to the plane you want to test. The plane equation of that new plane would be:
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A'x + B'y + C'z + D' = 0 ;
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Since the two planes are parallel, their surface normals are the same, so
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A' == A
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B' == B
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C' == C
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D' == D + distance_between_the_two_planes
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...the only thing that's different is their D values - which differ by the
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distance of your test point from the original plane.
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So, for a point (x,y,z), the distance from the plane (A,B,C,D) is
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dist = D' - D
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= -A'x - B'y - C'z - D
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= -Ax - By - Cz - D
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= -( [ABC]dot[xyz] + D )
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That's the general result - but culling to the view frustum is a very
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special case...if you are working in eye-relative coordinates (IMHO this
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is best), then since all top,bot,left,right planes of the frustum meet
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at the eye - and since the eye is at the origin (by definition), then
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D is always zero for those plane and that saves you a subtract.
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If you are feeling even more in need of optimisation - then you can save one
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multiply per plane by realising that (for rectangular screens) one of the
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three components of the plane equation will always be zero.
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eg for the LEFT clip plane, the Y component of the normal of the plane
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is zero, so the distance to the left or right plane is just
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- ( A x' + C z' )
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and to the top or bottom plane it's just:
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- ( A x' + B y' )
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Since you are only using this for culling, you don't need the minus sign
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so the cost of a test can be as little as two multiplies and one add
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per plane.
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Steve Baker 817-619-1361 (Vox-Lab)
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Hughes Training Inc. 817-619-8776 (Vox-Office/Vox-Mail)
|
||
|
|
2200 Arlington Downs Road 817-619-4028 (Fax)
|
||
|
|
Arlington, Texas. TX 76005-6171 Steve@MrEd.bgm.link.com (eMail)
|
||
|
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http://www.hti.com http://web2.airmail.net/sjbaker1 (personal)
|
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|
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||
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** Beware of Geeks bearing GIF's. **
|
||
|
|
|
||
|
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||
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--
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Author: Steve Baker
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INET: steve@mred.bgm.link.com
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|
||
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Fat City Network Services -- (619) 538-5030
|
||
|
|
San Diego, California -- Public Internet Access
|
||
|
|
-------------------------------------------------------------------
|
||
|
|
To REMOVE yourself from this mailing list, send an E-Mail message
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to: ListGuru@fatcity.com (note EXACT spelling of 'ListGuru') and in
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the message BODY, include a line containing: UNSUB OPENGL-GAMEDEV-L
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(or the name of mailing list you want to be removed from).
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From fatcity!root@news.cts.com Fri Aug 22 08:51:10 1997
|
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X-VM-v5-Data: ([nil nil nil nil nil nil nil nil nil]
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["2087" "Fri" "22" "August" "1997" "06:31:24" "-0800" "Steve Baker" "steve@mred.bgm.link.com" nil "47" "Re: culling with view FRUSTUM before view transformation" "^From:" nil nil "8" nil nil nil nil nil]
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nil)
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Received: from mailhub.cts.com (mailhub.cts.com [204.216.216.130])
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by meserv.me.umn.edu (8.8.6/8.8.6) with SMTP id IAA21767
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for <curt@me.umn.edu>; Fri, 22 Aug 1997 08:51:09 -0500 (CDT)
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Received: from donews.cts.com(really [192.188.72.21]) by mailhub.cts.com
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via smail with smtp
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id <m0x1u2d-000Wk7a@mailhub.cts.com>
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for <curt@me.umn.edu>; Fri, 22 Aug 97 06:46:19 -0700 (PDT)
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(Smail-3.1.92 1996-Mar-19 #3 built 1996-Apr-21)
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Received: from fatcity by donews.cts.com with uucp
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(Smail3.1.29.1 #5) id m0x1u2a-0000PHa; Fri, 22 Aug 97 06:46 PDT
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Received: by fatcity.com (06-Aug-97/v1.0d-b55/bab) via UUCP id 0C90C1C4; Fri, 22 Aug 1997 06:31:24 -0800
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Message-ID: <F001.0C90C1C4.19970822063124@fatcity.com>
|
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X-Comment: OpenGL Game Developers Mailing List
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X-Sender: steve@mred.bgm.link.com (Steve Baker)
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Reply-To: OPENGL-GAMEDEV-L@fatcity.com
|
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Errors-To: ML-ERRORS@fatcity.com
|
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Mime-Version: 1.0
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Content-Type: text/plain; charset="us-ascii"
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Content-Transfer-Encoding: 7bit
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Organization: Fat City Network Services, San Diego, California
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X-ListServer: v1.0d, build 55; ListGuru (c) 1996-1997 Bruce A. Bergman
|
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Precedence: bulk
|
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From: steve@mred.bgm.link.com (Steve Baker)
|
||
|
|
Sender: root@fatcity.com
|
||
|
|
To: Multiple recipients of list OPENGL-GAMEDEV-L <OPENGL-GAMEDEV-L@fatcity.com>
|
||
|
|
Subject: Re: culling with view FRUSTUM before view transformation
|
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Date: Fri, 22 Aug 1997 06:31:24 -0800
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> > > Actually, I find it easier to scale the whole such that D = 1, then you
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> > > only need to store the non-degenerate (A', B', C') vector (i.e., A' =
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> > > A/D, etc...). Storing a already-normalized (A, B, C) + D quadruple is
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> > > worth it only if you know before hand that the planes will never be
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> > > scaled before their normals are used. In the general case where a
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> > > scaling is possible, you'll have to renormalize anyway.
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> >
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> > Normally, I might agree with you - but in the case of Frustum clipping,
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> > scaling the normal by dividing by 'D' is a bad move since D==0 in the
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> > case of all four 'interesting' planes. :-)
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>
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> Interesting point. This is of course only true when the eyepoint is in
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> the origin, which brings us back to the issue of clipping before or
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> after the modelview transformation.
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Well - that's a dangerous thing to assume - I mean, the eye might just happen
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to move to the true world coordinate origin (by flook) and crash your program
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with a bunch of divide by zero errors.
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In any case, you are going to run into nasty precision problems as your eye
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approaches the origin - even if it doesn't reach it.
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> Learn something new every day.... :-)
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...Forget something important every night... :-)
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Steve Baker 817-619-1361 (Vox-Lab)
|
||
|
|
Hughes Training Inc. 817-619-8776 (Vox-Office/Vox-Mail)
|
||
|
|
2200 Arlington Downs Road 817-619-4028 (Fax)
|
||
|
|
Arlington, Texas. TX 76005-6171 Steve@MrEd.bgm.link.com (eMail)
|
||
|
|
http://www.hti.com http://web2.airmail.net/sjbaker1 (personal)
|
||
|
|
|
||
|
|
** Beware of Geeks bearing GIF's. **
|
||
|
|
|
||
|
|
|
||
|
|
--
|
||
|
|
Author: Steve Baker
|
||
|
|
INET: steve@mred.bgm.link.com
|
||
|
|
|
||
|
|
Fat City Network Services -- (619) 538-5030
|
||
|
|
San Diego, California -- Public Internet Access
|
||
|
|
-------------------------------------------------------------------
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To REMOVE yourself from this mailing list, send an E-Mail message
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to: ListGuru@fatcity.com (note EXACT spelling of 'ListGuru') and in
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the message BODY, include a line containing: UNSUB OPENGL-GAMEDEV-L
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(or the name of mailing list you want to be removed from).
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