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1. TC SetTemporalCoherence 3 Setup object callbacks or pointers Geometry and topology are not stored in the collision system in order to save some ram So when the system needs them to perform accurate intersection tests you re requested to provide the triangle vertices corresponding to a given face index This can be done through a callback static void ColCallback udword triangleindex VertexPointers amp triangle udword user_data Get back MeshO or Mesh you also can use 2 different callbacks Mesh MyMesh Mesh user_data Get correct triangle in the app controlled database const Triangle Tri MyMesh gt GetTriangle triangleindex Setup pointers to vertices for the collision system triangle Vertex 0 MyMesh gt GetVertex Tri gt mV Ref 0 triangle Vertex 1 MyMesh gt GetVertex Tri gt mVRef 1 triangle Vertex 2 MyMesh gt GetVertex Tri gt mV Ref 2 Setup callbacks TC SetCallbackO ColCallback udword Mesh0 TC SetCallback 1 ColCallback udword Mesh1 Of course you should make this callback as fast as possible And you re also not supposed to modify the geometry after the collision trees have been built The alternative was to store the geometry and topology in the collision system as well as in RAPID but we have found this approach to waste a lot of ram in many cases Since version 1 2 you can also use plain pointers It s a tiny bit faster but not as safe TC
2. 4 Perform a collision query static udword Cache bool IsOk RC Collide WorldRay OpcodeModel MeshWorldMatrix amp Cache Stabbed faces will be found in the user provided CollisionFaces array II 5 AABB vs mesh collision queries Introduced in OPCODE 1 2 Corresponding collider AABBCollider Brief description Box B Axis Aligned Bounding Box Mesh M Returns triangles from M touching or included in B Useful for rigid body simulation Steps 1 Build an OPCODE_Model using a creation structure 2 Create a box collider and setup it AABBCollider AC AC SetFirstContact AC SetTemporalCoherence 3 Setup object callback or pointers see the Mesh mesh case for more infos AC SetCallback AC SetPointers 4 Perform a collision query static AABBCache Cache bool IsOk AC Collide Cache LocalAABB OpcodeModel The contact status is given by BOOL Status AC GetContactStatusQ List of touched primitives is given by udword NbTouchedPrimitives AC GetNbTouchedPrimitives udword TouchedPrimitives AC GetTouchedPrimitives I 6 OBB vs mesh collision queries Introduced in OPCODE 1 2 Corresponding collider OBBCollider Brief description Box B Oriented Bounding Box Mesh M Returns triangles from M touching or included in B Useful for rigid body simulation Steps 1 Build an OPCODE_Model using a creation structure 2 Crea
3. The ODE library and its TriangleCollider contribution Q There is a check against exception handling if defined _CPPUNWIND what is the reason for that I have commented it out but wonder if that might break anything A No reason it s just that OPCODE uses some low level libraries of mine basically my engine and at some point I added the check because in my engine I wanted to disable exceptions As far as OPCODE is concerned you can put them back Q Some precision problems appeared in the SphereCollider when a sphere was collided against really big triangles A It happens when triangles are really too big It has been reported that using doubles instead of floats in the sphere triangle overlap test helped Tesselating your big triangles a bit is probably a better solution Q How do I deformable models Dynamic trees A You don t Seriously OPCODE is not very well suited to dynamic trees There are two different trees to begin with the source tree and the optimized tree Updating the former is relatively easy But updating the latter doesn t sound very exciting as it means re quantizing etc It can work but it probably won t be efficient Two notes if you need it to be fast say for deformable objects like cloth the best way is to update the source tree in the same way as Gino van den Bergen did in SOLID 2 0 or Thomas Larsson in his articles Check out his code it s pretty easy Then maybe rebuilding th
4. Status PC GetContactStatus List of touched primitives is given by udword NbTouchedPrimitives PC GetNbTouchedPrimitives udword TouchedPrimitives PC GetTouchedPrimitives IV Caches Caches are per couple entities The client is responsible for managing them In theory there should be one different cache for each possible couple of colliding entities where a couple can be a given sphere vs a given mesh or a given mesh vs another mesh In practice this is only true when temporal coherence is enabled If it s not just use a single cache structure filled with relevant data before each collision query When temporal coherence is enabled and caches are needed they re mainly filled with data computed during a collision query Those data are then reused in subsequent queries to possibly find collision results faster Cached data are query dependent i e they re not the same for sphere mesh or mesh mesh queries That s why you have SphereCache AABBCache etc V FAQ Q Am I allowed to use OPCODE in a commercial program A Yes The library is free No GPL or LGPL you can do whatever you want with the code I would appreciate it if you write somewhere in your product that you re using OPCODE but that s not mandatory I would also appreciate a notification by mail just to know how people are using the lib Q Why callbacks Why not simply pointers A Since OPCODE 1 2 you can select callbacks or pointers at comp
5. Temporal coherence in that case is implemented by caching the one and only previously touched primitive Then before everything else the cached primitive is tested for overlap in subsequent frames If it s still overlapping we can return immediately without doing the actual query which saves a lot of time Else we do a normal query and cache the newly touched primitive if it exists for next frames In All Contacts queries temporal coherence currently only works for several volume queries Sphere vs mesh and AABB vs mesh The idea is to use a larger fat volume for collision queries and compare normal ones against previously used fat volumes If the normal volume is included in the fat one we simply return the previous list of collided primitives This is a conservative test some returned primitives won t actually overlap the normal volume Yet it provides O 1 queries most of the time and can be very useful II 2 Mesh vs mesh collision queries Introduced in OPCODE 1 0 Corresponding collider AABBTreeCollider Brief description Mesh P Mesh Q Returns overlapping triangles TPG TQ i where TP belongs to P and TQ i belongs to Q Useful for virtual prototyping rigid body and mechanical simulation Steps 1 Build an OPCODE_Model using a creation structure 2 Create a tree collider and setup it AABBTreeCollider TC TC SetFirstContact TC SetFullBoxBoxTest TC SetFullPrimBoxTest
6. creation structure 2 Create a sphere collider and setup it SphereCollider SC SC SetFirstContact SC SetTemporalCoherence 3 Setup object callback or pointers see the Mesh mesh case for more infos SC SetCallback SC SetPointers 4 Perform a collision query static SphereCache Cache bool IsOk SC Collide Cache LocalSphere OpcodeModel Sphere WorldMatrix MeshWorldMatrix The contact status is given by BOOL Status SC GetContactStatus List of touched primitives is given by udword NbTouchedPrimitives SC GetNbTouchedPrimitives udword TouchedPrimitives SC GetTouchedPrimitives II 4 Ray vs mesh collision queries Introduced in OPCODE 1 1 Corresponding collider RayCollider Brief description Ray R Mesh M Returns triangles from M stabbed by R Useful for object picking raytracing shadow feelers in out tests sweep tests in rigid body simulation Steps 1 Build an OPCODE_Model using a creation structure 2 Create a ray collider and setup it RayCollider RC RC SetFirstContact RC SetTemporalCoherence RC SetClosestHitc RC SetCulling RC SetMaxDist CollisionFaces CF RC SetDestination amp CF Please refer to the comments in OPC_RayCollider cpp for more infos 3 Setup object callback or pointers see the Mesh mesh case for more infos RC SetCallback RC SetPointers
7. optimized trees III Colliders and collision queries III 1 Preliminaries Each collision query is handled by a different Collider You only need one Collider for each type of query not one per model So for example in your application you only need one SphereCollider to collide any sphere you want against any mesh you want Per model data are captured in caches in our example in a SphereCache not in Colliders Caches are briefly described in part IV Collision queries can be of two kinds First Contact and All Contacts In First Contact mode the query ends as soon as overlap is detected i e as soon as the first contact is found In All Contacts the query continues until all overlapping primitives have been found Collision queries can also use or not use temporal coherence When you perform successive collision queries say each frame using for example a sphere against a mesh most of the time the list of returned faces for frame N will be very similar to the previously returned list for frame N 1 To the limit the two lists will be the same This is called temporal coherence OPCODE takes advantage of temporal coherence in two different ways for First Contact queries for All Contacts queries In First Contact queries clients are only interested in a boolean answer does X overlap Y yes or no They usually don t care about the exact list of overlapping entities else they would have selected All Contacts
8. OPCODE User Manual Pierre Terdiman Last update august 18 2002 I Introduction OPCODE is a collision detection library freely available here ww codercorner com Opcode htm This is the user manual teaching you how to perform supported queries Mesh mesh Sphere mesh Ray mesh AABB mesh OBB mesh Planes mesh We call mesh a surface made of vertices and triangles A ray can be a segment or a half line not a line An AABB is an Axis Aligned Bounding Box An OBB is an Oriented Bounding Box What you need to do collision queries A collision structure a k a collision tree for each mesh A collider for each type of queries Various caches We ll describe these things successively in the rest of the document Frequently asked questions have been appended at the end II Collision trees II 1 Source trees and optimized trees To perform fast collision queries on a geometric surface you need to create a corresponding collision structure or collision tree for each surface mesh OPCODE creates collision trees in two passes first it creates a generic source tree discarded in the end then it transforms the source tree creating an optimized tree used in collision queries Four different optimized trees are supported Normal trees 2 N 1 nodes full size No leaf trees N 1 nodes full size Quantized trees 2 N 1 nodes half size Quantized no leaf trees N 1 nodes half size All of t
9. SetPointersO Mesh0 gt GetFaces Mesh0O gt GetVerts TC SetPointers Mesh1 gt GetFaces Mesh1 gt GetVerts You should setup callbacks or pointers each time before doing the collision query That way you can use the same collider for all meshes 4 Perform a collision query Setup cache static BVTCache ColCache ColCache Model0 amp Model0 ColCache Modell amp Modell Collision query bool IsOk TC Collide ColCache World0O World1 WorldO and World are world matrices for Model0 and Modell If a world matrix is identity you can pass a null pointer as a parameter instead of the matrix Then internally the library will use a special code path with less transforms and less computations Returned bool just says everything was ok It s not the collision status which is Get collision status gt if true objects overlap BOOL Status TC GetContactStatus If objects overlap you can further get the colliding pairs using Number of colliding pairs and list of pairs udword NbPairs TC GetNbPairs const Pair CollidingPairs TC GetPairs II 3 Sphere vs mesh collision queries Introduced in OPCODE 1 1 Corresponding collider SphereCollider Brief description Sphere S Mesh M Returns triangles from M touching or included in S Useful for camera vs world collision rigid body simulation finding faces lit by a point light Steps 1 Build an OPCODE_Model using a
10. e optimized tree from there is an option Else you can also use the source tree directly after all that s what SOLID does But then use SOLID not OPCODE if you don t need it to be fast say it only happens from time to time maybe you can simply discard the old trees and rebuild everything from scratch with the new geometry Q How do I do LSS vs mesh queries A There s no direct LSS query since LSS vs box is too slow Instead I use a box query gather touched faces then do a LSS triangle collision response on that But this is done out of OPCODE since it s not directly relevant to the lib Actually I may even be using a plane query instead of a box query It s usually faster yet more conservative Side note LSS Line Swept Sphere a capsule Q What are future features A have many things on my TODO list already implementing All Contacts temporal coherence for other volumes OBB maybe planes or ray moving some AABBTree queries from another part of my code to OPCODE queries on vanilla AABB trees not only on optimized trees distance queries already working but really too slow bottom up tree building implementing tree coherence as in GPG2 trying the P3 s prefetch instructions to speed up recursive queries doing some OpenGL samples better docs tutorials trying a huge list of remaining tricks to compress the trees better etc
11. ee OPCODE 1 0 vs RAPID Other volumes k Dops etc are interesting theoretically but practically they re not worth it So AABBs are the best compromise I think Q What about SIMD optimizations A Sure gimme a P3 Q Have you thought about adding an option for building the AABB trees bottom up A Yes and I think it would provide better trees and faster queries But I haven t had the time to play with them so far Later maybe In any case that s definitely something to test Q If you re using OPCODE in physics do you have a neat solution for turning intersecting triangle pairs into contact points normals for use in the physics A No I don t have a neat solution Q Did you read the article about compressed AABB trees in Game Programming Gems 2 A Yes and it s very nice It takes advantage of what I used to call tree coherence even if they don t name it that way It was planned for OPCODE at first but I would have to use Min Max boxes whereas I currently use Center Extents ones so I need to rewrite many things here and right now I can t The trees in GPG2 are more compressed than the ones in OPCODE I don t know how they behave as far as speed is concerned since there s no ready to use source code provided in the book Q I would like to port it to Linux any suggestions A Several people have done it and reported the port to be easy Two places to check The Flat Four engine http www 379 com f4
12. hem are exposed to clients through an OPCODE_Model II 2 Building an OPCODE_Model An OPCODE_Model is a wrapper for collision trees and various additional data This is the main interface between the user and the library You need to create one OPCODE_Model for each master mesh in your scene If your engine supports instancing and your master mesh has several instances you only need to create an OPCODE_Model for the master mesh Instances will have their own transform matrix that will be used later with the master OPCODE_Model to perform correct collision queries For now here s our OPCODE_Model OPCODE_Model Sample Before using it for collision queries you need to build initialize it using an OPCODECREATE creation structure 1 Initialize the creation structure OPCODECREATE OPCC Surface data OPCC NbTris OPCC NbVerts OPCC Tris OPCC Verts ae Tree building settings OPCC Rules OPCC NoLeaf OPCC Quantized oa Debug OPCC KeepOriginal ve 2 Build the model bool Status Sample Build OPCC The creation structure is temporary and can be discarded after the OPCODE_Model has been built Let s have a closer look at the creation structure and what it contains We can split the structure members in three parts Surface data Tree building settings and Debug Surface data The first members describe the surface i e the mesh NbTris a number of triangles MUST be greater than 1 NbVerts a number of ver
13. ile time At first callbacks have been used in sake of flexibility not to force clients to use the same geometry topology as I do With pointers clients must use indexed triangle lists with 32 bits indices With callbacks they can use 16 bits indices in the background or non indexed geometry provided they feed OPCODE with VertexPointers in the end Q Why not an extra callback to get triangle normals instead of computing them on the fly in overlap tests A Simply because that s the way I did it in the first place Since the goal was to save memory keeping an array of normals around didn t seem like a good idea Now an extra callback or extra pointers could be added but gains would be minimal if any so I probably won t bother But you can try it if you want and share your results with everyone Q How does OPCODE perform compared to other libs A Well it s always difficult to say since most of the time the same library is not always the fastest I assume we re speaking about speed here not memory usage In any case I d say it performs well as it seems to be overall faster than SOLID 2 0 RAPID and ColDet I didn t try against other libs QuickCD PQP Swift etc Give it a try and tell me Q Why AABB trees A Because they re easy to deal with efficient and memory friendly Spheres are more memory friendly but really too coarse and sphere trees are painful to build OBBs need more memory and are not that faster s
14. te a box collider and setup it OBBCollider OC OC SetFirstContact OC SetTemporalCoherence OC SetFullBoxBoxTest 3 Setup object callback or pointers see the Mesh mesh case for more infos OC SetCallback OC SetPointers 4 Perform a collision query static OBBCache Cache bool IsOk OC Collide Cache LocalOBB OpcodeModel BoxWorldMatrix MeshWorldMatrix The contact status is given by BOOL Status OC GetContactStatusQ List of touched primitives is given by udword NbTouchedPrimitives OC GetNbTouchedPrimitives udword TouchedPrimitives OC GetTouchedPrimitives II 7 Planes vs mesh collision queries Introduced in OPCODE 1 2 Corresponding collider PlanesCollider Brief description Planes P Mesh M Returns triangles from M enclosed by P on the positive sides of Ps Useful for view frustum culling mainly for software rendering projective texturing coarse OBB or LSS queries Steps 1 Build an OPCODE_Model using a creation structure 2 Create a planes collider and setup it PlanesCollider PC PC SetFirstContact PC SetTemporalCoherence 3 Setup object callback or pointers see the Mesh mesh case for more infos PC SetCallback PC SetPointers 4 Perform a collision query PlanesCache Cache bool IsOk PC Collide Cache Planes NbPlanes OpcodeModel MeshWorldMatrix The contact status is given by BOOL
15. tices Since we provide both i e since NbVerts NbTris 3 it means we re dealing with an indexed surface That s why we further provide Tris an array of triangle indices Verts an array of vertices This is fairly standard and similar to the way indexed primitives are defined in Direct3D The only difference is that we expect 32 bits indices here So you can t directly use the indices from your index buffers here since they re usually 16 bits Tree building settings The surface data source will be transformed to create the OPCODE_Model The transform has some parameters defined here Rules splitting rules This is a bitmask actually a combination of SplittingRules flags defined in OPC_TreeBuilders h They define the way splitting will be performed while creating the trees Please refer to the aforementioned file for more info In OPCODE the SPLIT_COMPLETE flag is mandatory since we only support complete optimized trees Hence the usual best splitting rules are the following Rules SPLIT_COMPLETE SPLIT_SPLATTERPOINTS SPLIT_GEOMCENTER Finally NoLeaf Quantized Those two flags define the type of tree used for the OPCODE_Model You ll have to stay consistent there since mesh mesh queries require the models to be of similar type Debug The last parameter KeepOriginal is here for debug purpose and you shouldn t care about it In short it keeps an internal copy of the source tree used to build

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