From -6100606290607151077 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,94e9c2bbb0060b48 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 2002-08-23 03:25:18 PST Path: archiver1.google.com!news1.google.com!newsfeed.stanford.edu!news-spur1.maxwell.syr.edu!news.maxwell.syr.edu!newsfeed-east.nntpserver.com!nntpserver.com!news-out.visi.com!hermes.visi.com!news.alt.net!comp-std-cpp-robomod!not-for-mail From: ros0230@iperbole.bologna.it (Natale Fietta) Newsgroups: comp.std.c++ Subject: Re: Proposals for small vectors? Date: 23 Aug 2002 10:25:18 GMT Organization: NETTuno Lines: 90 Approved: Fergus Henderson , moderator of comp.std.c++ Message-ID: <3d65e786.1110092@news.iperbole.bologna.it> References: <87wurryywb.fsf@grue.ucsd.edu> <3D38FB4F.1020909@animats.com> <877kjnpg3p.fsf@grue.ucsd.edu> <3d4f850b.146537@news.iperbole.bologna.it> Reply-To: ros0230@iperbole.bologna.it Return-Path: X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) Delivered-To: std-c++@ncar.ucar.edu X-Trace: panco.nettuno.it 1030088765 25601 193.207.4.115 (23 Aug 2002 07:46:05 GMT) X-Complaints-To: news@nettuno.it NNTP-Posting-Date: 23 Aug 2002 07:46:05 GMT X-Newsreader: Forte Free Agent 1.21/32.243 Xref: archiver1.google.com comp.std.c++:13355 On Tue, 20 Aug 2002 18:47:50 GMT, "Noah Stein" wrote: >I'm in the same field as you: video games, but I've noticed that my needs, >as well as those of others I know, varies from yours. The world is beautiful because it is so varied :-) Anyhow i was only pointing out my most unusual needs, it is not necessary to bother the C++ communitiy with all little details of my application domain... >I've also used a number of 2x2 and 3x2 matrices. There are a number of >computations that occur in the plane, and I much prefer not to "just ignore >z". A 3x3 matrix now has two roles: a homogeneous 2d transform as well as a >3d transform. I've never used 3x2 matrixes, interesting to know that someone find these useful. i was unsing 2x2 and 3x3 homo 2D in the past, i dont use them anymore, when i do make computations on a plane the plane can be inclinated so i dont want to ignore any coordinate. >however, in a general >n-dimensional setting, writing a version that works with VC6 will definitely >be a chore. Sorry, my english dictionary do not list the word "chore" what is the meaning of the term ? > And I do have use for dense matrices beyond 4x4, so it seems >quite a shame to give up n-dimensionality for just 2d, 3d, and 4d. I dont want to gave up n-dimensionality, i do want a different and more optimized version for some widely used little dimensions. If the optimized version can be made compatible with the more general version this is an added bonus, but i can live without it. >> It is also worth noting that for us 4x4 matrixes represent homogeneous >> coordinates transformations, so they always have the last row (or >> column) as 0,0,0,1; >I know a few people, in the same industry as you, who have found this >assumption to have thrown a giant monkey wrench in their plans. There are a >number of developers who use the perspective row (column). I think as a >general rule, this assumption would be a huge mistake to include. Yes, i was over-simplifiying for the sake of the general discussion, obviously i do also use the perspective row, so i use different multiplication implementation for perspective and non-perspective transformations. This has never caused bugs to me, i do use different matrix classes for perspective and non-perspective transformation, so i do have for example: PerspMatrix operator*(AnimMatrix, PerspMatrix); and the compiler will trigger an error if i erroneously assign the result to an AnimMatrix (i dont have implicit conversions from PerspMatrix to AnimMatrix). >> Also we often use 3-vector to represent normalized 4-vector in >> homogeneous coordinates, so we can multiply a 3-vector by a 4x4 matrix >> and obtain another 3-vector as result. (i suppose this sound very odd >> to mathematicians). > >This is another dangerous idea IMHO. Whether a vector represents a vector >or a point makes a big difference in its handling with a homogeneous matrix. >I would like to avoid jumping through hoops to deal with the issues of "is >this vector actually a point?". That would be a giant source of bugs. Yes, i do know, i use different "vector" classes for points and normals and versors. as for the previous issue i dont have mentioned it because to me it seem not significative in the general discussion. >> Probably my (ours ?) needs are too specific to be adressed by the >> standard C++ library... > >I don't think your (our) needs are too specific. I think, though, that to >have those needs met requires a significant amount of thoughtful design, not >an insurmountable amount. What do you think of tensorial formalism ? I am learning it, and i am only half-convinced that it can be useful to implement the library i dream of. Regards, Natale Fietta --- [ comp.std.c++ is moderated. 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