Quote:Original post by benryves
I cannot "learn" maths. I have a real mental block when it comes to it. I can add, subtract, multiply and divide and push the required buttons on a calculator - but don't ask me to do anything more advanced!

Well, you're going to need to learn sometime if you want to get anywhere far in 3D graphics, I'm not saying you have to learn from a book but you'll need to acquire the knowledge somehow. If that somehow means asking lots of questions on the GDNet forums then so be it, there will be plenty of people here who can help.

To answer your original question, yes, you can just rotate the normal. This is typically the way most APIs do things as well - the worst case is that the normal will be un-normalised if you've scaled it (when you look up matrix maths you'll understand how this might be possible) but obviously it's solvable by renormalising the vector.

-Mezz

Yes, maths is hard. I don't make any bones about this, and neither would most people - that's why they don't teach linear algebra to 5 year olds! It took me ages when I first started to get to grips with vectors, matrices and all the other shit that comes with 3D programming. I spent quite a bit of money and time reading books or papers on the internet, only to find that they didn't really explain things in a way which helped me understand them (I also have a problem learning new things from books... it's ok when I have some background in the field as I do now, but when I started it was impossible).

I also did fairly poorly in the advanced modules at A level maths too, although I got an A overall. You don't need to be a god of the pure maths side of things for 3D programming, it's actually more oriented on physics, mechanics and spatial transformation stuff. Don't expect to be integrating the inverse logarithm of the hyperbolic cosine of x much in this field.

That said, if you have a big issue learning from books or whatever, I don't know where I can direct you to learn this stuff, all I can say is that after many books, papers and internet sites I only found one which explained it in a way I understood, and it was infact Brian Hook who recommended it.

Computer Graphics: Mathematical First Steps ISBN: 0135995728

I hate sounding corny, but maybe it'll help you the way it helped me :)

-Mezz

Yeah, it is hard. I'm like you Benryves. I have real trouble with math. Just about passed it at school with a grade C, BUT, discovered at college that learning math (or any subject for that matter) is about the quality of the teacher. I had a math teacher at college (Mr Newton was his name believe it or not) that had the ability to make the subject realtively easy to understand. Many years later I now face the same issue again. I want to improve my 3D programming, but the math just numbs my brain. Got to find the right teacher again! Learning this stuff from books and online docs just doesnt work for me either - and that approach is the longer, more drawn out one also. Saying that though, I might still pick up the book Mezz mentioned though :-) Keep at it man. There's plenty of us out here in the same boat, and places like gamedev.net are excellent for receiving help and support when trying to get your head around something!

Note that you actually need to multiply the normals by the inverse transpose of the matrix used to transform the model vertexes though in practice if all your matrix does is rotations and translations taking the inverse transpose of it is a no-op so you can use the matrix directly.

Quote:Quote:Original post by Emmanuel Deloget

Quote:MxV (9 muls) is faster than U^V (3 muls) ? I'm pretty sure it is not the case :)

If you're talking about the cross product when using ^ (which is what you use to recalculate normals) then that generally requires six multiplies and three subtractions, not three muls. It also requires normalisation, though you need that with the matrix multiply as well if you allow non uniform scaling.

apart from the fact that it requires 6 muls (and not 3 like a dot product), this would only work for face normals, if you want vertex normals, you need much more work than that, like adding together the face normals of all the faces touching that vertex, with a final renormalization required per vertex (6 muls and one div)
matrices are definitely better, and the only place where re-computing the normals might have a slight advantage would be special cases anyway...