There are 2 disadvantages:
- additional calling overhead since D3DX functions are not inlined
- not portable to other platforms except DirectX and Xbox360
There a few other aspects to consider:
- With C++ math code, performance shouldn't get into the way of convenience. For instance, a proper operator+() always costs some performance because a temporary object must be constructed (the return value). But it's much more convenient and readable to use C++ operator overloading in generic game code instead of using (for instance) intrinsics. The point is to pay special attention to inner loops and use lower level code there when it actually makes sense.
- There should only be very few places in Nebula where heavy math code is actually executed on the CPU (in Nebula2 these are: particle systems, animation code, computing shadow caster geometry for skinned characters). In Nebula3 these tasks are either offloaded to the GPU, or will be obsolete). In general, the CPU should NEVER touch geometry per-frame and per-vertex.
Another basic change I wanted to do for some time was to differentiate between points and vectors. There is now a Math::point and a Math::vector class which both derive from the generic Math::float4 class. A point describes a position in 3d space and a vector describes a direction and length in 3d space, generalized to homogeneous 4d space (the w component of a point is always 1.0, the w component of a vector is always 0.0). By creating 2 different classes with the right operator overloading one can encode the computation rules for points and vectors right into the C++ code, so that the compiler throws an error when the rules are violated:
- (point + point) is illegal
- (point * scalar) is illegal
- point = point + vector
- vector = point - point
- vector = vector + vector
- vector = vector - vector
- vector = vector * scalar
So the new math lib basically looks like this:
The following low level classes directly call D3DX functions:
* matrix44 (D3DXMatrix functions)
* float4 (D3DXVec4 functions)
* quaternion (D3DXQuaternion functions)
* plane (D3DXPlane functions)
All other classes (like bbox, sphere, line, etc...) are generic and use functionality provided by the above low level classes. There is also a new scalar type (which is just a typedef'ed float), which helps in porting to some platforms (for instance, on NintendoDS, all math code is fixed point, so a scalar would be typedef'ed from one of the fixed point types). I still have to write a complete set of test and benchmark classes for the math library, but for now I'm quite happy that a very big chunk of work has been reduced to about 2 days of implementation time.