These constants are not part of the standard. We instead use our own
constexpr definitions in the filament::math namespace, as part of
the scalar.h include.
the C++ standard says:
if T is a class type with a default constructor that is neither
user-provided nor deleted (that is, it may be a class with an
implicitly-defined or defaulted default constructor), the object
is zero-initialized and then it is default-initialized if it has a
non-trivial default constructor
Unfortunately, MSVC always calls the default constructor, even if it
is trivial, which breaks constexpr-ness.
To workaround this, we're always zero-initializing TVecN<>
Also removed constexpr from default constructors, since they never can
be constexpr as they're not initializing the vector.
It used to be that operations e.g. like:
float3{} + double{} would be computed as
float3{} + float3{double{}} instead of
float3{} + double3{double{}}
I other words, when an implicit conversion was involved on the right
it would be converted to the left side’s type, possibly losing
precision.
Another problem was that swiping the operands could produce different
Results, e.g.:
float3{1} * 5.0 -> float3{5.0f}
5.0 * float3{1} -> double3{5.0}
This is no longer the case, now both expressions would return a double3.
Note:
float3 r{};
r *= 5;
Is now equivalent to:
r[0] *= 5;
r[1] *= 5;
r[2] *= 5;
Instead of before:
r[0] *= 5.0f;
r[1] *= 5.0f;
r[2] *= 5.0f;
It turns out that most of libmath couldn't be used in constexpr
expression due to our use of union{}. The C++ standard requires that
all accesses to a union{} in a constexpr expression be the same
element.
Also because libm and cmath are not constexpr some functions such
as length() or normalize() can't be constexpr. The same is true for
anything needing things like sqrt, cos, sin, ceil, floor.
This change mainly does the following:
- replace all accesses to vector elements by operator[]
(this ensure all of libmath uses the same union element)
- avoid use of std::min / std::max / std::abs
- avoid uninitialized variables, which can't be constexpr
- remove 'constexpr' keyword on functions that can never be
It is now possible to write things like:
constexpr mat4f I = inverse(
transpose(mat4f::translation(float3{ 1, 2, 3 })
* mat4f::scaling(4)));
