half datatype?

[Don Clugston]

On 18/11/12 12:21, Manu wrote:
> I've often wondered about having an official 'half' type.
> It's very common in rendering/image processing, supported by most video
> cards (so compression routines interacting with this type are common),
> and it's also supported in hardware by some cpu's.
>
> ARM for instance supports 'half's in hardware, and GCC has an __fp16
> type which would map nicely if D supported the type in the front end.
>
> The alternative us to use ushort everywhere, which is awkward, because
> it is neither unsigned, nor is it an integer, and it's not typesafe
> (allows direct assignment to ints and stuff)...
> It would be nice if: cast(half)someFloat would yield the proper value,
> even if it is performed in software in most architectures, it could be
> mapped to hardware for those that do it.
>
> It could be done in a library, but then GCC couldn't map it properly to
> the hardware type, and since D has no way to describe implicit casts
> (that I know of?) it becomes awkward to use.
> someFloat = someHalf <- doesn't work, because a cast operator expects an
> explicit cast, even though this is a lossless conversion and should be
> exactly the same as someDouble = someFloat.
>
> Thoughts?

I suspect that what you want in nearly all cases is conversion from

half[] <-> float[]

ie, a pack/unpack operation. I think it would be quite rare to want to 
operate on a single half.

Although it's supported natively on GPUs, for D's purposes it is more 
natural to view it as compressed data.