[1/3] dsound: Add support for 32-bit IEEE float buffers

[Henri Verbeet]

On 14 September 2010 03:00, Chris Robinson <chris.kcat at gmail.com> wrote:
> On Monday, September 13, 2010 12:55:43 pm Owen Rudge wrote:
>> +        if (*v < -1.0f) d = -128;
>> +        if (*v <  0.0f) d = *v * 128.0f;
>> +        if (*v >= 0.0f) d = *v * 127.0f;
>> +        if (*v >  1.0f) d = 127;
>
> The way these checks are written out can cause problems, particularly with the
> first two because the if(*v < 0.0f) occurs after the if(*v < -1.0f) check, so
> it won't properly clamp values below -1.0. Something like this:
>
>        if (*v < -1.0f) d = -128;
>        else if (*v <  0.0f) d = *v * 128.0f;
>        else if (*v >  1.0f) d = 127;
>        else if (*v >= 0.0f) d = *v * 127.0f;
>
> should do fine (along with the 16- and 32-bit variants; the 24-bit one looks
> fine). You also don't need to keep the float as a pointer, though I imagine
> the optimizer can handle that.
>
Is that correct in the first place though? This will produce a
non-uniform mapping for positive and negative values. IIRC the common
way to do these kinds of mappings is "((x * (2^n - 1)) - 1) / 2",
which would be "x * 127.5f - .5f" for an 8 bit signed format. The main
disadvantage of that method is that integer 0 doesn't correspond
exactly to floating-point 0.0f, but in practice few people seem to
care. Of course it all depends on how the target format is actually
defined.