[sldev] Blowing wind

[Philippe (Merov) Bossut]

Hi Aimee,

I haven't looked in the code, just assumed you copied the relevant parts :)

It seems to me that the intent of the code is to write in "newbuffer" by
packets of "sizeof(MIXBUFFERFORMAT_T) == stride" bytes using "cursamplep" as
a sliding pointer. That said then, having "stride == 4" when
"MIXBUFFERFORMAT_T" is defined as S16 seems wrong.

So the question is: why is "stride" set independently of
"sizeof(MIXBUFFERFORMAT_T)"? Are there any situation where those 2 things
need to be different? Looking at how "stride" is set should give you a hint.

Cheers,
- Merov

On Tue, Dec 8, 2009 at 11:53 AM, Aimee Trescothick <
aimee.trescothick at gmail.com> wrote:

> Can anyone explain how the below piece of the existing code from the wind
> noise synthesizer has ever actually worked properly, with
> MIXBUFFERFORMAT_T defined as S16 and stride set to 4 bytes?
>
>     U8 *cursamplep = (U8*)newbuffer;
> ...
>
>     MIXBUFFERFORMAT_T sample;
> ...
>
>   sample = llfloor(((F32)nextSample*32768.f*(1.0f -
> mCurrentPanGainR))+0.5f);
>     *(MIXBUFFERFORMAT_T*)cursamplep = llclamp(sample,
> (MIXBUFFERFORMAT_T)-32768,
>
>  (MIXBUFFERFORMAT_T)32767);
>     cursamplep += stride;
>
>     sample = llfloor(((F32)nextSample*32768.f*mCurrentPanGainR)+0.5f);
>     *(MIXBUFFERFORMAT_T*)cursamplep = llclamp(sample,
> (MIXBUFFERFORMAT_T)-32768,
>
>  (MIXBUFFERFORMAT_T)32767);
>     cursamplep += stride;
>
> On Windows and Linux MIXBUFFERFORMAT_T is defined as S16, and if the FMOD
> mixer mode is FSOUND_MIXER_QUALITY_FPU or FSOUND_MIXER_BLENDMODE, stride is
> set to 4 bytes, as these modes both use a 32-bit mix buffer.
>
> But as far as I can see, that's just going to write signed 16-bit integers
> into the first two bytes of the 4-byte sample, skipping over the other two
> bytes, leaving them at whatever they were before (zero, as wind is the first
> thing that goes in the buffer after it is cleared).  When FMOD tries to read
> the 4-bytes back as either 32-bit signed integers (FSOUND_MIXER_BLENDMODE), or 32-bit
> float (FSOUND_MIXER_QUALITY_FPU) that's just going to go horribly wrong.
>
> MIXBUFFERFORMAT_T is only defined as S32 on the Mac, on Windows and Linux
> it is S16 as one of FMOD's 16-bit MMX mixers is normally used. The only
> time I would imagine this situation occurs is on a PC without MMX (or when
> LL_VALGRIND is defined on Linux for testing, which specifically forces the
> mixer mode to FSOUND_MIXER_QUALITY_FPU).
>
> If this has never worked anyway, it would seem to make sense to turn off
> the wind generator for these mixer modes to avoid making a horrible noise,
> drop the striding which will simplify things elsewhere, and just declare
> cursamplep as MIXBUFFERFORMAT_T in the first place, removing the need to
> typecast it ... or am I missing something?
>
> (If you followed me this far you're doing better than I am :)
>
> Aimee.
>
>
>
>
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