[openal] Programmatic access to sounds played through OpenAL

[Aleksei Petrenko]

Hi Chris,

this is fantastic! Exactly the kind of advice I was looking for.
We'll try to make this work, I'll let you know how it goes!


Best regards,
Aleksei

On Sat, Jul 18, 2020 at 3:30 AM Chris Robinson <chris.kcat at gmail.com> wrote:

> On Sat, 18 Jul 2020 01:58:04 -0700
> Aleksei Petrenko <petrenko at usc.edu> wrote:
>
> > My question is, is it possible to get programmatic access to the
> > sounds played during every particular frame of the game?
> >
> > An example to be more concrete. Let's say the normal framerate of the
> > game is 60FPS. I am running the game 10 times faster than realtime,
> > so at 600FPS (10x is not important, I want to run the game as fast as
> > hardware allows, the exact speedup is not known). At some point, a
> > game event triggers a sound that under normal circumstances would
> > play for 1 second, so for 60 game ticks. In my fast simulation it
> > only lasts 0.1 seconds, and the same 60 game ticks.
>
> Hi.
>
> With OpenAL Soft's ALC_SOFT_loopback extension, it should be possible
> to do this, as long as you're able to modify your program's source. If
> the ALC_SOFT_loopback extension is supported, you can get the
> alcLoopbackOpenDeviceSOFT function (with alcGetProcAddress), and create
> an ALCdevice using that. Specify the render format you want when calling
> alcCreateContext, and the normal AL functions can be used as normal when
> the context is made current.
>
> At that point, you're in control of when and how fast samples are
> mixed. Call alcRenderSamplesSOFT to render samples and fill in your
> provided buffer. Ideally you'd render ~20ms worth of samples per call
> (512 to 1024 sample frames at 44100hz), but any size can work and it
> can be called as often as your CPU allows. So for example:
>
> #include "alext.h"
>
> LPALCLOOPBACKOPENDEVICESOFT alcLoopbackOpenDeviceSOFT;
> LPALCRENDERSAMPLESSOFT alcRenderSamplesSOFT;
>
> ...sound initialization...
> if(!alcIsExtensionPresent(NULL, "ALC_SOFT_loopback"))
> {
>     error("Loopback not supported");
>     abort();
> }
>
> alcLoopbackOpenDeviceSOFT = alcGetProcAddress(NULL,
>     "alcLoopbackOpenDeviceSOFT");
> alcRenderSamplesSOFT = alcGetProcAddress(NULL, "alcRenderSamplesSOFT");
>
> ALCdevice *device = alcLoopbackOpenDeviceSOFT(NULL);
> ALCint attrs[] = {
>     /* Standard 16-bit stereo 44.1khz. Can change as desired. */
>     ALC_FORMAT_TYPE_SOFT, ALC_SHORT_SOFT,
>     ALC_FORMAT_CHANNELS_SOFT, ALC_STEREO_SOFT,
>     ALC_FREQUENCY, 44100,
>
>     /* end-of-list */
>     0
> };
> ALCcontext *context = alcCreateContext(device, attrs);
> alcMakeContextCurrent(context);
>
> ...in main loop...
>     ..make normal AL update calls..
>
>     /* Ensure 'buffer' can hold 1024 sample frames when calling (4096
>      * bytes for 16-bit stereo). */
>     alcRenderSamplesSOFT(device, buffer, 1024);
>
>     ..1024 sample frames are now in 'buffer'..
> ...
>
> The above main loop can run faster than real-time. An AL call made after
> rendering 44100 sample frames would act as if it was made after one
> second of playback. If all the resulting samples are concatenated and
> played back at normal speed, it would sound indistinguishable from
> normal device playback. See
> <
> https://urldefense.com/v3/__https://openal-soft.org/openal-extensions/SOFT_loopback.txt__;!!LIr3w8kk_Xxm!83zmI7TW0wMP1j4irCuIr_UIc3DS4dOu99HuEnONDlUCVywFLZHZZskL-XP7CqY$
> > for more
> information about the extension.
>
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