Fix AudioStreamRandomizer.random_volume_offset_db not working

[jsjtxietian]

Fixes #82469

Note：I did not quite obey the rule of OOP here but I think it's fine. Add a dedicated virtual method for getting the volume seems a little overkill.

[rakkarage]

get_volume_variantion_range = get_volume_variation_range? :)

[ellenhp]

Looks good overall, just a naming suggestion. I feel kind of silly that I didn't hook that up originally but I'm glad someone noticed and fixed it 😅

[ellenhp]

Couldn't we just apply the volume shift here[1]? Leave the code that sets it where it is and just multiply the values returned by playing->mix by volume_scale

[1] https://github.com/godotengine/godot/blob/master/servers/audio/audio_stream.cpp#L811C3-L811C3

[jsjtxietian]

multiply the values returned by playing->mix by volume_scale

I tried, it didn't work out. The output sound is glitch.

[ellenhp]

I tried, it didn't work out. The output sound is glitch.

Just to confirm, what I was suggesting it to save the value of volume_scale that you're calculating here into a field of the playback object. Once it's set, it won't be updated ever again. If you're re-calculating the value each time _mix is called, that's going to cause some really glitchy audio. But calculating it once when the playback is started, and then re-using it each time _mix is called will work. I don't want to approve this with the cast_to because I don't want to special-case the randomizer stream.

[jsjtxietian]

@ellenhp Sorry for my misunderstanding and thank you for your suggestion.
What I mean is if I only add volume_scale * to this mix function like below (only change here without all my other changes), the sound is weird. But I might be wrong.

int AudioStreamPlaybackRandomizer::mix(AudioFrame *p_buffer, float p_rate_scale, int p_frames) {
	if (playing.is_valid()) {
		return volume_scale * playing->mix(p_buffer, p_rate_scale * pitch_scale, p_frames);
	} else {
		for (int i = 0; i < p_frames; i++) {
			p_buffer[i] = AudioFrame(0, 0);
		}
		return p_frames;
	}
}

And regarding special-case the randomizer stream, I do agree we should try our best to avoid cast-to because it's not clean and oop, also make the code hard to debug sometimes. I'm not familiar to the audio code but it seems to me _get_volume_vector is where we set the volume. I hope we can find the right place to insert some code and fix this problem :)

[ellenhp]

Ah! Okay, I see now. Can you try doing something more like this:

......
	if (playing.is_valid()) {
		int mixed_samples = playing->mix(p_buffer, p_rate_scale * pitch_scale, p_frames);
		for (int samp = 0; samp < mixed_samples; samp++) {
			p_buffer[samp] *= volume_scale;
		}
		return mixed_samples;
	} else {
......

The mix function returns an integer representing the number of samples it actually mixed into the provided buffer, and the samples themselves are written directly to the array provided in p_buffer. So to multiply the samples by volume_scale you have to go through that array and perform the multiplication for each one.

[jsjtxietian]

Tested locally, it worked！

[akien-mga]

Thanks!