godot/servers/audio/audio_stream.cpp

/*************************************************************************/
/*  audio_stream.cpp                                                     */
/*************************************************************************/
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/*                           GODOT ENGINE                                */
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/* Copyright (c) 2007-2017 Juan Linietsky, Ariel Manzur.                 */
/* Copyright (c) 2014-2017 Godot Engine contributors (cf. AUTHORS.md)    */
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#include "audio_stream.h"

//////////////////////////////

void AudioStreamPlaybackResampled::_begin_resample() {

	//clear cubic interpolation history
	internal_buffer[0] = AudioFrame(0.0, 0.0);
	internal_buffer[1] = AudioFrame(0.0, 0.0);
	internal_buffer[2] = AudioFrame(0.0, 0.0);
	internal_buffer[3] = AudioFrame(0.0, 0.0);
	//mix buffer
	_mix_internal(internal_buffer + 4, INTERNAL_BUFFER_LEN);
	mix_offset = 0;
}

void AudioStreamPlaybackResampled::mix(AudioFrame *p_buffer, float p_rate_scale, int p_frames) {

	float target_rate = AudioServer::get_singleton()->get_mix_rate() * p_rate_scale;

	uint64_t mix_increment = uint64_t((get_stream_sampling_rate() / double(target_rate)) * double(FP_LEN));

	for (int i = 0; i < p_frames; i++) {

		uint32_t idx = CUBIC_INTERP_HISTORY + uint32_t(mix_offset >> FP_BITS);
		//standard cubic interpolation (great quality/performance ratio)
		//this used to be moved to a LUT for greater performance, but nowadays CPU speed is generally faster than memory.
		float mu = (mix_offset & FP_MASK) / float(FP_LEN);
		AudioFrame y0 = internal_buffer[idx - 3];
		AudioFrame y1 = internal_buffer[idx - 2];
		AudioFrame y2 = internal_buffer[idx - 1];
		AudioFrame y3 = internal_buffer[idx - 0];

		float mu2 = mu * mu;
		AudioFrame a0 = y3 - y2 - y0 + y1;
		AudioFrame a1 = y0 - y1 - a0;
		AudioFrame a2 = y2 - y0;
		AudioFrame a3 = y1;

		p_buffer[i] = (a0 * mu * mu2 + a1 * mu2 + a2 * mu + a3);

		mix_offset += mix_increment;

		while ((mix_offset >> FP_BITS) >= INTERNAL_BUFFER_LEN) {

			internal_buffer[0] = internal_buffer[INTERNAL_BUFFER_LEN + 0];
			internal_buffer[1] = internal_buffer[INTERNAL_BUFFER_LEN + 1];
			internal_buffer[2] = internal_buffer[INTERNAL_BUFFER_LEN + 2];
			internal_buffer[3] = internal_buffer[INTERNAL_BUFFER_LEN + 3];
			_mix_internal(internal_buffer + 4, INTERNAL_BUFFER_LEN);
			mix_offset -= (INTERNAL_BUFFER_LEN << FP_BITS);
		}
	}
}