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math/rdft.h

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+/* ScummVM - Graphic Adventure Engine
+ *
+ * ScummVM is the legal property of its developers, whose names
+ * are too numerous to list here. Please refer to the COPYRIGHT
+ * file distributed with this source distribution.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+// Based on xoreos' (I)RDFT code which is in turn
+// Based upon the (I)RDFT code in FFmpeg
+// Copyright (c) 2009 Alex Converse <alex dot converse at gmail dot com>
+
+#ifndef MATH_RDFT_H
+#define MATH_RDFT_H
+
+#include "math/cosinetables.h"
+#include "math/sinetables.h"
+
+namespace Math {
+
+class FFT;
+
+/**
+ * @defgroup math_rdft RDFT algorithm
+ * @ingroup math
+ *
+ * @brief API for the Real Discrete Fourier Transform (RDFT) algorithm.
+ *
+ * @{
+ */
+
+/**
+ * @brief (Inverse) Real Discrete Fourier Transform.
+ *
+ * @details Used in audio:
+ *  - QDM2
+ *
+ * Used in engines:
+ *  - scumm
+ *
+ *
+ * It has four modes:
+ *
+ *   Below, n = 1 << bits
+ *
+ *   (I)DFT_R2C:
+ *     input:
+ *       n real floats
+ *     output:
+ *       n/2 complex floats (stored as real part followed by imag part).
+ *
+ *     The output represents the first half of the (I)DFT of the input.
+ *     If F is the complex (I)DFT of the input, then
+ *     output[0] = F[0] + i * F[n/2] and
+ *     output[k] = F[k] for k = 1 .. n/2-1.
+ *     Note that F[0] and F[k] are real since the input is real, and
+ *     the remaining values of F can be reconstructed from symmetry if desired.
+ *
+ *   (I)DFT_C2R:
+ *     input:
+ *       n/2 complex floats
+ *     output:
+ *       n real floats
+ *
+ *     The input encodes a complex vector x of length n that has the
+ *     required symmetry to have a real (I)DFT:
+ *     x[0] = Re(input[0])
+ *     x[k] = input[k] for k = 1 .. n/2-1
+ *     x[n/2] = Im(input[0])
+ *     x[k] = conj(input[n-k]) for k = n/2+1 .. n-1
+ *     The output is then the real (I)DFT of x, divided by 2.
+ *
+ *     TODO: Is this division by 2 intentional?
+ */
+
+class RDFT {
+public:
+	enum TransformType {
+		DFT_R2C,
+		IDFT_C2R,
+		IDFT_R2C,
+		DFT_C2R
+	};
+
+	RDFT(int bits, TransformType trans);
+	~RDFT();
+
+	void calc(float *data);
+
+private:
+	int _bits;
+	int _inverse;
+	int _signConvention;
+
+	SineTable _sin;
+	CosineTable _cos;
+	const float *_tSin;
+	const float *_tCos;
+
+	FFT *_fft;
+};
+
+/** @} */
+
+} // End of namespace Math
+
+#endif // MATH_RDFT_H