Simple lowpass filter implementation.

Originally committed as revision 14921 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/lowpass.c

+/*
+ * Lowpass IIR filter
+ * Copyright (c) 2008 Konstantin Shishkov
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file lowpass.c
+ * lowpass filter implementation
+ */
+
+#include "lowpass.h"
+
+/**********************
+ *         TODO:
+ * support filters with order != 4
+ * calculate coefficients for filter instead of taking approximate ones from the table
+ *********************/
+
+/** filter order */
+#define LOWPASS_FILTER_ORDER 4
+
+/**
+ * IIR filter global parameters
+ */
+typedef struct FFLPFilterCoeffs{
+    float gain;
+    float c[LOWPASS_FILTER_ORDER];
+}FFLPFilterCoeffs;
+
+/**
+ * filter data for 4th order IIR lowpass Butterworth filter
+ */
+static const FFLPFilterCoeffs lp_filter_coeffs[] = {
+    { 9.398085e-01, { -0.0176648009,  0.0000000000, -0.4860288221,  0.0000000000 } },
+    { 6.816645e-01, { -0.4646665999, -2.2127207402, -3.9912017501, -3.2380429984 } },
+    { 4.998150e-01, { -0.2498216698, -1.3392807613, -2.7693097862, -2.6386277439 } },
+    { 3.103469e-01, { -0.0965076902, -0.5977763360, -1.4972580903, -1.7740085241 } },
+    { 2.346995e-01, { -0.0557639007, -0.3623690447, -1.0304538354, -1.3066051440 } },
+    { 1.528432e-01, { -0.0261686639, -0.1473794606, -0.6204721225, -0.6514716536 } },
+    { 6.917529e-02, { -0.0202414073,  0.0780167640, -0.5277442247,  0.3631641670 } },
+    { 6.178391e-02, { -0.0223681543,  0.1069446609, -0.5615167033,  0.4883976841 } },
+    { 5.298685e-02, { -0.0261686639,  0.1473794606, -0.6204721225,  0.6514716536 } },
+    { 2.229030e-02, { -0.0647354087,  0.4172275190, -1.1412129810,  1.4320761385 } },
+    { 1.693903e-02, { -0.0823177861,  0.5192354923, -1.3444768251,  1.6365345642 } },
+    { 7.374053e-03, { -0.1481421788,  0.8650973862, -1.9894244796,  2.1544844308 } },
+    { 5.541768e-03, { -0.1742301048,  0.9921936565, -2.2090801108,  2.3024482658 } },
+};
+
+/** cutoff ratios for lp_filter_data[] */
+static const float lp_cutoff_ratios[] = {
+    0.5000000000, 0.4535147392, 0.4166666667, 0.3628117914,
+    0.3333333333, 0.2916666667, 0.2267573696, 0.2187500000,
+    0.2083333333, 0.1587301587, 0.1458333333, 0.1133786848,
+    0.1041666667,
+};
+
+/**
+ * IIR filter state
+ */
+typedef struct FFLPFilterState{
+    float x[LOWPASS_FILTER_ORDER];
+}FFLPFilterState;
+
+const struct FFLPFilterCoeffs* ff_lowpass_filter_init_coeffs(int order, float cutoff_ratio)
+{
+    int i, size;
+
+    //we can create only order-4 filters with cutoff ratio <= 0.5 for now
+    if(order != LOWPASS_FILTER_ORDER) return NULL;
+
+    size = sizeof(lp_cutoff_ratios) / sizeof(lp_cutoff_ratios[0]);
+    if(cutoff_ratio > lp_cutoff_ratios[0])
+        return NULL;
+    for(i = 0; i < size; i++){
+        if(cutoff_ratio >= lp_cutoff_ratios[i])
+            break;
+    }
+    if(i == size)
+        i = size - 1;
+    return &lp_filter_coeffs[i];
+}
+
+struct FFLPFilterState* ff_lowpass_filter_init_state(int order)
+{
+    if(order != LOWPASS_FILTER_ORDER) return NULL;
+    return av_mallocz(sizeof(FFLPFilterState));
+}
+
+#define FILTER(i0, i1, i2, i3)                  \
+    in =   *src * c->gain                       \
+         + c->c[0]*s->x[i0] + c->c[1]*s->x[i1]  \
+         + c->c[2]*s->x[i2] + c->c[3]*s->x[i3]; \
+    res =  (s->x[i0] + in      )*1              \
+         + (s->x[i1] + s->x[i3])*4              \
+         +  s->x[i2]            *6;             \
+    *dst = av_clip_int16(lrintf(res));          \
+    s->x[i0] = in;                              \
+    src += sstep;                               \
+    dst += dstep;                               \
+
+void ff_lowpass_filter(const struct FFLPFilterCoeffs *c, struct FFLPFilterState *s, int size, int16_t *src, int sstep, int16_t *dst, int dstep)
+{
+    int i;
+
+    for(i = 0; i < size; i += 4){
+        float in, res;
+
+        FILTER(0, 1, 2, 3);
+        FILTER(1, 2, 3, 0);
+        FILTER(2, 3, 0, 1);
+        FILTER(3, 0, 1, 2);
+    }
+}

libavcodec/lowpass.h

+/*
+ * Lowpass IIR filter
+ * Copyright (c) 2008 Konstantin Shishkov
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file lowpass.h
+ * lowpass filter interface
+ */
+
+#ifndef FFMPEG_LOWPASS_H
+#define FFMPEG_LOWPASS_H
+
+#include "avcodec.h"
+
+struct FFLPFilterCoeffs;
+struct FFLPFilterState;
+
+/**
+ * Initialize filter coefficients.
+ *
+ * @param order        filter order
+ * @param cutoff_ratio cutoff to input frequency ratio
+ *
+ * @return pointer to filter coefficients structure or NULL if filter cannot be created
+ */
+const struct FFLPFilterCoeffs* ff_lowpass_filter_init_coeffs(int order, float cutoff_ratio);
+
+/**
+ * Create new filter state.
+ *
+ * @param order filter order
+ *
+ * @return pointer to new filter state or NULL if state creation fails
+ */
+struct FFLPFilterState* ff_lowpass_filter_init_state(int order);
+
+#if 0 //enable with arbitrary order filter implementation, use av_free() for filter state only for now
+/**
+ * Free filter coefficients.
+ *
+ * @param coeffs pointer allocated with ff_lowpass_filter_init_coeffs()
+ */
+void ff_lowpass_filter_free_coeffs(struct FFLPFilterCoeffs *coeffs);
+
+/**
+ * Free filter state.
+ *
+ * @param state pointer allocated with ff_lowpass_filter_init_state()
+ */
+void ff_lowpass_filter_free_state(struct FFLPFilterState *state);
+#endif
+
+/**
+ * Perform lowpass filtering on input samples.
+ *
+ * @param coeffs pointer to filter coefficients
+ * @param state  pointer to filter state
+ * @param size   input length
+ * @param src    source samples
+ * @param sstep  source stride
+ * @param dst    filtered samples (destination may be the same as input)
+ * @param dstep  destination stride
+ */
+void ff_lowpass_filter(const struct FFLPFilterCoeffs *coeffs, struct FFLPFilterState *state, int size, int16_t *src, int sstep, int16_t *dst, int dstep);
+
+#endif /* FFMPEG_LOWPASS_H */