FFmpeg/libavcodec/dct.c

/*
 * (I)DCT Transforms
 * Copyright (c) 2009 Peter Ross <pross@xvid.org>
 * Copyright (c) 2010 Alex Converse <alex.converse@gmail.com>
 * Copyright (c) 2010 Vitor Sessak
 *
 * 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 St, Fifth Floor, Boston, MA  02110-1301  USA
 */

/**
 * @file libavcodec/dct.c
 * (Inverse) Discrete Cosine Transforms. These are also known as the
 * type II and type III DCTs respectively.
 */

#include <math.h>
#include "dsputil.h"

av_cold int ff_dct_init(DCTContext *s, int nbits, int inverse)
{
    int n = 1 << nbits;
    int i;

    s->nbits    = nbits;
    s->inverse  = inverse;

    ff_init_ff_cos_tabs(nbits+2);

    s->costab = ff_cos_tabs[nbits+2];

    s->csc2 = av_malloc(n/2 * sizeof(FFTSample));

    if (ff_rdft_init(&s->rdft, nbits, inverse) < 0) {
        av_free(s->csc2);
        return -1;
    }

    for (i = 0; i < n/2; i++)
        s->csc2[i] = 0.5 / sin((M_PI / (2*n) * (2*i + 1)));

    return 0;
}

/* sin((M_PI * x / (2*n)) */
#define SIN(s,n,x) (s->costab[(n) - (x)])

/* cos((M_PI * x / (2*n)) */
#define COS(s,n,x) (s->costab[x])

static void ff_dct_calc_c(DCTContext *ctx, FFTSample *data)
{
    int n = 1 << ctx->nbits;
    int i;

    if (ctx->inverse) {
        float next = data[n - 1];
        float inv_n = 1.0f / n;

        for (i = n - 2; i >= 2; i -= 2) {
            float val1 = data[i    ];
            float val2 = data[i - 1] - data[i + 1];
            float c = COS(ctx, n, i);
            float s = SIN(ctx, n, i);

            data[i    ] = c * val1 + s * val2;
            data[i + 1] = s * val1 - c * val2;
        }

        data[1] = 2 * next;

        ff_rdft_calc(&ctx->rdft, data);

        for (i = 0; i < n / 2; i++) {
            float tmp1 = data[i        ] * inv_n;
            float tmp2 = data[n - i - 1] * inv_n;
            float csc = ctx->csc2[i] * (tmp1 - tmp2);

            tmp1 += tmp2;
            data[i        ] = tmp1 + csc;
            data[n - i - 1] = tmp1 - csc;
        }
    } else {
        float next;
        for (i=0; i < n/2; i++) {
            float tmp1 = data[i        ];
            float tmp2 = data[n - i - 1];
            float s = SIN(ctx, n, 2*i + 1);

            s *= tmp1 - tmp2;
            tmp1 = (tmp1 + tmp2) * 0.5f;

            data[i    ] = tmp1 + s;
            data[n-i-1] = tmp1 - s;
        }

        ff_rdft_calc(&ctx->rdft, data);

        next = data[1] * 0.5;
        data[1] *= -1;

        for (i = n - 2; i >= 0; i -= 2) {
            float inr = data[i    ];
            float ini = data[i + 1];
            float c = COS(ctx, n, i);
            float s = SIN(ctx, n, i);

            data[i  ] = c * inr + s * ini;

            data[i+1] = next;

            next +=     s * inr - c * ini;
        }
    }
}

void ff_dct_calc(DCTContext *s, FFTSample *data)
{
    ff_dct_calc_c(s, data);
}

av_cold void ff_dct_end(DCTContext *s)
{
    ff_rdft_end(&s->rdft);
    av_free(s->csc2);
}