Function ac3_downmix_mask

libhb/decavcodec.c:192–244

}

static uint64_t ac3_downmix_mask(int hb_mixdown, int normalized, const AVChannelLayout *input_layout, const char **dmix_mode)
{
    /*
     * ac3/eac3 bitstreams contain mix levels for center, surround and LFE channels.
     *
     * libavcodec's decoder can use them to build a normalized downmix matrix:
     * libavcodec/ac3dec.c static int set_downmix_coeffs()
     *
     * and downmix to either mono or stereo specifically:
     * libavcodec/ac3dsp.c static void ac3_downmix_c()
     *
     * We only do a decoder downmix here for a minor speed boost, as the mix levels
     * are otherwise available to us via AV_FRAME_DATA_DOWNMIX_INFO, which we use
     * in decodeAudio() to build the "regular" (non-normalized) downmix matrix.
     *
     * Note: the decoder ignores Lt/Rt-specific mix levels and is otherwise incapable of
     * producing a Dolby Surround/PLII-compatible downmix: only use it for normal Stereo.
     */
    if (normalized == 1)
    {
        uint64_t mask = 0;
        switch (hb_mixdown)
        {
            case HB_AMIXDOWN_MONO:
            case HB_AMIXDOWN_STEREO:
                mask = hb_ff_mixdown_xlat(hb_mixdown, NULL);
                break;

            default:
                return 0;
        }
        if (mask)
        {
            AVChannelLayout mask_layout = {0};
            av_channel_layout_from_mask(&mask_layout, mask);

            if (downmix_required(&mask_layout, input_layout))
            {
                /*
                 * We also set the existing decoder option "dmix_mode" to 2 (AC3_DMIXMOD_LORO)
                 * which is currently ignored by the decoder but should (theoretically) ensure
                 * we always get a regular Lo/Ro downmix, if the decoder were to ever gain the
                 * ability to do a Dolby/PLII downmix in the future.
                 */
                *dmix_mode = "2";
                av_channel_layout_uninit(&mask_layout);
                return mask;
            }
            av_channel_layout_uninit(&mask_layout);
        }
    }
    return 0;
}

static uint64_t dca_downmix_mask(int hb_mixdown, int normalized, const AVChannelLayout *input_layout)
{