Function truehd_downmix_mask

libhb/decavcodec.c:265–347

}

static uint64_t truehd_downmix_mask(int hb_mixdown, int normalized, const AVChannelLayout *input_layout)
{
    /*
     * TrueHD bitstreams are made up of multiple "substreams" which are
     * combined in order to obtain the final output. Any given substream
     * depends on the previous substream(s), but not the next; by only
     * decoding up to specific substream, a TrueHD decoder can extract
     * an embedded downmix.
     */
    if (normalized == 0)
    {
        uint64_t mask = 0;
        switch (hb_mixdown)
        {
            /*
             * We cannot use an embedded Stereo downmix as we have no way
             * of knowing whether it is Dolby Surround or PLII-compatible.
             * Request 5.1 instead and let hb_audio_resample downmix that.
             */
            case HB_AMIXDOWN_DOLBY:
            case HB_AMIXDOWN_DOLBYPLII:
                mask = AV_CH_LAYOUT_5POINT1;
                break;

            default:
                mask = hb_ff_mixdown_xlat(hb_mixdown, NULL);
                break;
        }
        if (mask)
        {
            AVChannelLayout mask_layout = {0};
            av_channel_layout_from_mask(&mask_layout, mask);

            if (downmix_required(&mask_layout, input_layout))
            {
                if (mask == AV_CH_LAYOUT_STEREO)
                {
                    /*
                     * The majority of TrueHD tracks have a Stereo first
                     * substream, even when the second substream is Mono.
                     */
                    return mask;
                }
                if (hb_mixdown == HB_AMIXDOWN_MONO)
                {
                    /*
                     * It is unlikely that any substream configuration will
                     * give us an embedded Mono downmix (except in the case
                     * where the full input layout is Mono, but in said case
                     * a downmix is not required) however it may be possible
                     * to extract a Stereo downmix and let hb_audio_resample
                     * take care of downmixing that to Mono for final output.
                     *
                     * Do it after downmix_required() so we don't accidentally
                     * request a Stereo downmix when the input is already Mono.
                     */
                    return AV_CH_LAYOUT_STEREO;
                }