MODE_DROP_ODD      43 libavfilter/vf_tinterlace.c     {"drop_odd",          "drop odd fields",                              0, AV_OPT_TYPE_CONST, {.i64=MODE_DROP_ODD},          INT_MIN, INT_MAX, FLAGS, "mode"},
MODE_DROP_ODD     409 libavfilter/vf_tinterlace.c     case MODE_DROP_ODD:  /* only output even frames, odd  frames are dropped; height unchanged, half framerate */