avisynth: populate field order

Fixes Trac ticket #8757

Signed-off-by: Stephen Hutchinson <qyot27@gmail.com>

libavformat/avisynth.c

@@ -243,6 +243,23 @@ static int avisynth_create_stream_video(AVFormatContext *s, AVStream *st)
     st->nb_frames         = avs->vi->num_frames;
     avpriv_set_pts_info(st, 32, avs->vi->fps_denominator, avs->vi->fps_numerator);
 
+    av_log(s, AV_LOG_TRACE, "avs_is_field_based: %d\n", avs_is_field_based(avs->vi));
+    av_log(s, AV_LOG_TRACE, "avs_is_parity_known: %d\n", avs_is_parity_known(avs->vi));
+
+    /* The following typically only works when assumetff (-bff) and
+     * assumefieldbased is used in-script. Additional
+     * logic using GetParity() could deliver more accurate results
+     * but also decodes a frame which we want to avoid. */
+    st->codecpar->field_order = AV_FIELD_UNKNOWN;
+    if (avs_is_field_based(avs->vi)) {
+        if (avs_is_tff(avs->vi)) {
+            st->codecpar->field_order = AV_FIELD_TT;
+        }
+        else if (avs_is_bff(avs->vi)) {
+            st->codecpar->field_order = AV_FIELD_BB;
+        }
+    }
+
     switch (avs->vi->pixel_type) {
     /* 10~16-bit YUV pix_fmts (AviSynth+) */
     case AVS_CS_YUV444P10: