lavc/encode: pass through frame durations to encoded packets

The generic code can only handle the no-delay case. Encoders with delay
need to be handled individually, which will be done in the following
commits.

4 files changed, 22 insertions, 4 deletions

diff --git a/doc/APIchanges b/doc/APIchanges
index 148962cb40..bc52a07964 100644
--- a/doc/APIchanges
+++ b/doc/APIchanges
@@ -14,8 +14,8 @@ libavutil:     2021-04-27
 
 API changes, most recent first:
 
-2023-01-xx - xxxxxxxxxx - lavc 59.59.100 - avcodec.h
-  Add AV_CODEC_FLAG_COPY_OPAQUE.
+2023-01-29 - xxxxxxxxxx - lavc 59.59.100 - avcodec.h
+  Add AV_CODEC_FLAG_COPY_OPAQUE and AV_CODEC_FLAG_FRAME_DURATION.
 
 2023-01-13 - xxxxxxxxxx - lavu 57.44.100 - ambient_viewing_environment.h frame.h
   Adds a new structure for holding H.274 Ambient Viewing Environment metadata,
diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index 832b9d9575..90b437ccbe 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -268,6 +268,13 @@ typedef struct RcOverride{
  */
 #define AV_CODEC_FLAG_COPY_OPAQUE     (1 <<  7)
 /**
+ * Signal to the encoder that the values of AVFrame.duration are valid and
+ * should be used (typically for transferring them to output packets).
+ *
+ * If this flag is not set, frame durations are ignored.
+ */
+#define AV_CODEC_FLAG_FRAME_DURATION  (1 <<  8)
+/**
  * Use internal 2pass ratecontrol in first pass mode.
  */
 #define AV_CODEC_FLAG_PASS1           (1 <<  9)
diff --git a/libavcodec/encode.c b/libavcodec/encode.c
index 555bd3ff96..c92beaf8e1 100644
--- a/libavcodec/encode.c
+++ b/libavcodec/encode.c
@@ -231,10 +231,13 @@ int ff_encode_encode_cb(AVCodecContext *avctx, AVPacket *avpkt,
             if (avpkt->pts == AV_NOPTS_VALUE)
                 avpkt->pts = frame->pts;
 
-            if (avctx->codec->type == AVMEDIA_TYPE_AUDIO) {
-                if (!avpkt->duration)
+            if (!avpkt->duration) {
+                if (frame->duration)
+                    avpkt->duration = frame->duration;
+                else if (avctx->codec->type == AVMEDIA_TYPE_AUDIO) {
                     avpkt->duration = ff_samples_to_time_base(avctx,
                                                               frame->nb_samples);
+                }
             }
 
             ret = ff_encode_reordered_opaque(avctx, avpkt, frame);
@@ -463,6 +466,13 @@ FF_ENABLE_DEPRECATION_WARNINGS
             return ret;
     }
 
+    // unset frame duration unless AV_CODEC_FLAG_FRAME_DURATION is set,
+    // since otherwise we cannot be sure that whatever value it has is in the
+    // right timebase, so we would produce an incorrect value, which is worse
+    // than none at all
+    if (!(avctx->flags & AV_CODEC_FLAG_FRAME_DURATION))
+        dst->duration = 0;
+
     return 0;
 }
 
diff --git a/libavcodec/options_table.h b/libavcodec/options_table.h
index 4b38c42043..322ec7a156 100644
--- a/libavcodec/options_table.h
+++ b/libavcodec/options_table.h
@@ -59,6 +59,7 @@ static const AVOption avcodec_options[] = {
 {"qscale", "use fixed qscale", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_QSCALE }, INT_MIN, INT_MAX, 0, "flags"},
 {"recon_frame", "export reconstructed frames", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_RECON_FRAME}, .unit = "flags"},
 {"copy_opaque", "propagate opaque values", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_COPY_OPAQUE}, .unit = "flags"},
+{"frame_duration", "use frame durations", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_FRAME_DURATION}, .unit = "flags"},
 {"pass1", "use internal 2-pass ratecontrol in first  pass mode", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_PASS1 }, INT_MIN, INT_MAX, 0, "flags"},
 {"pass2", "use internal 2-pass ratecontrol in second pass mode", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_PASS2 }, INT_MIN, INT_MAX, 0, "flags"},
 {"gray", "only decode/encode grayscale", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_GRAY }, INT_MIN, INT_MAX, V|E|D, "flags"},