fftools/ffmpeg: Add new variant source_no_drop to the force_key_frames option

Suggested-By: ffmpeg@fb.com

doc/ffmpeg.texi

@@ -1011,6 +1011,7 @@ Deprecated see -bsf
 @item -force_key_frames[:@var{stream_specifier}] @var{time}[,@var{time}...] (@emph{output,per-stream})
 @item -force_key_frames[:@var{stream_specifier}] expr:@var{expr} (@emph{output,per-stream})
 @item -force_key_frames[:@var{stream_specifier}] source (@emph{output,per-stream})
+@item -force_key_frames[:@var{stream_specifier}] source_no_drop (@emph{output,per-stream})
 
 @var{force_key_frames} can take arguments of the following form:
 
@@ -1072,6 +1073,12 @@ starting from second 13:
 If the argument is @code{source}, ffmpeg will force a key frame if
 the current frame being encoded is marked as a key frame in its source.
 
+@item source_no_drop
+If the argument is @code{source_no_drop}, ffmpeg will force a key frame if
+the current frame being encoded is marked as a key frame in its source.
+In cases where this particular source frame has to be dropped,
+enforce the next available frame to become a key frame instead.
+
 @end table
 
 Note that forcing too many keyframes is very harmful for the lookahead

fftools/ffmpeg.c

@@ -1287,6 +1287,7 @@ static void do_video_out(OutputFile *of,
         }
     }
     ost->last_dropped = nb_frames == nb0_frames && next_picture;
+    ost->dropped_keyframe = ost->last_dropped && next_picture && next_picture->key_frame;
 
     /* duplicates frame if needed */
     for (i = 0; i < nb_frames; i++) {
@@ -1347,6 +1348,11 @@ static void do_video_out(OutputFile *of,
                    && in_picture->key_frame==1
                    && !i) {
             forced_keyframe = 1;
+        } else if (   ost->forced_keyframes
+                   && !strncmp(ost->forced_keyframes, "source_no_drop", 6)
+                   && !i) {
+            forced_keyframe = (in_picture->key_frame == 1) || ost->dropped_keyframe;
+            ost->dropped_keyframe = 0;
         }
 
         if (forced_keyframe) {

fftools/ffmpeg.h

@@ -504,6 +504,7 @@ typedef struct OutputStream {
     char *forced_keyframes;
     AVExpr *forced_keyframes_pexpr;
     double forced_keyframes_expr_const_values[FKF_NB];
+    int dropped_keyframe;
 
     /* audio only */
     int *audio_channels_map;             /* list of the channels id to pick from the source stream */