[FFmpeg-devel] [PATCH] avformat/img2dec: Option to play sequence backwards

[Sergio Acereda]

This patch should allow to play an image sequence in backwards 
direction, without needing to apply a reverse filter.

ffmpeg -i sequence%05d.png forward.mkv
ffmpeg -reverse 1 -i sequence%05d.png backward.mkv

Signed-off-by: Sergio Acereda <sergio.acereda at gmail.com>
---
  libavformat/img2.h    |  1 +
  libavformat/img2dec.c | 34 ++++++++++++++++++++++++----------
  2 files changed, 25 insertions(+), 10 deletions(-)

diff --git a/libavformat/img2.h b/libavformat/img2.h
index 5fd8ff77fc..3d01c00537 100644
--- a/libavformat/img2.h
+++ b/libavformat/img2.h
@@ -59,6 +59,7 @@ typedef struct VideoDemuxData {
  #endif
      int start_number;
      int start_number_range;
+    int reverse;
      int frame_size;
      int ts_from_file;
      int export_path_metadata; /**< enabled when set to 1. */
diff --git a/libavformat/img2dec.c b/libavformat/img2dec.c
index 8608252d83..7b85e01aca 100644
--- a/libavformat/img2dec.c
+++ b/libavformat/img2dec.c
@@ -105,10 +105,12 @@ static int is_glob(const char *path)
   * @param plast_index  pointer to index updated with the last number 
in the range
   * @param path         path which has to be matched by the image files 
in the range
   * @param start_index  minimum accepted value for the first index in 
the range
+ * @param start_index_range range for looking at the first sequence number
+ * @param reverse      play backwards
   * @return -1 if no image file could be found
   */
  static int find_image_range(AVIOContext *pb, int *pfirst_index, int 
*plast_index,
-                            const char *path, int start_index, int 
start_index_range)
+                            const char *path, int start_index, int 
start_index_range, int reverse)
  {
      char buf[1024];
      int range, last_index, range1, first_index;
@@ -152,8 +154,13 @@ static int find_image_range(AVIOContext *pb, int 
*pfirst_index, int *plast_index
              break;
          last_index += range;
      }
-    *pfirst_index = first_index;
-    *plast_index  = last_index;
+    if (reverse) {
+        *pfirst_index = last_index;
+        *plast_index  = first_index;
+    } else {
+        *pfirst_index = first_index;
+        *plast_index  = last_index;
+    }
      return 0;
   fail:
@@ -274,7 +281,7 @@ int ff_img_read_header(AVFormatContext *s1)
          }
          if ((s->pattern_type == PT_GLOB_SEQUENCE && !s->use_glob) || 
s->pattern_type == PT_SEQUENCE) {
              if (find_image_range(s1->pb, &first_index, &last_index, 
s->path,
-                                 s->start_number, 
s->start_number_range) < 0) {
+                                 s->start_number, 
s->start_number_range, s->reverse) < 0) {
                  av_log(s1, AV_LOG_ERROR,
                         "Could find no file with path '%s' and index in 
the range %d-%d\n",
                         s->path, s->start_number, s->start_number + 
s->start_number_range - 1);
@@ -307,7 +314,7 @@ int ff_img_read_header(AVFormatContext *s1)
          /* compute duration */
          if (!s->ts_from_file) {
              st->start_time = 0;
-            st->duration   = last_index - first_index + 1;
+            st->duration   = abs(last_index - first_index) + 1;
          }
      }
  @@ -413,11 +420,12 @@ int ff_img_read_packet(AVFormatContext *s1, 
AVPacket *pkt)
      AVCodecParameters *par = s1->streams[0]->codecpar;
       if (!s->is_pipe) {
+        int bad = s->reverse? (s->img_number < s->img_last) : 
(s->img_number > s->img_last);
          /* loop over input */
-        if (s->loop && s->img_number > s->img_last) {
+        if (s->loop && bad) {
              s->img_number = s->img_first;
          }
-        if (s->img_number > s->img_last)
+        if (bad)
              return AVERROR_EOF;
          if (s->pattern_type == PT_NONE) {
              av_strlcpy(filename_bytes, s->path, sizeof(filename_bytes));
@@ -554,8 +562,9 @@ int ff_img_read_packet(AVFormatContext *s1, AVPacket 
*pkt)
          }
          goto fail;
      } else {
+        int step = s->reverse? -1 : 1;
          s->img_count++;
-        s->img_number++;
+        s->img_number += step;
          s->pts++;
          return 0;
      }
@@ -594,9 +603,13 @@ static int img_read_seek(AVFormatContext *s, int 
stream_index, int64_t timestamp
          return 0;
      }
  -    if (timestamp < 0 || !s1->loop && timestamp > s1->img_last - 
s1->img_first)
+    int bad = s1->reverse? (timestamp  < s1->img_last) : (timestamp > 
s1->img_last);
+    if (timestamp < 0 || !s1->loop && bad)
          return -1;
-    s1->img_number = timestamp%(s1->img_last - s1->img_first + 1) + 
s1->img_first;
+    int dir = s1->reverse? -1 : 1;
+    int span = 1 + dir * (s1->img_last - s1->img_first);
+    int rel = timestamp % span;
+    s1->img_number = s1->img_first + dir * rel;
      s1->pts = timestamp;
      return 0;
  }
@@ -619,6 +632,7 @@ const AVOption ff_img_options[] = {
      { "none",         "disable pattern matching",            0, 
AV_OPT_TYPE_CONST,  {.i64=PT_NONE         }, INT_MIN, INT_MAX, DEC, 
"pattern_type" },
      { "start_number", "set first number in the sequence", 
OFFSET(start_number), AV_OPT_TYPE_INT,    {.i64 = 0   }, INT_MIN, 
INT_MAX, DEC },
      { "start_number_range", "set range for looking at the first 
sequence number", OFFSET(start_number_range), AV_OPT_TYPE_INT, {.i64 = 
5}, 1, INT_MAX, DEC },
+    { "reverse",      "reverse direction", 
OFFSET(reverse),      AV_OPT_TYPE_INT,    {.i64 = 0   }, 0, 1, DEC },
      { "ts_from_file", "set frame timestamp from file's one", 
OFFSET(ts_from_file), AV_OPT_TYPE_INT,    {.i64 = 0   }, 0, 2, 
DEC, "ts_type" },
      { "none", "none",                   0, AV_OPT_TYPE_CONST,    {.i64 
= 0   }, 0, 2,       DEC, "ts_type" },
      { "sec",  "second precision",       0, AV_OPT_TYPE_CONST,    {.i64 
= 1   }, 0, 2,       DEC, "ts_type" },
-- 
2.34.0