[FFmpeg-devel] [PATCH v2] avformat/img2dec: Option to play sequence backwards

[Sergio Acereda <sergio.acereda@gmail.com>]

This patch should allow playing 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@gmail.com>
---
 libavformat/img2.h    |  1 +
 libavformat/img2dec.c | 42 +++++++++++++++++++++++++++++++-----------
 2 files changed, 32 insertions(+), 11 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..37efb3d618 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   = FFABS(last_index - first_index) + 1;
         }
     }
 
@@ -402,6 +409,13 @@ static int add_filename_as_pkt_side_data(char *filename, AVPacket *pkt) {
     return 0;
 }
 
+static int out_bounds(const VideoDemuxData *s)
+{
+    if (s->reverse)
+        return s->img_number < s->img_last;
+    return s->img_number > s->img_last;
+}
+
 int ff_img_read_packet(AVFormatContext *s1, AVPacket *pkt)
 {
     VideoDemuxData *s = s1->priv_data;
@@ -414,10 +428,10 @@ int ff_img_read_packet(AVFormatContext *s1, AVPacket *pkt)
 
     if (!s->is_pipe) {
         /* loop over input */
-        if (s->loop && s->img_number > s->img_last) {
+        if (s->loop && out_bounds(s)) {
             s->img_number = s->img_first;
         }
-        if (s->img_number > s->img_last)
+        if (out_bounds(s))
             return AVERROR_EOF;
         if (s->pattern_type == PT_NONE) {
             av_strlcpy(filename_bytes, s->path, sizeof(filename_bytes));
@@ -554,8 +568,9 @@ 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;
     }
@@ -585,6 +600,7 @@ static int img_read_seek(AVFormatContext *s, int stream_index, int64_t timestamp
 {
     VideoDemuxData *s1 = s->priv_data;
     AVStream *st = s->streams[0];
+    int span;
 
     if (s1->ts_from_file) {
         int index = av_index_search_timestamp(st, timestamp, flags);
@@ -594,10 +610,13 @@ 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)
+    span = 1 + FFABS(s1->img_last - s1->img_first);
+    if (timestamp < 0 || !s1->loop && timestamp > span)
         return -1;
-    s1->img_number = timestamp%(s1->img_last - s1->img_first + 1) + s1->img_first;
-    s1->pts = timestamp;
+    else {
+        s1->img_number = s1->img_first + timestamp % span;
+        s1->pts = timestamp;
+    }
     return 0;
 }
 
@@ -619,6 +638,7 @@ 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" },
-- 
2.33.1

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