fftools/ffmpeg: add loopback decoding

This allows to send an encoder's output back to decoding and feed the
result into a complex filtergraph.

Changelog

@@ -33,6 +33,7 @@ version <next>:
 - ffprobe -show_stream_groups option
 - ffprobe (with -export_side_data film_grain) now prints film grain metadata
 - AEA muxer
+- ffmpeg CLI loopback decoders
 
 
 version 6.1:

doc/ffmpeg.texi

@@ -219,6 +219,40 @@ Since there is no decoding or encoding, it is very fast and there is no quality
 loss. However, it might not work in some cases because of many factors. Applying
 filters is obviously also impossible, since filters work on uncompressed data.
 
+@section Loopback decoders
+While decoders are normally associated with demuxer streams, it is also possible
+to create "loopback" decoders that decode the output from some encoder and allow
+it to be fed back to complex filtergraphs. This is done with the @code{-dec}
+directive, which takes as a parameter the index of the output stream that should
+be decoded. Every such directive creates a new loopback decoder, indexed with
+successive integers starting at zero. These indices should then be used to refer
+to loopback decoders in complex filtergraph link labels, as described in the
+documentation for @option{-filter_complex}.
+
+E.g. the following example:
+
+@example
+ffmpeg -i INPUT                                        \
+  -map 0:v:0 -c:v libx264 -crf 45 -f null -            \
+  -dec 0:0 -filter_complex '[0:v][dec:0]hstack[stack]' \
+  -map '[stack]' -c:v ffv1 OUTPUT
+@end example
+
+reads an input video and
+@itemize
+@item
+(line 2) encodes it with @code{libx264} at low quality;
+
+@item
+(line 3) decodes this encoded stream and places it side by side with the
+original input video;
+
+@item
+(line 4) combined video is then losslessly encoded and written into
+@file{OUTPUT}.
+
+@end itemize
+
 @c man end DETAILED DESCRIPTION
 
 @chapter Stream selection
@@ -2105,11 +2139,16 @@ type -- see the @option{-filter} options. @var{filtergraph} is a description of
 the filtergraph, as described in the ``Filtergraph syntax'' section of the
 ffmpeg-filters manual.
 
-Input link labels must refer to input streams using the
-@code{[file_index:stream_specifier]} syntax (i.e. the same as @option{-map}
-uses). If @var{stream_specifier} matches multiple streams, the first one will be
-used. An unlabeled input will be connected to the first unused input stream of
-the matching type.
+Input link labels must refer to either input streams or loopback decoders. For
+input streams, use the @code{[file_index:stream_specifier]} syntax (i.e. the
+same as @option{-map} uses). If @var{stream_specifier} matches multiple streams,
+the first one will be used.
+
+For decoders, the link label must be [dec:@var{dec_idx}], where @var{dec_idx} is
+the index of the loopback decoder to be connected to given input.
+
+An unlabeled input will be connected to the first unused input stream of the
+matching type.
 
 Output link labels are referred to with @option{-map}. Unlabeled outputs are
 added to the first output file.

fftools/cmdutils.c

@@ -528,7 +528,7 @@ static void check_options(const OptionDef *po)
 {
     while (po->name) {
         if (po->flags & OPT_PERFILE)
-            av_assert0(po->flags & (OPT_INPUT | OPT_OUTPUT));
+            av_assert0(po->flags & (OPT_INPUT | OPT_OUTPUT | OPT_DECODER));
 
         if (po->type == OPT_TYPE_FUNC)
             av_assert0(!(po->flags & (OPT_FLAG_OFFSET | OPT_FLAG_SPEC)));

fftools/cmdutils.h

@@ -144,8 +144,8 @@ typedef struct OptionDef {
 #define OPT_AUDIO       (1 << 4)
 #define OPT_SUBTITLE    (1 << 5)
 #define OPT_DATA        (1 << 6)
-/* The option is per-file (currently ffmpeg-only). At least one of OPT_INPUT or
- * OPT_OUTPUT must be set when this flag is in use.
+/* The option is per-file (currently ffmpeg-only). At least one of OPT_INPUT,
+ * OPT_OUTPUT, OPT_DECODER must be set when this flag is in use.
    */
 #define OPT_PERFILE     (1 << 7)
 
@@ -175,6 +175,9 @@ typedef struct OptionDef {
  * name is stored in u1.name_canon */
 #define OPT_HAS_CANON   (1 << 14)
 
+/* ffmpeg-only - OPT_PERFILE may apply to standalone decoders */
+#define OPT_DECODER     (1 << 15)
+
      union {
         void *dst_ptr;
         int (*func_arg)(void *, const char *, const char *);

fftools/ffmpeg.c

@@ -131,6 +131,9 @@ int         nb_output_files   = 0;
 FilterGraph **filtergraphs;
 int        nb_filtergraphs;
 
+Decoder     **decoders;
+int        nb_decoders;
+
 #if HAVE_TERMIOS_H
 
 /* init terminal so that we can grab keys */
@@ -340,6 +343,10 @@ static void ffmpeg_cleanup(int ret)
     for (int i = 0; i < nb_input_files; i++)
         ifile_close(&input_files[i]);
 
+    for (int i = 0; i < nb_decoders; i++)
+        dec_free(&decoders[i]);
+    av_freep(&decoders);
+
     if (vstats_file) {
         if (fclose(vstats_file))
             av_log(NULL, AV_LOG_ERROR,
@@ -404,6 +411,10 @@ InputStream *ist_iter(InputStream *prev)
 
 static void frame_data_free(void *opaque, uint8_t *data)
 {
+    FrameData *fd = (FrameData *)data;
+
+    avcodec_parameters_free(&fd->par_enc);
+
     av_free(data);
 }
 
@@ -430,6 +441,21 @@ static int frame_data_ensure(AVBufferRef **dst, int writable)
             const FrameData *fd_src = (const FrameData *)src->data;
 
             memcpy(fd, fd_src, sizeof(*fd));
+            fd->par_enc = NULL;
+
+            if (fd_src->par_enc) {
+                int ret = 0;
+
+                fd->par_enc = avcodec_parameters_alloc();
+                ret = fd->par_enc ?
+                      avcodec_parameters_copy(fd->par_enc, fd_src->par_enc) :
+                      AVERROR(ENOMEM);
+                if (ret < 0) {
+                    av_buffer_unref(dst);
+                    av_buffer_unref(&src);
+                    return ret;
+                }
+            }
 
             av_buffer_unref(&src);
         } else {

fftools/ffmpeg.h

@@ -331,6 +331,8 @@ typedef struct DecoderOpts {
 typedef struct Decoder {
     const AVClass   *class;
 
+    enum AVMediaType type;
+
     const uint8_t   *subtitle_header;
     int              subtitle_header_size;
 
@@ -606,6 +608,8 @@ typedef struct FrameData {
     int        bits_per_raw_sample;
 
     int64_t wallclock[LATENCY_PROBE_NB];
+
+    AVCodecParameters *par_enc;
 } FrameData;
 
 extern InputFile   **input_files;
@@ -617,6 +621,10 @@ extern int         nb_output_files;
 extern FilterGraph **filtergraphs;
 extern int        nb_filtergraphs;
 
+// standalone decoders (not tied to demuxed streams)
+extern Decoder     **decoders;
+extern int        nb_decoders;
+
 extern char *vstats_filename;
 
 extern float dts_delta_threshold;
@@ -734,6 +742,11 @@ void hw_device_free_all(void);
  */
 AVBufferRef *hw_device_for_filter(void);
 
+/**
+ * Create a standalone decoder.
+ */
+int dec_create(const OptionsContext *o, const char *arg, Scheduler *sch);
+
 /**
  * @param dec_opts Dictionary filled with decoder options. Its ownership
  *                 is transferred to the decoder.
@@ -748,12 +761,21 @@ int dec_init(Decoder **pdec, Scheduler *sch,
              AVFrame *param_out);
 void dec_free(Decoder **pdec);
 
+/*
+ * Called by filters to connect decoder's output to given filtergraph input.
+ *
+ * @param opts filtergraph input options, to be filled by this function
+ */
+int dec_filter_add(Decoder *dec, InputFilter *ifilter, InputFilterOptions *opts);
+
 int enc_alloc(Encoder **penc, const AVCodec *codec,
               Scheduler *sch, unsigned sch_idx);
 void enc_free(Encoder **penc);
 
 int enc_open(void *opaque, const AVFrame *frame);
 
+int enc_loopback(Encoder *enc);
+
 /*
  * Initialize muxing state for the given stream, should be called
  * after the codec/streamcopy setup has been done.