avformat/avformat: add an AVOutputFormat capability flag to signal fixed frame size is needed.

And set it on the IAMF muxer.

Signed-off-by: James Almer <jamrial@gmail.com>

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2025-03-28
 
 API changes, most recent first:
 
+2026-05-16 - xxxxxxxxxxx - lavf 62.16.100 - avformat.h
+  Add AVFMT_FIXED_FRAMESIZE.
+
 2026-05-16 - xxxxxxxxxxx - lavc 62.33.100 - avcodec.h
   Add AV_CODEC_FLAG2_FIXED_FRAME_SIZE.
 

libavformat/avformat.h

@@ -495,6 +495,7 @@ typedef struct AVProbeData {
                                         The user or muxer can override this through
                                         AVFormatContext.avoid_negative_ts
                                         */
+#define AVFMT_FIXED_FRAMESIZE 0x80000 /**< Format wants @ref AVCodecParameters.frame_size "fixed size audio frames." */
 
 #define AVFMT_SEEK_TO_PTS   0x4000000 /**< Seeking is based on PTS */
 
@@ -520,7 +521,7 @@ typedef struct AVOutputFormat {
      * can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER, AVFMT_EXPERIMENTAL,
      * AVFMT_GLOBALHEADER, AVFMT_NOTIMESTAMPS, AVFMT_VARIABLE_FPS,
      * AVFMT_NODIMENSIONS, AVFMT_NOSTREAMS,
-     * AVFMT_TS_NONSTRICT, AVFMT_TS_NEGATIVE
+     * AVFMT_TS_NONSTRICT, AVFMT_TS_NEGATIVE, AVFMT_FIXED_FRAMESIZE
      */
     int flags;
 

libavformat/iamfenc.c

@@ -197,5 +197,5 @@ const FFOutputFormat ff_iamf_muxer = {
     .write_packet      = iamf_write_packet,
     .write_trailer     = iamf_write_trailer,
     .p.codec_tag       = (const AVCodecTag* const []){ iamf_codec_tags, NULL },
-    .p.flags           = AVFMT_GLOBALHEADER | AVFMT_NOTIMESTAMPS,
+    .p.flags           = AVFMT_GLOBALHEADER | AVFMT_NOTIMESTAMPS | AVFMT_FIXED_FRAMESIZE,
 };

libavformat/version.h

@@ -31,8 +31,8 @@
 
 #include "version_major.h"
 
-#define LIBAVFORMAT_VERSION_MINOR  15
-#define LIBAVFORMAT_VERSION_MICRO 101
+#define LIBAVFORMAT_VERSION_MINOR  16
+#define LIBAVFORMAT_VERSION_MICRO 100
 
 #define LIBAVFORMAT_VERSION_INT AV_VERSION_INT(LIBAVFORMAT_VERSION_MAJOR, \
                                                LIBAVFORMAT_VERSION_MINOR, \