encode: implement --oset-metadata, and --oremove-metadata

This commit introduces a new --oset-metadata key-value-list option,
allowing the user to specify output metadata when encoding
(eg. --oset-metadata=title="Hello",comment="World").

A second option --oremove-metadata is added to exclude existing metadata
from the output file (assuming --ocopy-metadata is enabled).

Not all output formats support all tags, but luckily libavcodec
simply discards unsupported keys.

DOCS/man/encode.rst

@@ -153,3 +153,24 @@ You can encode files from one format/codec to another using this facility.
 ``--no-ocopy-metadata``
     Turns off copying of metadata from input files to output files when
     encoding (which is enabled by default).
+
+``--oset-metadata=<metadata-tag[,metadata-tag,...]>``
+    Specifies metadata to include in the output file.
+    Supported keys vary between output formats. For example, Matroska (MKV) and
+    FLAC allow almost arbitrary keys, while support in MP4 and MP3 is more
+    limited.
+
+    .. admonition:: Example
+
+        "``--oset-metadata=title="Output title",comment="Another tag"``"
+            adds a title and a comment to the output file.
+
+``--oremove-metadata=<metadata-tag[,metadata-tag,...]>``
+    Specifies metadata to exclude from the output file when copying from the
+    input file.
+
+    .. admonition:: Example
+
+        "``--oremove-metadata=comment,genre``"
+            excludes copying of the the comment and genre tags to the output
+            file.

common/encode.h

@@ -50,6 +50,8 @@ struct encode_opts {
     int video_first;
     int audio_first;
     int copy_metadata;
+    char **set_metadata;
+    char **remove_metadata;
 };
 
 // interface for mplayer.c

common/encode_lavc.c

@@ -56,6 +56,8 @@ const struct m_sub_options encode_config = {
         OPT_FLAG("ovfirst", video_first, M_OPT_FIXED),
         OPT_FLAG("oafirst", audio_first, M_OPT_FIXED),
         OPT_FLAG("ocopy-metadata", copy_metadata, M_OPT_FIXED),
+        OPT_KEYVALUELIST("oset-metadata", set_metadata, M_OPT_FIXED),
+        OPT_STRINGLIST("oremove-metadata", remove_metadata, M_OPT_FIXED),
         {0}
     },
     .size = sizeof(struct encode_opts),
@@ -278,8 +280,30 @@ struct encode_lavc_context *encode_lavc_init(struct encode_opts *options,
 void encode_lavc_set_metadata(struct encode_lavc_context *ctx,
                               struct mp_tags *metadata)
 {
-    if (ctx->options->copy_metadata)
+    if (ctx->options->copy_metadata) {
         ctx->metadata = metadata;
+    } else {
+        ctx->metadata = talloc_zero(ctx, struct mp_tags);
+    }
+
+    if (ctx->options->set_metadata) {
+        char **kv = ctx->options->set_metadata;
+        // Set all user-provided metadata tags
+        for (int n = 0; kv[n * 2]; n++) {
+            MP_VERBOSE(ctx, "setting metadata value '%s' for key '%s'\n",
+                       kv[n*2 + 0], kv[n*2 +1]);
+            mp_tags_set_str(ctx->metadata, kv[n*2 + 0], kv[n*2 +1]);
+        }
+    }
+
+    if (ctx->options->remove_metadata) {
+        char **k = ctx->options->remove_metadata;
+        // Remove all user-provided metadata tags
+        for (int n = 0; k[n]; n++) {
+            MP_VERBOSE(ctx, "removing metadata key '%s'\n", k[n]);
+            mp_tags_remove_str(ctx->metadata, k[n]);
+        }
+    }
 }
 
 int encode_lavc_start(struct encode_lavc_context *ctx)