taglib documentation.

README.md

@@ -26,7 +26,8 @@ For encoding, also install:
 
 The Opus encoder backend uses libopusenc directly.  The FLAC encoder backend
 uses libFLAC directly.  FLAC sample-rate conversion uses the existing FFmpeg
-swresample layer.
+swresample layer.  Metadata and cover-art copying use the TagLib wrapper; the
+public `taglib.rkt` API also supports read-write tag editing.
 
 ## macOS
 

scrbl/audio-encoder.scrbl

@@ -62,8 +62,8 @@ file extension.  Pass @racket['opus] or @racket['flac] to force a backend.
 When @racket[copy-tags?] is true, common textual tags and an embedded picture
 are copied from the source file to the destination file.  Opus comments and
 cover art are written before encoding starts through @tt{libopusenc}.  FLAC
-metadata is copied after the encoded file has been written, using the TagLib
+metadata is copied after the encoded file has been written, using the
-wrapper.
+read-write API from @racketmodname[racket-audio/taglib].
 
 When @racket[progress-callback] is a procedure, it is called with a progress
 hash during encoding.  Progress is based on the number of input frames read from

scrbl/taglib.scrbl

@@ -9,48 +9,72 @@
 @title{TagLib Metadata}
 @author[@author+email["Hans Dijkema" "hans@dijkewijk.nl"]]
 
-
 @defmodule[racket-audio/taglib]
 
 The @racketmodname[racket-audio/taglib] module provides the high level metadata
-reader used by the audio package.  It wraps the lower level TagLib FFI module
+API used by the audio package.  It wraps the lower level TagLib C FFI module and
-and presents a small, read-only Racket API for common tags, audio properties,
+presents a Racket API for common tags, generic properties, audio properties, and
-generic properties, and embedded cover art.
+embedded cover art.
 
-Calling @racket[id3-tags] opens the file through TagLib, copies the values that
+The module can be used in two modes.  The default mode is read-only and returns
-are needed on the Racket side, reads the optional embedded picture, frees the
+a snapshot of the metadata.  A handle opened with @racket[#:mode 'read-write]
-native TagLib objects, and returns an opaque tag handle.  The handle is
+keeps the native TagLib file open and can be modified with the setter
-therefore a snapshot of the metadata at the time it was read.  It does not keep
+procedures documented below.  Changes are written to the media file by calling
-the media file or the native TagLib handle open.
+@racket[tags-save!].
 
-The name @racket[id3-tags] is historical.  The module uses TagLib to open the
+The name @racket[id3-tags] is historical.  The implementation uses TagLib, so
-file, so the usable file types are the file types supported by the TagLib
+the usable file types are the file types supported by the TagLib library
-library available at run time.  This module is not a tag editor; it only reads
+available at run time.
-metadata.
 
-@section{Reading metadata}
+@section{Opening and closing tag handles}
 
-@defproc[(id3-tags [file path-string?]) any/c]{
+@defproc[(id3-tags [file path-string?]
-Reads metadata from @racket[file] and returns an opaque tag handle.  The
+                   [#:mode mode (or/c 'read 'read-only 'read-write 'write) 'read])
-argument may be a path or a string.  On Windows, the implementation retries
+         any/c]{
-with the wide-character TagLib open function when the normal open function does
+Opens @racket[file] through TagLib and returns an opaque tag handle.  In the
-not produce a valid TagLib file.
+default read-only mode, the module copies the values needed on the Racket side,
+frees the native TagLib objects, and returns a snapshot handle.
 
-The returned handle is passed to the other procedures in this module.  If the
+In read-write mode, the native TagLib file remains open.  Setter procedures may
-file cannot be opened, @racket[id3-tags] still returns a handle, but
+then be used to modify fields, properties, and pictures.  Call
-@racket[tags-valid?] returns @racket[#f].  Other accessors then return their
+@racket[tags-save!] to write changes and @racket[tags-close!] to close the
-default values, such as @racket[""], @racket[-1], @racket['()], or
+native handle.
-@racket[#f].}
 
-@defproc[(tags-valid? [tags any/c]) boolean?]{
+On Windows, the implementation retries with the wide-character TagLib open
-Returns @racket[#t] when @racket[id3-tags] successfully opened the file and
+function when the normal open function does not produce a valid TagLib file.}
-TagLib reported it as valid.}
+
+@defproc[(call-with-id3-tags [file path-string?]
+                             [proc procedure?]
+                             [#:mode mode (or/c 'read 'read-only 'read-write 'write) 'read])
+         any/c]{
+Opens @racket[file], calls @racket[proc] with the tag handle, and closes the
+handle afterwards with @racket[tags-close!].  This is most useful for
+read-write code because it avoids leaking the native TagLib file handle.}
+
+@deftogether[
+(@defproc[(tags-valid? [tags any/c]) boolean?]
+ @defproc[(tags-read-write? [tags any/c]) boolean?]
+ @defproc[(tags-closed? [tags any/c]) boolean?])]{
+Return handle state.  @racket[tags-valid?] reports whether TagLib opened the
+file successfully.  @racket[tags-read-write?] reports whether the handle was
+opened in read-write mode.  @racket[tags-closed?] reports whether the native
+TagLib file handle has been closed.}
+
+@deftogether[
+(@defproc[(tags-save! [tags any/c]) boolean?]
+ @defproc[(tags-close! [tags any/c]) void?])]{
+@racket[tags-save!] writes pending changes for a read-write handle to the media
+file.  @racket[tags-close!] closes the native TagLib file handle.  Closing a
+read-only snapshot is harmless.
+}
 
 @racketblock[
-(define tags (id3-tags "song.mp3"))
+(call-with-id3-tags "track.flac"
-
+  (lambda (tags)
     (when (tags-valid? tags)
-  (printf "~a - ~a\n" (tags-artist tags) (tags-title tags)))]
+      (tags-title! tags "New title")
+      (tags-save! tags)))
+  #:mode 'read-write)]
 
 @section{Common tag fields}
 
@@ -59,32 +83,52 @@ TagLib reported it as valid.}
  @defproc[(tags-album [tags any/c]) string?]
  @defproc[(tags-artist [tags any/c]) string?]
  @defproc[(tags-comment [tags any/c]) string?]
- @defproc[(tags-genre [tags any/c]) string?])]
+ @defproc[(tags-genre [tags any/c]) string?])]{
 Return the common textual fields from the TagLib tag interface.  Missing fields
-are returned as the empty string.
+are returned as the empty string.}
 
 @deftogether[
 (@defproc[(tags-year [tags any/c]) integer?]
- @defproc[(tags-track [tags any/c]) integer?])]
+ @defproc[(tags-track [tags any/c]) integer?])]{
 Return the year and track number from the common TagLib tag interface.  Missing
-numeric values are returned as @racket[-1].
+numeric values are returned as @racket[-1].}
 
 @deftogether[
-(@defproc[(tags-composer [tags any/c])
+(@defproc[(tags-title! [tags any/c] [value (or/c string? 'clear)]) void?]
-          (or/c string? (listof string?))]
+ @defproc[(tags-album! [tags any/c] [value (or/c string? 'clear)]) void?]
- @defproc[(tags-album-artist [tags any/c])
+ @defproc[(tags-artist! [tags any/c] [value (or/c string? 'clear)]) void?]
-          (or/c string? (listof string?))]
+ @defproc[(tags-comment! [tags any/c] [value (or/c string? 'clear)]) void?]
- @defproc[(tags-disc-number [tags any/c])
+ @defproc[(tags-genre! [tags any/c] [value (or/c string? 'clear)]) void?])]{
-          (or/c number? #f)])] 
+Set common textual fields on a read-write handle.  Passing @racket['clear]
-Return selected values from the generic TagLib property store.  The composer is
+clears the field.  Call @racket[tags-save!] to persist the change.}
-read from the lower-case @racket['composer] key, the album artist from
-@racket['albumartist], and the disc number from @racket['discnumber].
 
-Composer and album artist return a list of strings when the property is present
+@deftogether[
-and the empty string when it is missing.  The disc number is parsed from the
+(@defproc[(tags-year! [tags any/c] [value (or/c exact-nonnegative-integer? 'clear)]) void?]
-first property value and defaults to @racket[-1].  If the stored value cannot be
+ @defproc[(tags-track! [tags any/c] [value (or/c exact-nonnegative-integer? 'clear)]) void?])]{
-parsed as a number, the result may be @racket[#f].  Use @racket[tags-keys] and
+Set numeric common fields on a read-write handle.  Passing @racket['clear]
-@racket[tags-ref] for direct access to the complete generic property store.
+writes zero through the TagLib C API and updates the Racket-side cache to
+@racket[-1].}
+
+@section{Selected generic fields}
+
+@deftogether[
+(@defproc[(tags-composer [tags any/c]) (or/c string? (listof string?))]
+ @defproc[(tags-album-artist [tags any/c]) (or/c string? (listof string?))]
+ @defproc[(tags-disc-number [tags any/c]) (or/c number? #f)])]{
+Return selected values from the generic TagLib property store.  The composer is
+read from the @racket['composer] key, the album artist from
+@racket['albumartist], and the disc number from @racket['discnumber].  Use
+@racket[tags-keys] and @racket[tags-ref] for direct access to the complete
+generic property store.}
+
+@deftogether[
+(@defproc[(tags-composer! [tags any/c] [value (or/c string? 'clear)]) void?]
+ @defproc[(tags-album-artist! [tags any/c] [value (or/c string? 'clear)]) void?]
+ @defproc[(tags-disc-number! [tags any/c]
+                            [value (or/c exact-nonnegative-integer? string? 'clear)])
+          void?])]{
+Set selected generic properties on a read-write handle.  The disc number may be
+provided as a number or as the exact string that should be written.}
 
 @section{Audio properties}
 
@@ -92,10 +136,11 @@ parsed as a number, the result may be @racket[#f].  Use @racket[tags-keys] and
 (@defproc[(tags-length [tags any/c]) integer?]
  @defproc[(tags-sample-rate [tags any/c]) integer?]
  @defproc[(tags-bit-rate [tags any/c]) integer?]
- @defproc[(tags-channels [tags any/c]) integer?])] 
+ @defproc[(tags-channels [tags any/c]) integer?])]{
 Return audio properties reported by TagLib: length in seconds, sample rate in
-Hz, bit rate in kbit/s, and number of channels.  Missing values are returned as
+Hz, bit rate in kbit/s, and number of channels.  These values are read-only
-@racket[-1].
+properties of the media stream, not editable tags.  Missing values are returned
+as @racket[-1].}
 
 @section{Generic properties}
 
@@ -109,9 +154,39 @@ Returns the list of values associated with @racket[key], or @racket[#f] when the
 property was not found.  Use lower-case symbol keys, matching the values
 returned by @racket[tags-keys].}
 
+@defproc[(tags-set! [tags any/c]
+                    [key (or/c symbol? string?)]
+                    [value (or/c string? 'clear)])
+         void?]{
+Sets a generic property on a read-write handle.  Symbol keys are converted to
+upper-case TagLib property names; string keys are passed as supplied.  Passing
+@racket['clear] clears the property.}
+
+@defproc[(tags-set-values! [tags any/c]
+                           [key (or/c symbol? string?)]
+                           [values (or/c (listof string?) 'clear)])
+         void?]{
+Replaces a generic property with zero or more values.  Passing @racket['clear]
+removes the property.}
+
+@defproc[(tags-append! [tags any/c]
+                       [key (or/c symbol? string?)]
+                       [value string?])
+         void?]{
+Appends a value to a generic property on a read-write handle.}
+
+@defproc[(tags-clear! [tags any/c]
+                      [key (or/c symbol? string?)])
+         void?]{
+Clears a generic property on a read-write handle.}
+
 @racketblock[
-(for ([key (in-list (tags-keys tags))])
+(call-with-id3-tags "track.flac"
-  (printf "~a: ~s\n" key (tags-ref tags key)))]
+  (lambda (tags)
+    (tags-set-values! tags 'composer '("Johann Sebastian Bach"))
+    (tags-set! tags 'discnumber "1")
+    (tags-save! tags))
+  #:mode 'read-write)]
 
 Generic properties may contain multiple values for a single key.  The API keeps
 those values as lists instead of joining them into one string.
@@ -120,8 +195,8 @@ those values as lists instead of joining them into one string.
 
 The module represents embedded artwork as an opaque @deftech{picture value}.
 The picture value is returned by @racket[tags-picture] and can be inspected with
-the picture procedures documented below.  When no picture is available, the
+the picture procedures documented below.  It can also be written to another
-picture-related procedures return @racket[#f].
+file with @racket[tags-picture!] or @racket[tags-append-picture!].
 
 @defproc[(tags-picture [tags any/c]) (or/c any/c #f)]{
 Returns the embedded picture value, or @racket[#f] when the file has no picture
@@ -130,14 +205,15 @@ that the underlying FFI layer could read.}
 @deftogether[
 (@defproc[(tags-picture->kind [tags any/c]) (or/c integer? #f)]
  @defproc[(tags-picture->mimetype [tags any/c]) (or/c string? #f)]
+ @defproc[(tags-picture->description [tags any/c]) (or/c string? #f)]
  @defproc[(tags-picture->size [tags any/c]) (or/c integer? #f)]
- @defproc[(tags-picture->ext [tags any/c]) (or/c symbol? #f)])] 
+ @defproc[(tags-picture->ext [tags any/c]) (or/c symbol? #f)])]{
 Return selected information about the embedded picture.  The kind is the
 numeric picture type reported by the FFI layer.  The MIME type is the stored
 MIME type, such as @racket["image/jpeg"] or @racket["image/png"].  The size is
 the number of bytes in the embedded image.  The extension helper returns
 @racket['jpg], @racket['png], or @racket[#f] when the MIME type is not
-recognized.
+recognized.}
 
 @defproc[(tags-picture->bitmap [tags any/c])
          (or/c (is-a?/c bitmap%) #f)]{
@@ -150,41 +226,85 @@ Reads the embedded picture bytes with @racket[read-bitmap] and returns a
          boolean?]{
 Writes the embedded picture bytes to @racket[path] in binary mode, replacing an
 existing file.  The procedure returns @racket[#t] when a picture was written and
-@racket[#f] when the tag handle has no picture.  The file name is not adjusted
+@racket[#f] when the tag handle has no picture.}
-automatically; use @racket[tags-picture->ext] when the caller wants to choose an
+
-extension from the MIME type.}
+@defproc[(make-tags-picture [mimetype string?]
+                            [kind integer?]
+                            [data (or/c bytes? (is-a?/c bitmap%))]
+                            [#:description description string? ""])
+         id3-picture?]{
+Creates a picture value from encoded image bytes or from a @racket[bitmap%].
+The MIME type should normally be @racket["image/jpeg"] or @racket["image/png"].}
+
+@defproc[(make-tags-picture-from-bitmap [bitmap (is-a?/c bitmap%)]
+                                        [kind integer?]
+                                        [#:mimetype mimetype string? "image/png"]
+                                        [#:description description string? ""])
+         id3-picture?]{
+Creates a picture value by encoding @racket[bitmap] as PNG or JPEG.}
+
+@deftogether[
+(@defproc[(tags-picture! [tags any/c]
+                          [picture (or/c id3-picture? 'clear)])
+          void?]
+ @defproc[(tags-append-picture! [tags any/c]
+                                 [picture id3-picture?])
+          void?]
+ @defproc[(tags-clear-picture! [tags any/c]) void?])]{
+Set, append, or clear embedded artwork on a read-write handle.  The procedures
+use TagLib complex properties underneath.  Call @racket[tags-save!] to persist
+the change.}
 
 @racketblock[
-(define ext (tags-picture->ext tags))
+(define cover
+  (make-tags-picture "image/jpeg" 3 (file->bytes "cover.jpg")
+                     #:description "Cover"))
 
-(when ext
+(call-with-id3-tags "track.flac"
-  (tags-picture->file tags
+  (lambda (tags)
-                      (format "cover.~a" ext)))]
+    (tags-picture! tags cover)
+    (tags-save! tags))
+  #:mode 'read-write)]
 
 @section{Picture values}
 
 @deftogether[
-(@defproc[(id3-picture-mimetype [picture any/c]) string?]
+(@defproc[(id3-picture? [v any/c]) boolean?]
- @defproc[(id3-picture-kind [picture any/c]) integer?]
+ @defproc[(id3-picture-mimetype [picture id3-picture?]) string?]
- @defproc[(id3-picture-size [picture any/c]) integer?]
+ @defproc[(id3-picture-kind [picture id3-picture?]) integer?]
- @defproc[(id3-picture-bytes [picture any/c]) bytes?])] 
+ @defproc[(id3-picture-size [picture id3-picture?]) integer?]
-Access the fields of a picture value returned by @racket[tags-picture].  These
+ @defproc[(id3-picture-bytes [picture id3-picture?]) bytes?]
-procedures are useful when the caller wants to process the image bytes directly
+ @defproc[(id3-picture-description [picture id3-picture?]) string?])]{
-instead of converting them to a bitmap or writing them to a file.
+Access the fields of a picture value.  These procedures are useful when the
+caller wants to process the image bytes directly or pass a picture to another
+component.}
 
 @section{Converting to a hash}
 
 @defproc[(tags->hash [tags any/c]) hash?]{
 Returns a mutable hash containing the core values copied from the tag handle.
-The hash contains the keys @racket['valid?], @racket['title], @racket['album],
+The hash contains the keys @racket['valid?], @racket['read-write?],
-@racket['artist], @racket['comment], @racket['composer], @racket['genre],
+@racket['closed?], @racket['title], @racket['album], @racket['artist],
-@racket['year], @racket['track], @racket['length], @racket['sample-rate],
+@racket['comment], @racket['composer], @racket['genre], @racket['year],
-@racket['bit-rate], @racket['channels], @racket['picture], and @racket['keys].
+@racket['track], @racket['length], @racket['sample-rate], @racket['bit-rate],
+@racket['channels], @racket['picture], and @racket['keys].
 
 The hash is intended as a convenient snapshot for application code.  Generic
 property values are not expanded into the hash; use @racket[tags-ref] for those
 values.}
 
+@section{Copying tags and pictures}
+
+The encoder pipeline uses this module for metadata transfer.  For FLAC output,
+@racket[audio-encode] first writes the audio stream and then opens the resulting
+file with @racket[id3-tags] in read-write mode to copy tags and pictures through
+TagLib.  For Opus output, comments and pictures are supplied to
+@tt{libopusenc} before encoding starts, because OpusTags are written at the
+start of the Ogg Opus stream.
+
+Applications that need explicit metadata editing should use the read-write API
+directly, as in the examples above.
+
 @section{Example}
 
 @racketblock[
@@ -209,10 +329,10 @@ This chapter documents the public @racketmodname["taglib.rkt"] layer.  The
 native TagLib calls are delegated to @racketmodname["taglib-ffi.rkt"], but
 callers normally should not use that lower level module directly.
 
-The tag handle is implemented as a small Racket object with a private dispatch
+A read-only tag handle is a Racket-side snapshot.  A read-write tag handle keeps
-procedure.  The native TagLib file is not stored in the handle.  This keeps the
+the native TagLib file open until @racket[tags-close!] is called.  Setter
-public API simple and prevents native resources from leaking into application
+procedures update both the native file and the Racket-side cache; the changes
-code.
+are persisted only after @racket[tags-save!] succeeds.
 
 The implementation normalizes generic property names by lower-casing TagLib
 property keys and converting them to symbols.  Values remain lists of strings