fix: re-encode MP3 and multichannel Opus audio when converting

.sparkle/notes/next.md

@@ -1,2 +1,3 @@
 - Fixed: The Audio Track and Subtitle menus said “None” for files whose tracks they could not see, including the Play Spatial Audio Sample file
 - Fixed: Tracks with no language tag were listed as “Unknown language” rather than by their name or number
+- Fixed: Handling files with multichannel Opus or MP3 audio tracks

Front Row Tests/Conversion/RemuxPlannerTests.swift

@@ -232,8 +232,8 @@ struct RemuxPlannerTests {
         #expect(plan == .unsupported(.noPlayableStreams))
     }
 
-    /// Both are common in Matroska and decode from an MP4 intact, so re-encoding them to AAC threw
-    /// audio away for nothing.
+    /// Both decode from an MP4 intact - FLAC at any channel count, Opus at the stereo the helper
+    /// defaults to - so re-encoding them to AAC threw audio away for nothing.
     @Test(arguments: ["flac", "opus"])
     func losslessAndModernAudioIsCopiedRatherThanReEncoded(codec: String) {
         let plan = RemuxPlanner.plan(for: [video(0, "h264"), audio(1, codec)])
@@ -242,6 +242,33 @@ struct RemuxPlannerTests {
         if case .remux = plan {} else { Issue.record("Expected a straight remux, got \(plan)") }
     }
 
+    /// ffmpeg can only mux MP3 under the `mp4a` tag, and AVFoundation drops that track on open, so
+    /// copying it produced a file that played silently.
+    @Test
+    func mp3IsReEncodedRatherThanCopied() {
+        let plan = RemuxPlanner.plan(for: [video(0, "h264"), audio(1, "mp3")])
+
+        #expect(plan.recipe?.transcodesAudio == true)
+    }
+
+    /// Above two channels Opus carries a channel mapping family AVFoundation will not load, and
+    /// the whole MP4 fails to open rather than merely losing its audio.
+    @Test(arguments: [6, 8])
+    func multichannelOpusIsReEncoded(channels: Int) {
+        let plan = RemuxPlanner.plan(for: [video(0, "h264"), audio(1, "opus", channels: channels)])
+
+        #expect(plan.recipe?.transcodesAudio == true)
+    }
+
+    /// Whether an Opus track can be copied hinges on its channel count, so a missing one is
+    /// re-encoded rather than assumed to be stereo.
+    @Test
+    func opusWithNoChannelCountIsReEncoded() {
+        let plan = RemuxPlanner.plan(for: [video(0, "h264"), audio(1, "opus", channels: nil)])
+
+        #expect(plan.recipe?.transcodesAudio == true)
+    }
+
     /// PCM decodes as well, but copying it keeps a stream far larger than the AAC it would become.
     @Test
     func pcmIsStillReEncoded() {

Front Row UI Tests/ConversionUITests.swift

@@ -46,8 +46,8 @@ final class ConversionUITests: FrontRowUITestCase {
     ///
     /// Written by ffmpeg rather than committed: nothing else on the machine can write Matroska, and
     /// a test that needs ffmpeg installed to reach the conversion at all has no use for a fixture
-    /// that exists to avoid needing it. FLAC because it has to be re-encoded, which is the plan the
-    /// offer alert describes in the most detail.
+    /// that exists to avoid needing it. FLAC copies into an MP4 untouched, so the offer alert
+    /// describes a straight remux.
     private func makeMatroska(named name: String) throws -> URL {
         guard let ffmpeg = locateTool("ffmpeg") else {
             throw XCTSkip("ffmpeg is not installed, so the app has no conversion to offer")

Front Row/Conversion/Models/RemuxPlanner.swift

@@ -34,12 +34,12 @@ enum RemuxPlanner {
 
     /// Audio codecs AVFoundation decodes from an MP4. Everything else is re-encoded to AAC.
     ///
-    /// FLAC and Opus are here because they are common in Matroska and decode from an MP4 intact -
-    /// copying them keeps the original audio where re-encoding it to AAC would throw some away for
-    /// no gain. PCM is deliberately not: it decodes too, but it is far larger than the AAC it
-    /// would become.
+    /// FLAC and Opus are here because they decode from an MP4 intact, and re-encoding them to AAC
+    /// would throw audio away for no gain. PCM and MP3 are deliberately absent: PCM decodes but is
+    /// far larger than the AAC it would become, and MP3 only muxes under the `mp4a` tag, which
+    /// AVFoundation drops on open for a file that plays silently.
     private static let copyableAudioCodecs: Set<String> = [
-        "aac", "mp3", "alac", "ac3", "eac3", "flac", "opus",
+        "aac", "alac", "ac3", "eac3", "flac", "opus",
     ]
 
     /// Subtitle codecs that carry text, and so can become `mov_text`.
@@ -98,7 +98,7 @@ enum RemuxPlanner {
                 index: stream.index,
                 codecName: stream.codecName,
                 channels: stream.channels,
-                transcodes: !copyableAudioCodecs.contains(stream.codecName ?? "")
+                transcodes: !isCopyable(audio: stream)
             )
         }
 
@@ -132,6 +132,18 @@ enum RemuxPlanner {
         }
     }
 
+    /// Opus is capped at stereo. Above two channels it needs channel mapping family 1, which
+    /// AVFoundation will not load at all - the whole file fails to open, not just the track.
+    /// A stream with no channel count is re-encoded rather than guessed at.
+    private static func isCopyable(audio stream: ProbedStream) -> Bool {
+        guard let codecName = stream.codecName, copyableAudioCodecs.contains(codecName) else {
+            return false
+        }
+
+        guard codecName == "opus" else { return true }
+        return stream.channels.map { $0 <= 2 } ?? false
+    }
+
     /// A stream with no pixel format, or one this doesn't recognise, is refused rather than
     /// guessed at. The profile name is not read as a fallback: HEVC 4:2:2 reports itself as
     /// `Rext`, so the profile misses the very case it would be consulted for.

README.md

@@ -50,7 +50,7 @@ brew install ffmpeg
 Open an MKV file and Front Row checks what's inside it, then offers to convert:
 
 - If the video and audio are both formats it can play, they're copied into an MP4 as they are — nothing is re-encoded, and it takes a few seconds
-- If only the audio is unsupported (DTS, TrueHD, FLAC, Opus), the video is still copied and just the audio is re-encoded to AAC
+- If only the audio is unsupported (DTS, TrueHD, MP3, multichannel Opus), the video is still copied and just the audio is re-encoded to AAC
 - Text subtitles come along as `mov_text`. Bitmap subtitles (Blu-ray, DVD) can't be stored in an MP4, and the dialog warns you before you start
 - If the video itself can't be decoded (VP9, for example), Front Row won't offer to convert the file because it will usually take much longer time