Omit data streams when outputting to matroska (#253)

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Unreleased (0.8.1)
 * Support negative `--preset` args.
 * Add `--vmaf-fps`: Frame rate override used to analyse both reference & distorted videos.
+* Omit data streams when outputting to matroska (.mkv or .webm).
 * mpeg2video: map `--crf` to ffmpeg `-q` and set default crf range to 2-30.
 
 # v0.8.0

src/ffmpeg.rs

@@ -132,8 +132,8 @@ pub fn encode(
     let output_ext = output.extension().and_then(|e| e.to_str());
 
     let add_faststart = output_ext == Some("mp4") && !oargs.contains("-movflags");
-    let add_cues_to_front =
-        matches!(output_ext, Some("mkv") | Some("webm")) && !oargs.contains("-cues_to_front");
+    let matroska = matches!(output_ext, Some("mkv") | Some("webm"));
+    let add_cues_to_front = matroska && !oargs.contains("-cues_to_front");
 
     let audio_codec = audio_codec.unwrap_or(if downmix_to_stereo && has_audio {
         "libopus"
@@ -163,6 +163,7 @@ pub fn encode(
         .arg2_opt("-vf", vfilter)
         .arg2("-c:s", "copy")
         .arg2("-c:a", audio_codec)
+        .arg_if(matroska, "-dn") // "Only audio, video, and subtitles are supported for Matroska"
         .arg2_if(downmix_to_stereo, "-ac", 2)
         .arg2_if(set_ba_128k, "-b:a", "128k")
         .arg2_if(add_faststart, "-movflags", "+faststart")

src/process.rs

@@ -283,6 +283,9 @@ pub trait CommandExt {
     /// Adds two arguments if `condition` otherwise noop.
     fn arg2_if(&mut self, condition: bool, a: impl ArgString, b: impl ArgString) -> &mut Self;
 
+    /// Adds an argument if `condition` otherwise noop.
+    fn arg_if(&mut self, condition: bool, a: impl ArgString) -> &mut Self;
+
     /// Convert to readable shell-like string.
     fn to_cmd_str(&self) -> String;
 }
@@ -305,6 +308,13 @@ impl CommandExt for tokio::process::Command {
         }
     }
 
+    fn arg_if(&mut self, condition: bool, a: impl ArgString) -> &mut Self {
+        match condition {
+            true => self.arg(a.arg_string()),
+            false => self,
+        }
+    }
+
     fn to_cmd_str(&self) -> String {
         let cmd = self.as_std();
         cmd.get_args().map(|a| a.to_string_lossy()).fold(