[postprocessor:ugoira] auto-select demuxer by default

docs/configuration.rst

@@ -3239,7 +3239,7 @@ ugoira.ffmpeg-demuxer
 Type
     ``string``
 Default
-    ``concat``
+    ``auto``
 Description
     FFmpeg demuxer to read and process input files with. Possible values are
 
@@ -3247,6 +3247,9 @@ Description
     * "`image2 <https://ffmpeg.org/ffmpeg-formats.html#image2-1>`_" (accurate timecodes, not usable on Windows)
     * "mkvmerge" (accurate timecodes, only WebM or MKV, requires `mkvmerge <ugoira.mkvmerge-location_>`__)
 
+    `"auto"` will select `mkvmerge` if possible and fall back to `image2` or
+    `concat` depending on the local operating system.
+
 
 ugoira.ffmpeg-location
 ----------------------

gallery_dl/postprocessor/ugoira.py

@@ -43,23 +43,28 @@ class UgoiraPP(PostProcessor):
         mkvmerge = options.get("mkvmerge-location")
         self.mkvmerge = util.expand_path(mkvmerge) if mkvmerge else "mkvmerge"
 
-        rate = options.get("framerate", "auto")
-        if rate != "auto":
-            self.calculate_framerate = lambda _: (None, rate)
-
         demuxer = options.get("ffmpeg-demuxer")
-        if demuxer == "image2":
-            self._process = self._process_image2
-            self._finalize = None
-            self.log.debug("using image2 demuxer")
-        elif demuxer == "mkvmerge":
+        if demuxer is None or demuxer == "auto":
+            if self.extension in ("webm", "mkv") and (
+                    mkvmerge or shutil.which("mkvmerge")):
+                demuxer = "mkvmerge"
+            else:
+                demuxer = "concat" if util.WINDOWS else "image2"
+
+        if demuxer == "mkvmerge":
             self._process = self._process_mkvmerge
             self._finalize = self._finalize_mkvmerge
-            self.log.debug("using image2+mkvmerge demuxer")
+        elif demuxer == "image2":
+            self._process = self._process_image2
+            self._finalize = None
         else:
             self._process = self._process_concat
             self._finalize = None
-            self.log.debug("using concat demuxer")
+        self.log.debug("using %s demuxer", demuxer)
+
+        rate = options.get("framerate", "auto")
+        if rate != "auto":
+            self.calculate_framerate = lambda _: (None, rate)
 
         if options.get("libx264-prevent-odd", True):
             # get last video-codec argument