fix: ffmpeg v6 does not support VFR mode with a fixed framerate

This change keeps the existing VFR behaviour when the framerate is not set.

Note vsync=2 is the old syntax for pfs_mode=vfr.

lib/video/ffargs.js

@@ -25,10 +25,12 @@ exports.prepare = function (source, target, options) {
   // output framerate
   if (options.framerate !== 0) {
     args.push('-r', options.framerate || DEFAULT_VIDEO_FPS)
+  } else {
+    args.push('-fps_mode', 'vfr')
   }
 
   // misc options
-  args.push('-vsync', '2', '-movflags', '+faststart')
+  args.push('-movflags', '+faststart')
 
   // remove metadata if required
   if (options.keepMetadata) {

test/unit/ffargs.test.js

@@ -57,18 +57,25 @@ describe('ffargs', () => {
       const args = ffargs.prepare('source.mts', 'target.mp4', {})
       const str = args.join(' ')
       should(str).match(/-r 25/)
+      // these are not compatible with -r anymore with ffmpeg v6
+      // we simply default to -fps_mode=auto
+      should(str).not.match(/-vsync/)
+      should(str).not.match(/-fps_mode/)
     })
 
-    it('can specify a value', () => {
+    it('can specify a framerate value', () => {
       const args = ffargs.prepare('source.mts', 'target.mp4', { framerate: 60 })
       const str = args.join(' ')
       should(str).match(/-r 60/)
+      should(str).not.match(/-vsync/)
+      should(str).not.match(/-fps_mode/)
     })
 
     it('keeps the source framerate if set to 0', () => {
       const args = ffargs.prepare('source.mts', 'target.mp4', { framerate: 0 })
       const str = args.join(' ')
       should(str).not.match(/-r/)
+      should(str).match(/-fps_mode vfr/)
     })
   })
 })