Fix transcoding example about timebase (#69)

* Fix transcoding example about timebase

* applied review feedback

* remove comment

examples/transcoding/main.go

@@ -275,7 +275,7 @@ func openOutputFile() (err error) {
 			} else {
 				s.encCodecContext.SetSampleFormat(s.decCodecContext.SampleFormat())
 			}
-			s.encCodecContext.SetTimeBase(s.decCodecContext.TimeBase())
+			s.encCodecContext.SetTimeBase(astiav.NewRational(1, s.encCodecContext.SampleRate()))
 		} else {
 			s.encCodecContext.SetHeight(s.decCodecContext.Height())
 			if v := s.encCodec.PixelFormats(); len(v) > 0 {
@@ -284,7 +284,7 @@ func openOutputFile() (err error) {
 				s.encCodecContext.SetPixelFormat(s.decCodecContext.PixelFormat())
 			}
 			s.encCodecContext.SetSampleAspectRatio(s.decCodecContext.SampleAspectRatio())
-			s.encCodecContext.SetTimeBase(s.decCodecContext.TimeBase())
+			s.encCodecContext.SetTimeBase(s.decCodecContext.Framerate().Invert())
 			s.encCodecContext.SetWidth(s.decCodecContext.Width())
 		}
 
@@ -376,7 +376,7 @@ func initFilters() (err error) {
 			args = astiav.FilterArgs{
 				"pix_fmt":      strconv.Itoa(int(s.decCodecContext.PixelFormat())),
 				"pixel_aspect": s.decCodecContext.SampleAspectRatio().String(),
-				"time_base":    s.decCodecContext.TimeBase().String(),
+				"time_base":    s.inputStream.TimeBase().String(),
 				"video_size":   strconv.Itoa(s.decCodecContext.Width()) + "x" + strconv.Itoa(s.decCodecContext.Height()),
 			}
 			buffersrc = astiav.FindFilterByName("buffer")

rational.go

@@ -50,3 +50,7 @@ func (r Rational) String() string {
 	}
 	return strconv.Itoa(r.Num()) + "/" + strconv.Itoa(r.Den())
 }
+
+func (r Rational) Invert() Rational {
+	return NewRational(r.Den(), r.Num())
+}

rational_test.go

@@ -10,6 +10,7 @@ func TestRational(t *testing.T) {
 	r := NewRational(2, 1)
 	require.Equal(t, 2, r.Num())
 	require.Equal(t, 1, r.Den())
+	require.Equal(t, 0.5, r.Invert().Float64())
 	r.SetNum(1)
 	r.SetDen(2)
 	require.Equal(t, 1, r.Num())