"Output file does not contain any stream" encountered while trying to create a SubtitleScene in Manim

[vanleefxp]

While I am using Manim to generate a video subtitle by creating a custom SubtitleScene I encountered a strange behavior.

My subtitle is stored in a JSON file which looks like this:

[
    {
        "content": "Line 1",
        "startTime": 0,
        "endTime": null
    },
    {
        "content": "Line 2",
        "startTime": 3229,
        "endTime": null
    },
    {
        "content": "Line 3",
        "startTime": 6275,
        "endTime": null
    },
    {
        "content": "Line 4",
        "startTime": 8893,
        "endTime": 11909
    },
}

Each line of the subtitle contains three attributes. content is the text to display. startTime and endTime are the start and end time of current line, in milliseconds, where endTime can be null if current line ends right before previous line.

I have written two versions of code to create something like a SubtitleScene that displays a subtitle on the bottom of video view and I compile my Manim file using command like manim -t ./scene.py SubtitleScene. However, only the first version generates the correst subtitle video. The second version generates a blank video and an Output file does not contain any stream warning is shown after compile. So what's the matter? It's so weird that I can't explain it any way.

Here goes the first version:

class SubtitleScene ( Scene ):
    def construct ( self ):
        data = [ ]
        for lineData in json.loads ( ( DIR/"assets/sub.json" ).read_text ( encoding = "utf-8" ) ):
            data.append ( SubtitleLine ( **lineData ) )
        
        mob_subtitle = None
        for i, line in enumerate ( data ):
            print ( f"Line {i+1}: {line.content}" )
            if mob_subtitle is not None:
                self.remove ( mob_subtitle )
            mob_subtitle = Tex ( line.content, **latexConfig )\
                .to_corner ( DOWN, buff = 0.1 )
            mob_subtitleBg = SurroundingRectangle ( mob_subtitle, buff = 0.1 )\
                .set_fill ( color = BLACK, opacity = 0.5 )\
                .set_stroke ( width = 0, opacity = 0 )  
            mob_subtitle = VGroup ( mob_subtitleBg, mob_subtitle )
            self.add ( mob_subtitle )
            start, end = line.startTime, line.endTime
            if end is not None:
                self.wait ( ( end - start ) / 1000 )
                self.remove ( mob_subtitle )
                if i < len ( data ) - 1:
                    nextLine = data [ i + 1 ]
                    nextStart = nextLine.startTime
                    self.wait ( ( nextStart - end ) / 1000 )
            else:
                nextLine = data [ i + 1 ]
                nextStart = nextLine.startTime
                self.wait ( ( nextStart - start ) / 1000 )

And the second version that generates blank video.

class SubtitleScene ( Scene ):
    def construct ( self ):
        import json
        data = json.loads ( 
            ( DIR/"assets/subtitle.json" )\
                .read_text ( encoding = "utf-8" )
        ) [ :10 ]
        
        lastStart = lastEnd = 0
        mob_subtitle = None
        
        for i, line in enumerate ( data ):
            start = line.get ( "startTime" )
            end = line.get ( "endTime" )
            if lastEnd is None:
                lastEnd = start
            print ( lastEnd - lastStart )
            self.wait ( ( lastEnd - lastStart ) / 1000 )
            
            if mob_subtitle is not None:
                self.remove ( mob_subtitle )
            self.wait ( ( start - lastEnd ) / 1000 )
                
            print ( f"Line {i}: {line [ "content" ]}" )
            
            mob_subtitle = Tex ( line [ "content" ], **latexConfig )\
                .to_corner ( DOWN, buff = 0.25 )
            mob_subtitleBg = SurroundingRectangle ( mob_subtitle )\
                .set_fill ( color = BLACK, opacity = 0.5 )\
                .set_stroke ( width = 0, opacity = 0 )
            mob_subtitle = VGroup ( mob_subtitleBg, mob_subtitle )
            
            self.add ( mob_subtitle )
            
            lastStart, lastEnd = start, end