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• Freeze when writing ffmpeg execution result to io.Pipe

  7 November 2023, by alex

  I am writing a wrapper to work with exec.Cmd to work with io.Pipe. The wrapper is intended to simplify work with the file system, I want to convert a video, write the result of the conversion to io.Pipe, read it in the goroutine and transfer it to make a preview of the new converted file. For this purpose I use FFmpeg. But I've encountered a problem, when I start the test the execution just hangs and I can't get any result, but there are no errors, everything just hangs.

  


  CmdRunner source code

  


  type CmdRunner struct {
    commander Commander
    StdIn io.ReadCloser
    StdoutWriter io.WriteCloser
    StdErrWriter io.Writer
}

func (c *CmdRunner) RunByPipe(ctx context.Context) error {
    //done := make(chan error)
    name, args := c.commander.Command()
    cmd := exec.CommandContext(ctx, name, args...)

    if c.StdIn != nil {
        fmt.Print("RunByPipe STDIN\n")
        cmd.Stdin = c.StdIn
    }

    if c.StdoutWriter != nil {
        fmt.Print("RunByPipe STDOUT\n")
        cmd.Stdout = c.StdoutWriter
    }

    stderr := bytes.Buffer{}
    cmd.Stderr = &stderr

    if err := cmd.Start(); err != nil {
        return err
    }

    if err := cmd.Wait(); err != nil {
        return err
    }

    if stderr.String() != "" {
        return fmt.Errorf("want empty stderr, but got %s", stderr.String())
    }

    return nil
}



  


  Unit test code

  


  type TestCommander struct {
    name string
    args []string
}

func (c *TestCommander) SetCommand(name string, args []string) {
    c.name = name
    c.args = args
}

func (c *TestCommander) Command() (string, []string) {
    return c.name, c.args
}

func TestConvert(t *testing.T) {
    ctx := context.Background()
    filePath := "testdata/input_mp4.mp4"
    data, err := convertFile(filePath, ctx)
    outFile := "testdata/output_mp4.mp4"
    if err != nil {
        fmt.Print("ERR: ", err, "\n")
    }

    os.WriteFile(outFile, data, 0644)

}

func convertFile(filePath string, ctx context.Context) (bytes []byte, err error) {
    // Create a CmdRunner instance with your custom Commander.
    runner := &CmdRunner{}
    commander := &TestCommander{}
    args := []string{
        "-nostats",
        "-i", filePath,
        "-y",
        "-loglevel", "0",
        "-filter:v", "fps=30, crop=trunc(iw/2)*2:trunc(ih/2)*2",
        "-c:v", "libx264",
        "-c:a", "aac",
        "-pix_fmt", "yuv420p",
        "-movflags", "frag_keyframe+faststart",
        "-bufsize", "24M",
        "-maxrate", "12M",
        "-f", "mp4",
        "pipe:1",
    }

    commander.SetCommand("ffmpeg", args)
    runner.SetCommander(commander)
    outputPipeReader, outputPipeWriter := io.Pipe()
    runner.SetStdOutWriter(outputPipeWriter)

    wg := &sync.WaitGroup{}
    wg.Add(1)
    go func() {
        defer outputPipeReader.Close()
        defer wg.Done()

        // Read data from output pipe
        bytes, err = io.ReadAll(outputPipeReader)
        if err != nil {
            fmt.Print("\nReadAll err: ", err, "\n")
        }
    }()

    err = runner.RunByPipe(ctx)
    if err != nil {
        fmt.Print("\nRunByPipe err: ", err, "\n")
        return
    }

    wg.Wait()

    return
}


  


  I can't find a clue as to what might have gone wrong.
P.S: But if I remove if err := cmd.Wait(); err != nil { return err } Then the problem goes away, but there will be no data, because nothing is written to io.Pipe.Why?

  


• avutil/opt: Don't cast when the result might be misaligned

  25 March 2024, by Andreas Rheinhardt

  avutil/opt: Don't cast when the result might be misaligned
  A pointer conversion is UB if the resulting pointer is not
  
  correctly aligned for the resultant type, even if no
  
  load/store is ever performed through that pointer (C11 6.3.2.3 (7)).
  This may happen in opt_copy_elem(), because the pointers are
  
  converted even when they belong to a type that does not guarantee
  
  sufficient alignment.
  Fix this by deferring the cast after having checked the type.
  
  Also make the casts -Wcast-qual safe and avoid an indirection
  
  for src.
  Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavutil/opt.c

• FFmpeg: canvas and crop work separately but result in black screen when combined

  25 January, by didi00

  I'm working on a video processing pipeline with FFmpeg, where I:

  


  

  • Create a black canvas using the color filter.

  


  • Crop a region from my video input.

  


  • Overlay the cropped region onto the black canvas.

  


  


  Both the canvas and the crop display correctly when tested individually. However, when I attempt to combine them (overlay the crop onto the canvas), the result is a black screen.
What Works:

  


  Black Canvas Alone:

  


  ffmpeg -filter_complex "color=c=black:s=1920x1080[out]" -map "[out]" -f nut - | ffplay 
-


  


  This shows a plain black screen, as expected.

  


  Cropped Region Alone:

  


  ffmpeg -f v4l2 -input_format yuyv422 -framerate 60 -video_size 1920x1080 -i /dev/video0 
\ -vf "crop=1024:192:0:0" -f nut - | ffplay -


  


  This shows the cropped region of the video correctly.

  


  When I combine these steps to overlay the crop onto the black canvas, I get a black screen:

  


  ffmpeg -f v4l2 -input_format yuyv422 -framerate 60 -video_size 1920x1080 -i /dev/video0 
\-filter_complex "color=c=black:s=1920x1080,format=yuv420p[background]; \
[0:v]crop=1024:192:0:0,format=yuv420p[region0]; \
[background][region0]overlay=x=0:y=0[out]" \
-map "[out]" -f nut - | ffplay -


  


  Environment:

  


  

  • OS: Linux (Debian-based)

  


  • FFmpeg Version: [Insert version, e.g., 4.x or 5.x]

  


  • Capture Card Format: yuyv422

  


  


  Question:

  


  Why does the pipeline result in a black screen when combining the canvas and the crop, even though both work separately? Is this an issue with pixel format compatibility, or is there something I'm overlooking in the overlay filter setup?