video thumbnails.go

func videoThumbnail(ps *pixelstore.PixelStore, sourceFile pixelstore.FileID) (image.Image, error) {
	// To generate thumbnails from video files we use FFMPEG. One challenge is
	// that FFMPEG needs to seek to the end of the video to decode the stream
	// metadata. Saving the whole file to disk would be inefficient. Luckily
	// FFMPEG supports reading (and seeking) videos over HTTP. Here we start a
	// HTTP server on a random TCP port, then the address of the listener is
	// given to FFMPEG to read the file from. When FFMPEG exits we shut down the
	// listener and the HTTP server. The resulting thumbnail image is piped to
	// stdout and saved in a buffer. The buffer is then decoded and returned
	// from the function
	var start = time.Now()
	var listener, err = net.Listen("tcp", "127.0.0.1:0")
	if err != nil {
		return nil, fmt.Errorf("failed to listen on TCP: %w", err)
	}
	defer listener.Close()

	var mux = http.NewServeMux()
	mux.HandleFunc("/", func(rw http.ResponseWriter, r *http.Request) {
		file, err := ps.Read(sourceFile)
		if err != nil {
			log.Error("Failed to read video file: %s", err)
			rw.WriteHeader(http.StatusInternalServerError)
			return
		}

		http.ServeContent(rw, r, "video_file", time.Time{}, file)
		file.Close()
	})

	// Start the HTTP server in the background. This function returns when the
	// TCP listener is closed
	go http.Serve(listener, mux)

	// The start of a video is often black, so we take the frame at half a
	// second for our thumbnail
	cmd := exec.Command(
		"ffmpeg",
		"-i", "http://"+listener.Addr().String(),
		"-ss", "00:00:01.000",
		"-vframes", "1",
		"-c:v", "png",
		"-f", "image2pipe",
		"pipe:1",
	)

	// Create a buffer for holding the image data
	var buf = &bytes.Buffer{}
	cmd.Stdout = buf
	cmd.Stderr = io.Discard

	if err := cmd.Run(); err != nil {
		log.Debug("ffmpeg returned error: %s", err)
		return nil, errThumbnailFileTypeIncompatible
	}

	log.Debug("Generated thumbnail for %s with FFMPEG in %s", sourceFile, time.Since(start))

	// Decode the original file into raw image data
	return imaging.Decode(buf, imaging.AutoOrientation(true))
}