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• Save a partial video file locally using NodeJS

  25 octobre 2017, par Sami

  I have a serverless web application that is deployed on AWS and I have to take a screenshot from an uploaded video to S3. I am using ffmpeg to extract the screenshot but the only drawback is that I have to download the video file first in order to let ffmpeg work with it.
  Knowing the fact I am using AWS Lambda and I don’t have limits for video length users might upload large files which makes AWS Lambda to hit the storage limit.
  To overcome this I thought of downloading a small chunk of the video and use it with ffmpeg to extract the thumbnail so using the S3.getOjbect method with range params I was able to download a chunk of the file but ffmpeg couldn’t understand it.
  Here is my code :

  s3.getObject({
  
    Bucket: bucketName,
  
    Key: key,
  
    Range: 'bytes=0-1048576'
  
  }, (err, data) => {
  
    fs.writeFile(fileName, data.Body, error => {
  
      if (error)
  
        console.log(error)
  
      else
  
        console.log('File saved');
  
    })
  
  })

  And the code to extract the thumbnail :

  const ffmpeg = require('fluent-ffmpeg');
  
  new ffmpeg(fileName).screenshots({
  
    timestamps: [0],
  
    filename: 'thumb.png',
  
    folder: '.'
  
  })

  And I am getting this error from ffmpeg

  Error: ffmpeg exited with code 1: ./test.mp4: Invalid data found when processing input

  I know there is a problem in saving the file like this but I couldn’t find any solution that solves my problem. If anybody has one that would be much appreciated.

  UPDATE :
  It turns out that ffmpeg does this for me, I just gave it the url and it downloaded what it needs to render the screenshot without the need to download the file locally and the code looks like this :

  const ffmpeg = require('fluent-ffmpeg');
  
  new ffmpeg(url).screenshots({
  
    timestamps: [0],
  
    filename: 'thumb.png',
  
    folder: '.'
  
  })

• Moov atom in android MeidaRecorder recorded data

  11 mars 2013, par mmmaaak

  I have a problem :

  I record data from camera using MediaRecorder in my Android app. I save it in socket, not in file. This data's length may be different. At the other side of socket connection, I save it in file. Connection may be interrupted at any unexpected moment. So after android socket disconnection I try to decode received data using ffmpeg. But as I understood, it cant find moov atom in this file. I've read some info about moov, so I think that MediaRecorder puts moov atom in the end of file. But if recording was interrupted, writing moov atom was skipped.

  I also have read that for data with unknown length (progressive loading, streaming) it is possible to write moov atom at the begining of the file.

  How to write my own moov data into the stream ? May I use MediaRecorder for this ? Or it is neccessary to do it manualy ? How to generate valid moov data ? If anybody has already solved this problem, please give me advice..

• How to save animations with tight layout, transparency and in high quality with matplotlib ?

  29 novembre 2019, par mapf

  I am trying to implement an option in my GUI to save an image sequence displayed using matplotlib. The code looks something like this :

  import matplotlib.pyplot as plt
  
  from matplotlib.backends.backend_qt5agg import \
  
      FigureCanvasQTAgg as FigureCanvas
  
  from matplotlib.animation import FuncAnimation
  
  from PIL import Image
  
  
  
  
  
  plt.rcParams['savefig.bbox'] = 'tight' 
  
  
  
  
  
  class Printer:
  
      def __init__(self, data):
  
          self.fig, self.ax = plt.subplots()
  
          self.canvas = FigureCanvas(self.fig)
  
  
  
          # some irrelevant color adjustment here
  
          #self.ax.spines['bottom'].set_color('#f9f2d7')
  
          #self.ax.spines['top'].set_color('#f9f2d7')
  
          #self.ax.spines['right'].set_color('#f9f2d7')
  
          #self.ax.spines['left'].set_color('#f9f2d7')
  
          #self.ax.tick_params(axis='both', colors='#f9f2d7')
  
          #self.ax.yaxis.label.set_color('#f9f2d7')
  
          #self.ax.xaxis.label.set_color('#f9f2d7')
  
          #self.fig.subplots_adjust(left=0.1, right=0.975, bottom=0.09, top=0.98)
  
          self.fig.patch.set_alpha(0)
  
          self.fig.patch.set_visible(False)
  
          self.canvas.setStyleSheet("background-color:transparent;")
  
          self.fig.set_size_inches(10, 10, True)
  
          self.fig.tight_layout()
  
  
  
          self.data = data
  
          self.image_artist = self.ax.imshow(data[0])
  
  
  
      def animate(self, i):
  
          self.image_artist.set_data(self.data[i])
  
          self.canvas.draw()
  
  
  
  
  
  def save_animation():
  
      data = [
  
          Image.open("test000.png"),
  
          Image.open("test001.png"),
  
      ]
  
      file = 'test.gif'
  
      printer = Printer(data)
  
  
  
      ani = FuncAnimation(
  
          printer.fig, printer.animate, interval=100, frames=len(data),
  
      )
  
      # writer = animation.writers['pillow'](bitrate=1000)
  
      ani.save(
  
          file, writer='pillow', savefig_kwargs={'transparent': True, 'bbox_inches': 'tight'}
  
      )
  
  
  
  
  
  save_animation()

  Transparency :

  As you can see I have already tried several different approaches as suggested elsewhere (1, 2), but didn’t manage to find a solution. All of the settings and arguments patch.set_alpha(0), patch.set_visible(False), canvas.setStyleSheet("background-color:transparent;"), savefig_kwargs={'transparent': True} seem to have no effect at all on the transparency. I found this post but I didn’t get the code to work (for one I had to comment out this %matplotlib inline, but then I ended up getting some error during the MovieWriter.cleanup out = TextIOWrapper(BytesIO(out)).read() TypeError: a bytes-like object is required, not 'str'). Here, it was suggested that this is actually a bug, but the proposed workaroud doesn’t work for me since I would have to rely on third-party software. There also exists this bug report which was supposedly solved, so maybe it is unrelated.

  Tight layout

  I actually couldn’t really find much on this, but all the things I tried (plt.rcParams['savefig.bbox'] = 'tight', fig.tight_layout(), savefig_kwargs={'bbox_inches': 'tight'}) don’t have any effect or are even actively discarded in the case of the bbox_inches argument. How does this work ?

  High quality

  Since I cannot use ImageMagick and can’t get ffmpeg to work (more on this below), I rely on pillow to save my animation. But the only argument in terms of quality that I can pass on seems to be the bitrate, which doesn’t have any effect. The files still have the same size and the animation still looks like mush. The only way that I found to increase the resolution was to use fig.set_size_inches(10, 10, True), but this still doesn’t improve the overall quality of the animation. It still looks bad. I saw that you can pass on codec and extra_args so maybe that is something that might help, but I have no idea how to use these because I couldn’t find a list with allowed arguments.

  ffmpeg

  I can’t get ffmpeg to work. I installed the python package from here and can import it into a python session but I don’t know how I can get matplotlib to use that. I also got ffmpeg from here (Windows 64-bit version) and set the plt.rcParams['animation.ffmpeg_path'] to where I saved the files (there was no intaller to run, not sure if I did it correctly). But this didn’t help either. Also this is of course also third-party software, so if somebody else were to use my code/program it wouldn’t work.