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• How can I speed up the generation of an MP4 using matplotlib's Animation Writer ?

  18 février 2019, par Victor 'Chris' Cabral

  I am using matplotlib to generate a graphical animation of some data. The data has about 4 hours of collection time so I expect the animation to be about 4 hours. However, generating a smaller 60 second video takes approximately 15 minutes. Thus, the total estimated run time for generating the 4 hour video is 2.5 days. I assume I am doing something incredibly inefficient. How can I speed up the creation of an animation with matplotlib ?

  create_graph.py

  import matplotlib.pyplot as plt
  
  import matplotlib.animation as animation
  
  import matplotlib
  
  import pandas as pd
  
  import numpy as np
  
  
  
  matplotlib.use("Agg")
  
  
  
  frame = pd.read_csv("tmp/total.csv")
  
  min_time = frame.iloc[0]["time"]
  
  max_time = frame.iloc[-1]["time"]
  
  total_time = max_time - min_time
  
  
  
  hertz_rate = 50
  
  window_length = 5
  
  save_count = hertz_rate * 100
  
  
  
  def data_gen():
  
      current_index_of_matching_ts = 0
  
      t = data_gen.t
  
      cnt = 0
  
      while cnt < save_count:
  
          print("Done: {}%".format(cnt/save_count*100.0))
  
          predicted = cnt * (1.0/hertz_rate)
  
          while frame.iloc[current_index_of_matching_ts]["time"] - min_time <= predicted and current_index_of_matching_ts < len(frame) - 1:
  
              current_index_of_matching_ts = current_index_of_matching_ts + 1
  
  
  
          y1 = frame.iloc[current_index_of_matching_ts]["var1"]
  
          y2 = frame.iloc[current_index_of_matching_ts]["var2"]
  
          y3 = frame.iloc[current_index_of_matching_ts]["var3"]
  
          y4 = frame.iloc[current_index_of_matching_ts]["var4"]
  
          y5 = frame.iloc[current_index_of_matching_ts]["var5"]
  
          y6 = frame.iloc[current_index_of_matching_ts]["var6"]
  
          y7 = frame.iloc[current_index_of_matching_ts]["var7"]
  
          y8 = frame.iloc[current_index_of_matching_ts]["var8"]
  
          y9 = frame.iloc[current_index_of_matching_ts]["var9"]
  
          t = frame.iloc[current_index_of_matching_ts]["time"] - min_time
  
          # adapted the data generator to yield both sin and cos
  
          yield t, y1, y2, y3, y4, y5, y6, y7, y8, y9
  
          cnt+=1
  
  
  
  data_gen.t = 0
  
  
  
  # create a figure with two subplots
  
  fig, (ax1, ax2, ax3, ax4, ax5, ax6, ax7, ax8, ax9) = plt.subplots(9,1,figsize=(7,14)) # produces a video of 700 × 1400
  
  
  
  # intialize two line objects (one in each axes)
  
  line1, = ax1.plot([], [], lw=2, color='b')
  
  line2, = ax2.plot([], [], lw=2, color='b')
  
  line3, = ax3.plot([], [], lw=2, color='b')
  
  line4, = ax4.plot([], [], lw=2, color='g')
  
  line5, = ax5.plot([], [], lw=2, color='g')
  
  line6, = ax6.plot([], [], lw=2, color='g')
  
  line7, = ax7.plot([], [], lw=2, color='r')
  
  line8, = ax8.plot([], [], lw=2, color='r')
  
  line9, = ax9.plot([], [], lw=2, color='r')
  
  line = [line1, line2, line3, line4, line5, line6, line7, line8, line9]
  
  
  
  # the same axes initalizations as before (just now we do it for both of them)
  
  for ax in [ax1, ax2, ax3, ax4, ax5, ax6, ax7, ax8,  ax9]:
  
      ax.set_ylim(-1.1, 1.1)
  
      ax.grid()
  
  
  
  # initialize the data arrays 
  
  xdata, y1data, y2data, y3data, y4data, y5data, y6data, y7data, y8data, y9data = [], [], [], [], [], [], [], [], [], []
  
  
  
  my_gen = data_gen()
  
  for index in range(hertz_rate*window_length-1):
  
      t, y1, y2, y3, y4, y5, y6, y7, y8, y9 = my_gen.__next__()
  
      xdata.append(t)
  
      y1data.append(y1)
  
      y2data.append(y2)
  
      y3data.append(y3)
  
      y4data.append(y4)
  
      y5data.append(y5)
  
      y6data.append(y6)
  
      y7data.append(y7)
  
      y8data.append(y8)
  
      y9data.append(y9)
  
  
  
  
  
  def run(data):
  
      # update the data
  
      t, y1, y2, y3, y4, y5, y6, y7, y8, y9 = data
  
      xdata.append(t)
  
      y1data.append(y1)
  
      y2data.append(y2)
  
      y3data.append(y3)
  
      y4data.append(y4)
  
      y5data.append(y5)
  
      y6data.append(y6)
  
      y7data.append(y7)
  
      y8data.append(y8)
  
      y9data.append(y9)
  
  
  
      # axis limits checking. Same as before, just for both axes
  
      for ax in [ax1, ax2, ax3, ax4, ax5, ax6, ax7, ax8, ax9]:
  
          ax.set_xlim(xdata[-1]-5.0, xdata[-1])
  
  
  
      # update the data of both line objects
  
      line[0].set_data(xdata, y1data)
  
      line[1].set_data(xdata, y2data)
  
      line[2].set_data(xdata, y3data)
  
      line[3].set_data(xdata, y4data)
  
      line[4].set_data(xdata, y5data)
  
      line[5].set_data(xdata, y6data)
  
      line[6].set_data(xdata, y7data)
  
      line[7].set_data(xdata, y8data)
  
      line[8].set_data(xdata, y9data)
  
  
  
      return line
  
  
  
  ani = animation.FuncAnimation(fig, run, my_gen, blit=True, interval=20, repeat=False, save_count=save_count)
  
  
  
  Writer = animation.writers['ffmpeg']
  
  writer = Writer(fps=hertz_rate, metadata=dict(artist='Me'), bitrate=1800)
  
  ani.save('lines.mp4', writer=writer)

• Decoding bmp images from stdout using go image library

  11 février 2019, par n3wton

  I am trying to decode a bmp image using the image and golang.org/x/image/bmp libraries. The image is output by ffmpeg into stdout. This is the code to get the frame :

  cmd := exec.Command("ffmpeg", "-accurate_seek", "-ss", strconv.Itoa(index), "-i",
  
      filename, "-frames:v", "1", "-hide_banner", "-loglevel", "0", "pipe:.bmp")
  
  var out bytes.Buffer
  
  cmd.Stdout = &out
  
  err := cmd.Run()
  
  if err != nil {
  
      log.Fatal(err)
  
  }
  
  
  
  o := bufio.NewReader(&out)

  and then I decode it using img, _, err := image.Decode(o)
  However this gives an error of "image : unknown format". I have already registered the bmp format in the main method, and I have successfully decoded actual BMP files from disk previously, just not from stdout.

  I have tried just using bmp.Decode instead of image.Decode but this just gives the error "EOF".

  I thought maybe I was not getting the stdout in the correct way, but if I just write it straight to a file :

  o := bufio.NewReader(&out)
  
  
  
  outputfile, err := os.Create("test.bmp")
  
  if err != nil {
  
      log.Fatal(err)
  
  }
  
  defer outputfile.Close()
  
  io.Copy(outputfile, o)

  then it works fine and I can open it.

  Edit : code

• Node.js child_process TypeError : Cannot read property '_writableState' of undefined

  15 mai 2018, par functorial

  I am trying to wrap a piece of ffmpeg’s functionality in a Node.js API, using the child_process library, but when I attempt to send any data to ffmpeg’s stdin pipe, I get an error TypeError: Cannot read property '_writableState' of undefined.

  import {spawn} from "child_process"
  
  
  
  export default frames => {
  
      // Spawn ffmpeg process
  
      const ffmpeg = spawn("ffmpeg", ["-f", "image2pipe", "-i", "-", "output.mkv"])
  
      // Send frames to ffmpeg as stdin
  
      frames.forEach(ffmpeg.stdin.write)
  
  
  
      // Listen for output and errors
  
      return new Promise((resolve, reject) => {
  
          const chunks = []
  
  
  
          ffmpeg.stdout.on("data", chunks.push)
  
          ffmpeg.stderr.on("data", reject(data))
  
          ffmpeg.on("close", code =>
  
              resolve(Buffer.concat(chunks))
  
          )
  
      })
  
  }

  Error :

  TypeError: Cannot read property '_writableState' of undefined
  
      at Writable.write (_stream_writable.js:270:20)
  
      at Array.forEach (<anonymous>)
  
      at exports.default (/home/fiendfan1/workspace/nodejs/declare/dist/app/common/encodeVideo.js:21:12)
  
      at _callee$ (/home/fiendfan1/workspace/nodejs/declare/dist/app/tests/video.js:34:84)
  
      at tryCatch (/home/fiendfan1/workspace/nodejs/declare/node_modules/regenerator-runtime/runtime.js:65:40)
  
      at Generator.invoke [as _invoke] (/home/fiendfan1/workspace/nodejs/declare/node_modules/regenerator-runtime/runtime.js:303:22)
  
      at Generator.prototype.(anonymous function) [as next] (/home/fiendfan1/workspace/nodejs/declare/node_modules/regenerator-runtime/runtime.js:117:21)
  
      at Generator.tryCatcher (/home/fiendfan1/workspace/nodejs/declare/node_modules/bluebird/js/release/util.js:16:23)
  
      at PromiseSpawn._promiseFulfilled (/home/fiendfan1/workspace/nodejs/declare/node_modules/bluebird/js/release/generators.js:97:49)
  
      at Promise._settlePromise (/home/fiendfan1/workspace/nodejs/declare/node_modules/bluebird/js/release/promise.js:574:26)
  
      at Promise._settlePromise0 (/home/fiendfan1/workspace/nodejs/declare/node_modules/bluebird/js/release/promise.js:614:10)
  
      at Promise._settlePromises (/home/fiendfan1/workspace/nodejs/declare/node_modules/bluebird/js/release/promise.js:693:18)
  
      at Async._drainQueue (/home/fiendfan1/workspace/nodejs/declare/node_modules/bluebird/js/release/async.js:133:16)
  
      at Async._drainQueues (/home/fiendfan1/workspace/nodejs/declare/node_modules/bluebird/js/release/async.js:143:10)
  
      at Immediate.Async.drainQueues [as _onImmediate] (/home/fiendfan1/workspace/nodejs/declare/node_modules/bluebird/js/release/async.js:17:14)
  
      at runCallback (timers.js:696:18)
  
  </anonymous>