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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)

• Decompressing time-To-Sample table (STTS) in an MP4 file

  30 mai 2016, par man-r

  i have an mp4 video byte array and i need to generate a thumbnail for it using its middle frame (e.g. if the video length is 10 seconds then i need to get the picture from 5th second).

  i managed to parse through the file and extract its boxes (atom). i have also managed to get the video length from the mvhd box. also i managed to extract
  1. the time-To-Sample table from stts box,
  2. the sample-To-Chunk table from stcs box,
  3. the chunk-Offset table from stco box,
  4. the sample Size table from stsz box,
  5. the Sync Sample table from stss box

  i know that all the actual media are available in the mdat box and that i need to correlate the above table to find the exact frame offset in the file but my question is how ? the tables data seems to be compressed (specially the time-To-Sample table) but i don’t know how decompress them.

  any help is appreciated.

  below are code samples

  code to convert byte to hex

  public static char[] bytesToHex(byte[] bytes) {
  
      char[] hexChars = new char[bytes.length * 2];
  
      for ( int j = 0; j < bytes.length; j++ ) {
  
          int v = bytes[j] & 0xFF;
  
  
  
          hexChars[j * 2] = hexArray[v >>> 4];
  
          hexChars[j * 2 + 1] = hexArray[v & 0x0F];            
  
      }
  
      return hexChars;
  
  }

  code for getting the box offset

  final static String MOOV                          = "6D6F6F76";
  
  final static String MOOV_MVHD                     = "6D766864";
  
  final static String MOOV_TRAK                     = "7472616B";
  
  final static String MOOV_TRAK_MDIA                = "6D646961";
  
  final static String MOOV_TRAK_MDIA_MINF           = "6D696E66";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL      = "7374626C";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSD = "73747364";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STTS = "73747473";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSS = "73747373";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSC = "73747363";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STCO = "7374636F";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSZ = "7374737A";
  
  
  
  static int getBox(char[] s, int offset, String type) {
  
      int typeOffset = -1;
  
      for (int i = offset*2; i-1) {
  
                  break;
  
              } 
  
          }
  
          i+=(size*2);
  
      }
  
  
  
      return typeOffset;
  
  }

  code for getting the duration and timescale

  static int[] getDuration(char[] s) {
  
      int mvhdOffset = getBox(s, 0, MOOV_MVHD);
  
      int timeScaleStart = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4)*2;
  
      int timeScaleEnd   = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4)*2;
  
  
  
      int durationStart  = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4)*2;
  
      int durationEnd    = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4 + 4)*2;
  
  
  
      String timeScaleHex = new String(Arrays.copyOfRange(s, timeScaleStart, timeScaleEnd));
  
      String durationHex = new String(Arrays.copyOfRange(s, durationStart, durationEnd));
  
  
  
      int timeScale = Integer.parseInt(timeScaleHex, 16);
  
      int duration = Integer.parseInt(durationHex, 16);
  
  
  
      int[] result = {duration, timeScale};
  
      return result;
  
  }

  code to get the time-To-Sample table

  static int[][] getTimeToSampleTable(char[] s, int trakOffset) {
  
      int offset = getBox(s, trakOffset, MOOV_TRAK_MDIA_MINF_STBL_STTS);
  
      int sizeStart = offset*2;
  
      int sizeEnd   = offset*2 + (4)*2;
  
  
  
      int typeStart = offset*2 + (4)*2;
  
      int typeEnd   = offset*2 + (4 + 4)*2;
  
  
  
      int noOfEntriesStart = offset*2 + (4 + 4 + 1 + 3)*2;
  
      int noOfEntriesEnd   = offset*2 + (4 + 4 + 1 + 3 + 4)*2;
  
  
  
      String sizeHex = new String(Arrays.copyOfRange(s, sizeStart, sizeEnd));
  
      String typeHex = new String(Arrays.copyOfRange(s, typeStart, typeEnd));
  
      String noOfEntriesHex = new String(Arrays.copyOfRange(s, noOfEntriesStart, noOfEntriesEnd));
  
  
  
      int size = Integer.parseInt(sizeHex, 16);
  
      int noOfEntries = Integer.parseInt(noOfEntriesHex, 16);
  
  
  
      int[][] timeToSampleTable = new int[noOfEntries][2];
  
  
  
      for (int i = 0; icode>

• MP4 file data structure

  16 décembre 2015, par man-r

  i have an mp4 video byte array and i need to generate a thumbnail for it using its middle frame (e.g. if the video length is 10 seconds then i need to get the picture from 5th second).

  i managed to parse through the file and extract its boxes (atom). i have also managed to get the video length from the mvhd box. also i managed to extract
  1. the time-To-Sample table from stts box,
  2. the sample-To-Chunk table from stcs box,
  3. the chunk-Offset table from stco box,
  4. the sample Size table from stsz box,
  5. the Sync Sample table from stss box

  i know that all the actual media are available in the mdat box and that i need to correlate the above table to find the exact frame offset in the file but my question is how ? the tables data seems to be compressed (specially the time-To-Sample table) but i don’t know how decompress them.

  any help is appreciated.

  below are code samples

  code to convert byte to hex

  public static char[] bytesToHex(byte[] bytes) {
  
      char[] hexChars = new char[bytes.length * 2];
  
      for ( int j = 0; j < bytes.length; j++ ) {
  
          int v = bytes[j] & 0xFF;
  
  
  
          hexChars[j * 2] = hexArray[v >>> 4];
  
          hexChars[j * 2 + 1] = hexArray[v & 0x0F];            
  
      }
  
      return hexChars;
  
  }

  code for getting the box offset

  final static String MOOV                          = "6D6F6F76";
  
  final static String MOOV_MVHD                     = "6D766864";
  
  final static String MOOV_TRAK                     = "7472616B";
  
  final static String MOOV_TRAK_MDIA                = "6D646961";
  
  final static String MOOV_TRAK_MDIA_MINF           = "6D696E66";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL      = "7374626C";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSD = "73747364";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STTS = "73747473";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSS = "73747373";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSC = "73747363";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STCO = "7374636F";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSZ = "7374737A";
  
  
  
  static int getBox(char[] s, int offset, String type) {
  
      int typeOffset = -1;
  
      for (int i = offset*2; i-1) {
  
                  break;
  
              } 
  
          }
  
          i+=(size*2);
  
      }
  
  
  
      return typeOffset;
  
  }

  code for getting the duration and timescale

  static int[] getDuration(char[] s) {
  
      int mvhdOffset = getBox(s, 0, MOOV_MVHD);
  
      int timeScaleStart = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4)*2;
  
      int timeScaleEnd   = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4)*2;
  
  
  
      int durationStart  = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4)*2;
  
      int durationEnd    = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4 + 4)*2;
  
  
  
      String timeScaleHex = new String(Arrays.copyOfRange(s, timeScaleStart, timeScaleEnd));
  
      String durationHex = new String(Arrays.copyOfRange(s, durationStart, durationEnd));
  
  
  
      int timeScale = Integer.parseInt(timeScaleHex, 16);
  
      int duration = Integer.parseInt(durationHex, 16);
  
  
  
      int[] result = {duration, timeScale};
  
      return result;
  
  }

  code to get the time-To-Sample table

  static int[][] getTimeToSampleTable(char[] s, int trakOffset) {
  
      int offset = getBox(s, trakOffset, MOOV_TRAK_MDIA_MINF_STBL_STTS);
  
      int sizeStart = offset*2;
  
      int sizeEnd   = offset*2 + (4)*2;
  
  
  
      int typeStart = offset*2 + (4)*2;
  
      int typeEnd   = offset*2 + (4 + 4)*2;
  
  
  
      int noOfEntriesStart = offset*2 + (4 + 4 + 1 + 3)*2;
  
      int noOfEntriesEnd   = offset*2 + (4 + 4 + 1 + 3 + 4)*2;
  
  
  
      String sizeHex = new String(Arrays.copyOfRange(s, sizeStart, sizeEnd));
  
      String typeHex = new String(Arrays.copyOfRange(s, typeStart, typeEnd));
  
      String noOfEntriesHex = new String(Arrays.copyOfRange(s, noOfEntriesStart, noOfEntriesEnd));
  
  
  
      int size = Integer.parseInt(sizeHex, 16);
  
      int noOfEntries = Integer.parseInt(noOfEntriesHex, 16);
  
  
  
      int[][] timeToSampleTable = new int[noOfEntries][2];
  
  
  
      for (int i = 0; icode>