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• ffmpeg can't transcode DVD ac3 audio stream, but VLC can play it

  21 février 2020, par RalphORama

  I’m attempting to transcode a DVD to a single MKV file. I’ve had success in the past with other DVDs, but I’m running into an error I haven’t seen before.

  First I concatenate the VOB files I want to transcode :

  cat VTS_02_1.VOB VTS_02_2.VOB VTS_02_3.VOB > WMAV.VOB

  ffprobe output :

  $ ffprobe -analyzeduration 100M -probesize 100M WMAV.VOB                                                                         Input #0, mpeg, from 'WMAV.VOB':
  
    Duration: 01:05:19.42, start: 0.300300, bitrate: 5686 kb/s
  
      Stream #0:0[0x1bf]: Data: dvd_nav_packet
  
      Stream #0:1[0x1e0]: Video: mpeg2video (Main), yuv420p(tv, smpte170m, top first), 720x480 [SAR 32:27 DAR 16:9], 29.97 fps, 29.97 tbr, 90k tbn, 59.94 tbc
  
      Stream #0:2[0x80]: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s
  
  Unsupported codec with id 100357 for input stream 0

  Then I run this command to transcode the file :

  ffmpeg -analyzeduration 100M -probesize 100M \
  
    -i WMAV.VOB \
  
    -map 0:1 -map 0:2 \
  
    -c:v libx264 -preset slow -tune film -crf 21 \
  
    -c:a aac -b:a 192k \
  
    wmav.mkv

  However, when I include -c:a aac, I get thousands of errors like this :

  Error while decoding stream #0:2: Error number -16976906 occurred
  
  [ac3 @ 000002bd24d8eec0] expacc 127 is out-of-range
  
  [ac3 @ 000002bd24d8eec0] error decoding the audio block

  There doesn’t seem to be any issue with the audio stream since it plays back fine in VLC. The transcode succeeds if I use -c:a copy.

  What is causing this error and how could I fix the problem ?

• What does FFmpeg expect me to send to the first rawvideo input pipe ?

  23 octobre 2023, par Somebody

  I'm using two named pipes, in order :

  


  

  1. video_pipe


     -f rawvideo
-video_size 1x1
-pix_fmt gray
-r 1


     


  


  2. audio_pipe


     -f s16le
-ar 32000
-channels 1


     


  


  


  I thought FFmpeg needed to read individual frames from a rawvideo pipe but I must be mistaken cause it doesn't start reading from the second pipe until I feed 11 bytes to the first pipe although, in the example given, a grayscale frame of one pixel is exactly one byte. I have experimenting by increasing video_size and here's the table I could infer :

  


  





  


  


  


  


  



  


  


  


  


  


  


  


  


  


  


  


  


  


  


  


  


  


  


  


  


  


  


  


  


  Actual frame size in bytes	Bytes needed to be sent before to move on
  1	11
  2	17
  3	25
  4	33
  5	41
  6	49

  


  


  I can't just send multiple frames as I want to output a 1 second video.
I tested most of the parameters in this page : https://github.com/FFmpeg/FFmpeg/blob/ff5a3575fec2d49d5fae4ec1198a939e203314db/libavformat/options_table.h
but none of them solved it. (I also used "-re" with no luck).

  


  This is an example command in case you want to reproduce the issue :
ffmpeg -y -re -f rawvideo -video_size 1x1 -pixel_format gray -framerate 1 -i \\.\pipe\video_pipe -f s16le -ar 32000 -channels 1 -i \\.\pipe\audio_pipe -map 0:v:0 -map 1:a:0 out.mp4

  


  Any idea of how I could send the exact frame bytes amount instead of being forced to send way more bytes ?

  


• How to set background transparency for animation with ffmpeg

  23 octobre 2023, par Jan Turowski

  I am creating animated physics graphs with a transparent background for later use in a NLE. On my old machine at work they display and render with background transparency just fine. The exact same code however loses background transparency in the ffmpeg render on both my Linux and my Windows machine at home. The animations are displayed just fine on all machines.

  


  As I first thought it was a Linux issue, I tried to run the code on my Windows machine expecting it to work again. Unfortunately it did not.

  


  Reduced code :

  


  import numpy as np
import matplotlib.pylab as plt
from matplotlib.animation import FuncAnimation
import matplotlib.animation as animation
from matplotlib.pyplot import figure
from matplotlib import style
import locale
from matplotlib.ticker import (MultipleLocator, AutoMinorLocator)
# Set to German locale to get comma decimal separater
locale.setlocale(locale.LC_NUMERIC, "de_DE")
# Tell matplotlib to use the locale we set above
plt.rcParams['axes.formatter.use_locale'] = True

# plt.clf()
# plt.rcdefaults()

# Style und Font definieren

style.use('dark_background')

# Pfeile erstellen
def arrowed_spines(fig, ax):

    xmin, xmax = ax.get_xlim()
    ymin, ymax = ax.get_ylim()

    # removing the default axis on all sides:
    for side in ['bottom','right','top','left']:
        ax.spines[side].set_visible(False)

    # removing the axis ticks
    # plt.xticks([]) # labels
    # plt.yticks([])
    # ax.xaxis.set_ticks_position('none') # tick markers
    # ax.yaxis.set_ticks_position('none')

    # get width and height of axes object to compute
    # matching arrowhead length and width
    dps = fig.dpi_scale_trans.inverted()
    bbox = ax.get_window_extent().transformed(dps)
    width, height = bbox.width, bbox.height

    # manual arrowhead width and length
    hw = 1./20.*(ymax-ymin)
    hl = 1./20.*(xmax-xmin)
    lw = 1. # axis line width
    ohg = 0.3 # arrow overhang

    # compute matching arrowhead length and width
    yhw = hw/(ymax-ymin)*(xmax-xmin)* height/width
    yhl = hl/(xmax-xmin)*(ymax-ymin)* width/height

    # draw x and y axis
    ax.arrow(xmin, 0, xmax-xmin, 0., fc='w', ec='w', lw = lw,
             head_width=hw, head_length=hl, overhang = ohg,
             length_includes_head= True, clip_on = False)

    ax.arrow(0, ymin, 0., ymax-ymin, fc='w', ec='w', lw = lw,
             head_width=yhw, head_length=yhl, overhang = ohg,
             length_includes_head= True, clip_on = False)

# Meine easing-Funktion
def ease(n):
    if n < 0.0:
        return 0
    elif n > 1.0:
        return 1
    else:
        return 3*n**2-2*n**3

# Meine Floor/Warte Funktion
def wait(n):
    if n < 0.0:
        return 0
    else:
        return n

# Canvas erstellen
fig = plt.figure()
ax = fig.add_subplot(111)
fig.set_size_inches([8,9])

def f(x):
    return -0.05*x**2+125
xlin = np.linspace(0,60,100)


# Beschriftung und Optik

plt.xlabel(r"$x$ in $\rm{m}$", horizontalalignment='right', x=1.0)
plt.ylabel(r"$y$ in $\rm{m}$", horizontalalignment='right', y=1.0)
ax.set_xlim(0,100)
ax.set_ylim(0,139)
plt.grid(alpha=.4)
plt.xticks(np.arange(0, 100, 20))
plt.yticks(np.arange(0, 140, 20))
ax.yaxis.set_minor_locator(MultipleLocator(10))
ax.xaxis.set_minor_locator(MultipleLocator(10))
ax.tick_params(axis='x', direction = "inout", length= 10.0, which='both', width=3)
ax.tick_params(axis='y', direction = "inout", length= 10.0, which='both', width=3)


xsub = np.array([0])

# statische Linien definieren
line2, = ax.plot(xsub,f(xsub),linewidth=5,zorder=0,c = 'b')
arrowed_spines(fig, ax)
plt.tight_layout()

# Linien animieren
def animate(i):

    xsub = xlin[0:wait(i-20)]
    global line2
    line2.remove()
    line2, = ax.plot(xsub, f(xsub), linewidth=5, zorder=0,c = "b")
    plt.tight_layout()

animation = FuncAnimation(fig, animate, np.arange(0, 130, 1), interval=100)

plt.show()

# animation.save(r"YOUR\PATH\HERE\reduced_x-y.mov", codec="png",
         dpi=100, bitrate=-1,
         savefig_kwargs={'transparent': True, 'facecolor': 'none'})