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• Matplotlib : animation saved with trace

  25 mai 2020, par Thiziri yoor

  In the following Python code, I am trying to make an animation of a 2 rotating vectors around the origin. I am using matplotlib 3.2.1 and Python 3.8.2 on Ubuntu 20.04.

  



  
import numpy as np
import matplotlib.pyplot as plt

from matplotlib.animation import FuncAnimation


r = 2.0

def circle(phi):
    return np.array([r*np.cos(phi), r*np.sin(phi)])

fig, ax = plt.subplots(figsize=(10,6))
ax.axis([-3.5*r,3.5*r,-2.5*r,2.5*r])
ax.get_xaxis().set_visible(False)
ax.get_yaxis().set_visible(False)

# set equal aspect
ax.set_aspect("equal")

point, = ax.plot(0, r, marker="o")


traj = plt.Circle((0,0), r, fill=False, color='black')
ax.add_artist(traj) # draw the circular trajectory

def update(phi):

    x, y = circle(phi)
    point.set_data([x], [y])
    er_vec = np.array([0.5*x, 0.5*y])
    eθ_vec = np.array([-0.5*y, 0.5*x])


    er_arr = plt.arrow(x, y, dx=er_vec[0], dy=er_vec[1], head_width=0.1, head_length=0.2, color='gray')
    eθ_arr = plt.arrow(x, y, dx=eθ_vec[0], dy=eθ_vec[1], head_width=0.1, head_length=0.2, color='grey')

    annot_er = plt.text(1.7*x, 1.7*y, r'$\mathbf{e}_r$', fontsize=11)
    annot_eθ = plt.text(1.1*(x-0.5*y), 1.1*(y+0.5*x), r'$\mathbf{e}_\theta$', fontsize=11) 

    ax.add_artist(er_arr)    
    ax.add_artist(eθ_arr)
    ax.add_artist(annot_er)
    ax.add_artist(annot_eθ)
    return point, er_arr, eθ_arr, annot_er, annot_eθ

anim = FuncAnimation(fig, update, interval=10, blit=True, repeat=False, frames=np.linspace(0, 2.0*np.pi, 360, endpoint=False))
plt.show()


  



  The code above runs smoothly and without any issues.
This is a screenshot of the animation :

  



  

  



  However, when I try to save the animation to an mp4 video :

  



  anim.save('anim-issue.mp4', writer='ffmpeg')


  



  the animation in the video appears with traces which, something like this screenshot :

  



  

  



  Could someone help me fix that issue with the video animation ?

  



  I appreciate your help.

  



  Edit 1 : According to this answer this is due to blit=True. But that doesn't solve the issue here, since the arrows have no set_position method.

  



  Edit 2 : I found another related question with the same issue I described above but I don't know how to adapt my code to make it work as expected in both cases (plt.show, anim.save).

  


• Android Studio - Append two videos but showing black screen

  6 septembre 2020, par Darkhmar

  First of all hello,

  


  When I add another mp4 file at the end of an mp4 file, the video looks completely black screen but I can get the video sounds.

  


  If I add a copy of a video to the end of it, I saw that there was no problem, but this is not a scenario I want.

  


  I am getting the "text relocations" error when merging with the FFmpeg library. (This problem is fixed when I change this library version, but I have other problems. Therefore, mp4parser is an alternative I see.)

  


  The piece of code that allows the videos to be merged is as follows. Thank you from now.

  


  private void appendTwoVideos(String firstVideoPath, String secondVideoPath) {&#xA;    try {&#xA;        Movie[] inMovies = new Movie[2];&#xA;&#xA;        inMovies[0] = MovieCreator.build(firstVideoPath);&#xA;        inMovies[1] = MovieCreator.build(secondVideoPath);&#xA;&#xA;        List<track> videoTracks = new LinkedList&lt;>();&#xA;        List<track> audioTracks = new LinkedList&lt;>();&#xA;&#xA;        for (Movie m : inMovies) {&#xA;            for (Track t : m.getTracks()) {&#xA;                if (t.getHandler().equals("soun")) {&#xA;                    audioTracks.add(t);&#xA;                }&#xA;                if (t.getHandler().equals("vide")) {&#xA;                    videoTracks.add(t);&#xA;                }&#xA;            }&#xA;        }&#xA;&#xA;        Movie result = new Movie();&#xA;&#xA;        if (audioTracks.size() > 0) {&#xA;            result.addTrack(new AppendTrack(audioTracks&#xA;                    .toArray(new Track[audioTracks.size()])));&#xA;        }&#xA;        if (videoTracks.size() > 0) {&#xA;            result.addTrack(new AppendTrack(videoTracks&#xA;                    .toArray(new Track[videoTracks.size()])));&#xA;        }&#xA;&#xA;        BasicContainer out = (BasicContainer) new DefaultMp4Builder().build(result);&#xA;&#xA;        @SuppressWarnings("resource")&#xA;        FileChannel fc = new RandomAccessFile(DIRECTORY_PATH &#x2B; "output.mp4", "rw").getChannel();&#xA;        out.writeContainer(fc);&#xA;        fc.close();&#xA;    } catch (FileNotFoundException e) {&#xA;        e.printStackTrace();&#xA;    } catch (IOException e) {&#xA;        e.printStackTrace();&#xA;    }&#xA;}&#xA;</track></track>

  


• Why can't Chrome 83 play this AV1 encoded video ?

  25 mai 2020, par Slbox

  The file plays great in VLC.

  



  I've tried adding the codecs="av01.0.05M.08" to the video source, but no luck - no browser will play it, despite what's stated here : https://caniuse.com/#feat=av1

  



  The video was encoded like so :

  



  ffmpeg -i input.avi -pix_fmt yuva420p -f nut -c:v libaom-av1 -minrate 1k -b:v 350k -maxrate 2000k -strict experimental output.mp4


  



  Browser console :

  



  


  • Chrome gives no hint whatsoever as to why it won't play. Firefox gives some minor clues.

  


  • Firefox shows the below

  


  



  Media resource https://example.com/example.mp4 could not be decoded. example.mp4


  



  Media resource https://example.com/example.mp4 could not be decoded, error: Error Code: NS_ERROR_DOM_MEDIA_METADATA_ERR (0x806e0006)
Details: static MP4Metadata::ResultAndByteBuffer __cdecl mozilla::MP4Metadata::Metadata(mozilla::ByteStream *): Cannot parse metadata


  



  Any advice on where I'm going wrong, either in understanding or in encoding arguments, would be greatly appreciated !