Create Video with FFmpeg

#notes #video #FFmpeg #Gnuplot

This blog shows how to create a video from numerical simulation data.

Sources and Tools

Sources

The basic sources are numerical simulation data, which include the time-series vtk files (each file represents the whole simulated system at a single fixed time) and an time augmentation file (a file where each line summarizes the basic geometric information of the system).

Tools

  1. Paraview: import vtk files and export images
  2. Gnuplot: create images from ASCII data
  3. FFmpeg: create video from generated images
  4. bash: formated data generation

Basic steps

Use Paraview to get a time-series images for all vtk files

In paraview, choose Save Animation and ‘png’ (for example) as the exported Files of type, paraview will automatically generate a series of images files.

Make the first video with ffmpeg

Suppose the images obatined from last step are saved with name like figure_#####.png, then you can make a video with these images as

    ffmpeg -framerate 20 -i ./figure_%05d.png -c:v libx264 -profile:v high444 -refs 16 -crf 0 ./video_name.mp4

You may check your video size with

    ffprobe -v error -select_streams v:0 -show_entries stream=width,height -of default=nw=1 ./video_name.mp4

and you may get some outputs like this

    width=2127
    height=1195

If they size are not even, you may need crop it, otherwise you’ll get error in following step when you want embed one video in another. For this, you can just do

    ffmpeg -i ./video_name.mp4 -filter:v "crop=2126:1194:1:1" ./video_name_cropped.mp4

Prepare data for second video

Suppose the time augmentation file named Geo.txt, in which, each line summarizes some geometric information about the simulated system. Now, we need to seperate this file to time-series files (like the vtks), where each file contains the information from start to this real time step. This can be done as

    #!/bin/bash

    # here 2126 is the total number of lines
    for i in {2..2116}; do
      let im1=i-1
      sed -n 2,${i}p Geo.txt  > "$(printf './trace_increment/trace_%05d.txt' "${im1}")"
    done

Make the second video

First, we will use gnuplot to convert each trace_#####.txt to png images, with the following main gnuplot script

    # gnuplot script, trace.gp

    set terminal png size 800,600 enhanced font 'Times-Roman,14'
    Xmax = 10000
    unset xtics
    unset ytics
    unset ztics
    unset xlabel
    unset ylabel
    unset zlabel
    set border 0
    set view 100,19
    set xrange [:Xmax*1.05]
    set yrange [-0.05:0.05]
    set zrange [-0.05:0.05]
    
    set parametric
    set iso 2,100
    set samp 1000
    set urange [0:Xmax*1.05]
    set vrange [0:2*pi]
    unset key
    # http://gnuplot.sourceforge.net/demo/pm3dcolors.html
    # https://stackoverflow.com/questions/12325410/gnuplot-line-opacity-transparency
    #set pm3d
    set pal gray negative
    
    set output 'filename.png'
    set label "d" at 1,0.3,-0.0 font ',50'
    set label "X" at 0,0.3,0.05 font ',45'
    splot u,0.0,0.0 notitle w l lt 8 dt 2 lw 3.0 , \
          filename u 2:3:4 notitle '' with l lt 1 dt 1 lc rgb "red" lw 4.0

and a bash script to call it

    #!/bin/bash
    # bash script, plotFig.sh

    for s in $@; do
        echo $s
        gnuplot -e "filename='$s'" trace.gp
        filename2="${s/txt/png}"
        mv filename.png $filename2
    done

And this video can be made by

    ./plotfig.sh trace_*.txt
    ffmpeg -framerate 20 -i ./trace_%05d.png -c:v libx264 -profile:v high444 -refs 16 -crf 0 ./trace.mp4

And if you want resize the video (for example, from size 800x470 to size of width 600 and with same aspect ratio), you can do

    ffmpeg -i ./trace.mp4 -filter:v scale=600:-1 -c:a copy ./trace_reduceSize.mp4

The -1 will tell ffmpeg to automatically choose the correct height in relation to the provided width to preserve the aspect ratio. -1 can also be used for width if you provide a given height.

Embed one video over another

To embed to second video over the first one, you can do just

    # https://superuser.com/questions/938458/how-to-put-a-video-in-a-corner-of-another-video
    ffmpeg -i video_name_cropped.mp4 -i trace.mp4 -filter_complex "overlay=700:700"  combined.mp4

Now, you finish all steps, and have you final video.

Publish a shorter video

Well, if you want to make your video online avaiable, you may want to make it shorter, here is it

    ffmpeg -i combined.mp4 -ss 00:00:20 -t 00:01:46 -async 1 my_video.mp4