Merged and Reverse Videos

Merged and Reverse Videos

Occasionally it can be useful to play videos in reverse. Visually, this can help the viewer see certain details a bit more clearly, such as a pedestrian’s head contacting the A pillar in a Virtual CRASH generated pedestrian impact animation, as the video scrubs backward and forward in time. In this post, we’ll discuss how to make merged and reverse videos.

First, you’ll need to download the free application ffmpeg. You can download ffmpeg here: https://www.ffmpeg.org/download.html

What is ffmpeg? “FFmpeg is the leading multimedia framework, able to decode, encode, transcode, mux, demux, stream, filter and play pretty much anything that humans and machines have created. It supports the most obscure ancient formats up to the cutting edge. No matter if they were designed by some standards committee, the community or a corporation. It is also highly portable: FFmpeg compiles, runs, and passes our testing infrastructure FATE across Linux, Mac OS X, Microsoft Windows, the BSDs, Solaris, etc. under a wide variety of build environments, machine architectures, and configurations.”

Using ffmpeg is also discussed in the vCRASH Academy Blog post: https://www.vcrashusa.com/blog/2017/11/24/360-video-and-virtual-reality-come-to-virtual-crash-4

ffmpeg is a very useful application that allows Virtual CRASH users to convert output avi files to mp4. mp4 files generated with ffmpeg can be much smaller than avi files (depending on what codec is used). 

We like to store the ffmpeg application in the same folder as our avi files. This way, we can simply go to the Windows Command Prompt, and type: 

ffmpeg – i myFileName.avi myFileName.mp4

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That’s all you need to do to convert your avi file to mp4. Below we see the resulting forward video:

To create a reverse video, simply type:

ffmpeg – i myFileName.avi -vf reverse myFileName_reverse.mp4

We see the resulting reverse video below:

Of course, the two videos can also be combined using ffmpeg. This can create the boomerang effect discussed at the beginning of the blog. To concatenate videos with ffmpeg, first make a text file using Notepad. It should have the following format:

Save the file as “list.txt” in the same folder as ffmpeg and your avi (and mp4) files. Note you can concatenate more than two videos. Just add the additional file names to the list.

Next, type the command:

ffmpeg -safe 0 -f concat -i list.txt -c copy myFileName_final.mp4

We see the result below: