Category Archives: K’NEX

Video editing with PiTiVi

For my last video project I used Blender. It is a very capable video editor but it has a steep learning curve and is complex. I wanted something easier that I could show the kids how to use. They have had some exposure to iMovie at school. Ubuntu 10.04 (Lucid) comes with a few more video editing options than past releases. I decided to give PiTiVi a try.

Over all PiTiVi is fairly good for such a young product. It has lots of potential to be both powerful and easy to use. The documentation is good, straight forward, and easy to understand. It does leave out some background or supplemental information which I had to figure out on my own. This is not a tutorial, just some information that may be useful to others trying it out.

The video project I choose is a marble timer built with K’NEX that my daughter Isabella and I made for a school project of hers last year. The video was shot with a Flip HD camera.

I installed the latest PiTiVi version. There really is good reason to use a newer version than the one that comes with the Lucid software sources. There are many improvements and new features. The PiTiVi download page gives instructions on how install from the Ubuntu PPA (Personal Package Archive). I used version 0.14.0-2.

The Flip takes nice video but the HD resolution (1280×720) can make it difficult to work with on underpowered computers. I have an old Z61m ThinkPad with a 64MB ATI Mobility Radeon x1300 video card. It has no problem playing back HD video but it could not handle editing.

The PiTiVi documentation talks about friendly video formats. It says frame-independent compression formats such as MJPEG will perform better. I figured editing may work better if I convert my Flip video, which uses a H.264 / AVC codec to MJPEG or even uncompressed. I tried both. The uncompressed AVI files were huge and didn’t perform any better so I stuck with MJPEG. It took me a while to figure out how to convert the files. Since PiTiVi is based on GStreamer I decided to try using it. There may be other tools for this. I had to learn a lot about Gstreamer and its command line program gst-launch. The command line I finally figured out is this (just change the src and sink file names):


gst-launch filesrc location=VID00001.MP4 ! decodebin name=demux demux. ! queue ! ffmpegcolorspace ! jpegenc ! mux. demux. ! queue ! mux. avimux name=mux ! filesink location=vid0001.avi

The performance was better with these files. If there was only one clip I could scrub and play back with no problem but as soon as I added another clip the video was choppy. I could have tried reducing the resolution of the video but I really wanted the result to be in HD otherwise what is the point of taking video in HD in the first place. I decided since my video would be fairly simple and short that I could manage to edit it with choppy video. I could advance a frame at a time to check fades and transitions. Audio playback was in real time.

The titles were made with GIMP. When added as a clip they are automatically turned into video.

The most complicated thing about my video is the timer overlay. To do this I created a solid green frame using GIMP brought it into a new PiTiVi project added it as a clip and made it 30.5 seconds long. Then I rendered this to MJPEG. This gave me a green screen video 30 and a half seconds long. Then I used the pango timeoverlay plugin in the following command to add the timer.


gst-launch filesrc location=green.avi ! decodebin ! timeoverlay halign=right valign=bottom xpad=100 ypad=50 font-desc="serif bold 20px" ! ffmpegcolorspace ! jpegenc ! avimux ! filesink location=timer.avi

I added the resulting timer.avi file as a clip above the marble track video and used the alpha filter effect so that the green was transparent. This gives a counting timer overlaid on the marble video. All that remained was to start it when the marbles started and trim it when the last marble hit the table.

PiTiVI has plenty of effects. The one I wanted to use was Bulge. I would have used it on the main title screen but I wanted it to move across the screen. The effects will be much better once they can be animated over time with the key frames. Also if the pango plugin was available in the effect library perhaps I wouldn’t have needed to use a command line.

One problem the video has that I couldn’t find a way to fix is an audio glitch at the beginning and end of each voice over. The voice over quality in general is not that good probably due to a cheap microphone. The voice was recorded separate from the video. I used Audacity to cleanup the audio some.

I tried rendering in a few different formats. 1) MP4, FFmpeg PEG-4 part 2, ACC. This produced a very small file (16MB) and rendered quickly but the video quality was very poor. 2) Ogg, Theora, Vorbis. This produced very good quality video. Render time was about 20min. The file size is 187MB. 3) AVI, FFmpeg motion jpeg, ACC. The video quality was not very good the file was large (127MB) but the render time was very fast (~7min). 4) WebM, On2 VP8, Vorbis. The video quality was very good, the file was small (22MB) but it took about 44 min to render. Uploading the WebM format to youtube didn’t work so I uploaded the Ogg format. There are many other rendering options but these are the only ones I have tried so far. The audio glitch was present in all rendering formats.

My wish list for PiViTi:

The documentation should include more information on preferred video formats for editing and how to use external tools to do the conversion. Even better would be built-in tools do do the conversion.

The documentation should give advice on different rendering combinations. There are many containers each with different options for video and audio codecs. It would be nice to have a simplified dialog that chooses a recommended output format based on high level choices such as what do you plan to do with the video (burn to DVD, upload to YouTube etc.) and other choices like do you prefer open standards.

The render dialog should remember the last folder I rendered into.

Here is a direct link to the video.

Lego Climbing Robot

Way back in January I watched a Google tech talk video Lego Engineering: from kindergarten to college. The talk stresses the importance of teaching engineering in schools. A few days later at breakfast I started talking with my oldest daughter, Alexandria, about engineering and what it is that engineers do. I said that engineers use what they know about science and how things work to find solutions to challenges. I threw out a challenge as an example: Build the tallest possible tower that fits in our house and then build a robot to climb it. It was just an example but she got really excited about the idea. “Can we build it. Can we. Can we.” she said. I said “sure”.

In March I posted a preview of the robot with a picture and promises of more details to come. Its taken me way to long to finally write it up. The robot has been done since May and has made a dozen or so ascents. Making the video is what caused the delay.

Here is what I remember of the process.
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