Felipe Contreras

A while ago I wrote the “official” video transcoding for Maemo how-to based on the wiki page and some heuristics.

The interesting thing here is the intelligent script that automatically finds the right parameters to use for the encoding.

Some of the considerations include:

• As higher framerate as possible. For smooth videos.
• Keep aspect-ratio. So the videos don’t look weird.
• No cropping. You might miss something.
• Single pass. Bigger files, but constant CPU usage.

Is this thing really smart? Let’s try it.

Crappy clip from YouTube

transcode.rb -v -i youtube.flv
* Input: 320x240 [4:3], 29.917 fps, 0 kbps.
* Output: 320x240 [4:3], 29.917 fps, 400 kbps.
mencoder youtube.flv -o youtube_it.avi -srate 44100 -oac mp3lame -lameopts vbr=0:br=128 -af volnorm -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=400 -ofps 29.917000

Nothing special, the script maintained all the parameters, so there shouldn’t be any problems. Also, the aspect-ratio is not so different from the one of the device. This one is OK.

DVD

transcode.rb -v -i dvd://1
* Input: 720x480 [3:2], 29.97 fps, 7500000 kbps.
* Output: 400x240 [5:3], 29.97 fps, 400 kbps.
mencoder dvd://1 -o 1_it.avi -srate 44100 -oac mp3lame -lameopts vbr=0:br=128 -af volnorm -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=400 -ofps 29.970000 -vf-add scale=400:240

In this case the aspect-ratio is slightly modified (less than 10%), but the target aspect-ratio is exactly the same as the one of the device, so, it will look good.

So far the script does seem intelligent

High Definition

Now let’s try a non-standard resolution, high bitrate, and H.264 format.

transcode.rb -v -i starcraft.divx -f h264 -q 8
* Input: 1024x436 [256:109], 23.99 fps, 1917920 kbps.
* Output: 352x144 [22:9], 23.99 fps, 1500 kbps.
mencoder starcraft.divx -o starcraft_it.avi -srate 44100 -oac mp3lame -lameopts vbr=0:br=128 -af volnorm -ovc x264 -x264encopts bitrate=1500:nocabac -ofps 23.990000 -vf-add scale=352:144

The aspect ratio now is about 4% different, that’s good. Unfortunately the aspect-ratio is quite different from the one of the device, but that’s not too bad.

This is how it would look like:

note H.264 videos look pretty good on the device.

Conclusion

So far there hasn’t been a single video where I had to modify the parameters that this algorithm suggested; sometimes the quality is not that good but increasing it with the -q option does the trick.

Wondering if it would find the right parameter for your clips? Why don’t you try it and find out

The script is here. And the garage project here.

Continuing my previous GStreamer introduction this new tutorial will guide you on your first GStreamer application written in C.

For anxious people the code is here.

The whole thing is here.

It’s in bluwiki so if you want to modify it, feel free to do so

I feel a little bit ashamed of posting such simple things, but there doesn’t seem to be anything for the really newbies.