Firstly, I don't understand how can melt encode... it's a player.

I made my own preset as such -

-acodec mp2 -ar 22100 -ac 1 -threads 3 -vpre libx264-veryslow -vcodec libx264

It appears to work very well on ffmpeg, but when I render using kdenlives, it appears to ignore the parameters... no matter what I change, the quality remains the same; i.e the quality is not even close to what I get with ffmpeg.

melt is more than a player ... it is a command line video editor. For encoding it uses ffmpeg through a module named "Avformat Consumer"

MLT's documentation for the Avformat Consumer tells : "MLT's preset options require the full path to the preset file and not just the basename"

so it seems you have to add -vpre /path/to/presets/libx264-veryslow

sunab.

Actually, that rule about presets changed with the last MLT release 0.6.2. Now, it tries to be smarter by using the libavformat install prefix (/usr) plus 'ffmpeg/share' plus video codec name (libx264) plus '-' plus property value (veryslow) plus '.ffpreset' = /usr/share/ffmpeg/libx264-veryslow.ffpreset
That means if he has the most recent version vpre=veryslow should work but not libx264-veryslow.

I've 0.5.10 -

[D] media-libs/mlt
Available versions: 0.4.6 0.5.4 ~0.5.4-r2 ~0.5.10 {compressed-lumas debug dv ffmpeg frei0r gtk jack kde libsamplerate lua melt mmx python qt4 quicktime ruby sdl sse sse2 vdpau vorbis xine xml}
Installed versions: 0.5.10(18:41:39 02/09/11)(dv ffmpeg kde libsamplerate melt mmx qt4 quicktime sdl sse sse2 vorbis xine xml -compressed-lumas -debug -frei0r -gtk -jack -lua -python -ruby -vdpau)
Homepage: http://www.mltframework.org/
Description: An open source multimedia framework, designed and developed for television broadcasting

No no... it's not working... I tried virtually all combinations, without the =, with the =, with the full path and without the full path, i.e. only with the preset name without libx264

If you are using the "-name value" syntax with melt (or kdenlive render profile) as described in the first message to pass the encoding parameters, then it is incorrect syntax. The correct syntax for this part of the melt command line is name=value. See the other kdenlive render profiles. Using the parameters in your first message that will be:

acodec=mp2 ar=22100 ac=1 threads=3 vpre=/usr/share/ffmpeg/libx264-veryslow.ffpreset vcodec=libx264

Ok, this appears to be working, thanks. But melt takes too much memory.. it literally hanged my system. However ffmpeg does not.

Unable to reproduce.