libvpx optimization

[vincent...@gmail.com]

Hi,
I'm new to Webm and I'm experimenting with FFmpeg and libvpx 1.3.0. I have short video clips (5s), fast paced action (10-30 frames per scene, quick fade-to black and direct transitions) and I want to convert them for the web (low bitrate, no sound, 720p).

I'm fairly confident with x264, and I already converted my clips to mp4 format. (about 600kB per file).

I have some problems using libvpx, first, I have heavy blockiness at the beginning of the scenes. Isn't the loop filter supposed to eliminate all blocky artifacts? (and introduce blur instead) How do I tweak the settings to get rid of the blockiness?

Second, I can't figure out how to use the crf value.  For some reason, ffmpeg ignore crf if b:v is not set and fall back to a ultra-low quality mode. Is b:v is set, changing the crf value doesn't seems to change anything about the quality. With x264 the crf value can be set without bothering about bitrates, can libvpx do the same?

for reference, here is the command line I used :

ffmpeg -i src/1.mp4 -c:v libvpx -quality best -cpu-used 0 -crf 4 -qmin 4 -qmax 42 -maxrate 4M -bufsize 2000k -threads 4 -b:v 800k -g 10 -an 1.webm

This produce a 600kB file, the quality is ok except for the blockiness of the first few frames of each scenes.

Thank you for your help.