ffmpeg how to "lossless" convert video codec to libx264 ? CRF & qp value

[feelart]

I wish to convert in a single pass videos (often mpeg4 that I will crop) to x264.

How to best achieve this with FFmpeg 1.x ?

Quality encoding x264 : http://ffmpeg.org/trac/ffmpeg/wiki/x264EncodingGuide

Lossless H.264
You can use -qp 0 or -crf 0 to encode a lossless output.

Lossless Example (fastest encoding but not best compression)

ffmpeg -i input -c:v libx264 -preset ultrafast -qp 0 output.mkv

Lossless Example (best compression)

ffmpeg -i input -c:v libx264 -preset veryslow -qp 0 output.mkv

Now, if I set -crf 0 even on a cropped video, I see the resulting output larger in Mb than the orginal!!! I have to choose something like -crf 20 to see a compression.

What's a non trial an error method ?
I try with preset slower only, this is good compression, but is it the best near lossless compression???

ffmpeg -i input -c:a copy -c:v libx264 -preset slower output.mkv

Or should I use some bitrate specifier such as -b:v 1800k

I'm looking for some -sameq specifier...

Thanks.

P.S. http://mewiki.project357.com/wiki/X264_Settings#qp

[jagabo]

Lossless output will vastly increase the size of any video that's compressed with a lossy codec. Filtering video requires that the source be decompressed, then filtered. If you then compress with a lossless codec you will get something only a little smaller than a completely uncompressed video.

[bat999]

I'm looking for some -sameq specifier...

Forget it.

[sanlyn]

You inflict even more damage by filtering with lossy compressed video. For cleanup you decompress and save with a lossless codec (or uncompressed), then re-encode when you're finished. You don't have to keep the lossless work files, just keep the final output.