I am going to add a late answer to this one after spending all day trying to figure out how to get YUV 4:4:4 pixels into x264. While x264 does accept raw 4:2:0 pixels in a file, it is really quite difficult getting 4:4:4 pixels passed in. With recent versions of ffmpeg, the following works for completely lossless encoding and extraction to verify the encoding.
First, write your raw yuv 4:4:4 pixels to a file in a planar format. The planes are a set of Y bytes, then the U and V bytes where U and V use 128 as the zero value. Now, invoke ffmpeg and pass in the size of the raw YUV frames as use the “yuv444p” pixel format twice, like so:
ffmpeg -y -s 480x480 -pix_fmt yuv444p -i Tree480.yuv \
-c:v libx264 -pix_fmt yuv444p -profile:v high444 -crf 0 \
-preset:v slow \
Tree480_lossless.m4v
Once the encoding to h264 and wrapping as a Quicktime file is done, one can extract the exact same bytes like so:
ffmpeg -y -i Tree480_lossless.m4v -vcodec rawvideo -pix_fmt yuv444p \
Tree480_m4v_decoded.yuv
Finally, verify the two binary files with diff:
$ diff -s Tree480.yuv Tree480_m4v_decoded.yuv
Files Tree480.yuv and Tree480_m4v_decoded.yuv are identical
Just keep in mind that you need to write the YUV bytes to a file yourself, do not let ffmpeg do any conversion of the YUV values!