[FFmpeg-user] Question about chroma sample location

[Oliver Fromme]

Hi,

I've got a question about the chroma sample location in h.265 (HEVC)
video using ffmpeg with libx265.

I'm transcoding a source file with ffmpeg.  According to ffprobe, the
source file's chroma location is "left" (integer value 0).  I assume
that information is taken from the VUI data of the HEVC stream.
(Or maybe there is no such information, and ffprobe assumes "left" by
default.)

When transcoding, I do *not* specify any chroma sample location on
the command line, therefore it should be taken from the source file
and be transferred to the target file unchanged.  Indeed, when I run
ffprobe on the target file, it reports "left" again.  Additionally,
ffmpeg sets the optional "chroma_siting" tag in the MKV stream header
to "1,2" which means the same as "left".

*However*, when I look at the encoder settings stored in the target
file (with mediainfo, or simply with a hexdump), I can see that there
is the libx265 option chromaloc=1.  But the number 1 means "center",
according to the libx265 documentation.

Can somebody explain this?

(BTW, I'm using ffmpeg 6.1.1, but I guess it's not an issue with the
ffmpeg version, but rather an issue with my head ...)

Best regards
Oliver