From my own experience, if you want absolutely no loss in quality, --lossless is what you are looking for.
Not sure about
avconv but the command you typed looks identical to what I do with
FFmpeg you can pass the parameter like this:
ffmpeg -i INPUT.mkv -c:v libx265 -preset ultrafast -x265-params lossless=1 OUTPUT.mkv
x265 switches (options with no value) can be specified like this (except those CLI-only ones, those are only used with
x265 binary directly).
With that out of the way, I'd like to share my experience with
x265 encoding. For most videos (be it WMV, or MPEG, or AVC/H.264) I use
x265 decides the rest of the parameters and usually it does a good enough job.
However often before I commit to transcoding a video in its entirety, I test my settings by converting a small portion of the video in question. Here's an example, suppose an mkv file with stream 0 being video, stream 1 being DTS audio, and stream 2 being a subtitle:
ffmpeg -hide_banner \
-ss 0 \
-i "INPUT.mkv" \
-attach "COVER.jpg" \
-map_metadata 0 \
-map_chapters 0 \
-metadata title="TITLE" \
-map 0:0 -metadata:s:v:0 language=eng \
-map 0:1 -metadata:s:a:0 language=eng -metadata:s:a:0 title="Surround 5.1 (DTS)" \
-map 0:2 -metadata:s:s:0 language=eng -metadata:s:s:0 title="English" \
-metadata:s:t:0 filename="Cover.jpg" -metadata:s:t:0 mimetype="image/jpeg" \
-c:v libx265 -preset ultrafast -x265-params \
-c:a copy \
-c:s copy \
-t 120 \
Note that the backslashes signal line breaks in a long command, I do it to help me keep track of various bits of a complex CLI input. Before I explain it line-by-line, the part where you convert only a small portion of a video is the second line and the second last line:
-ss 0 means seek to 0 second before starts decoding the input, and
-t 120 means stop writing to the output after 120 seconds. You can also use hh:mm:ss or hh:mm:ss.sss time formats.
FFmpeg from showing build information on start. I just don' want to see it when I scroll up in the console;
-ss 0 seeks to 0 second before start decoding the input. Note that if this parameter is given after the input file and before the output file, it becomes an output option and tells
ffmpeg to decode and ignore the input until x seconds, and then start writing to output. As an input option it is less accurate (because seeking is not accurate in most container formats), but takes almost no time. As an output option it is very precise but takes a considerable amount of time to decode all the stream before the specified time, and for testing purpose you don't want to waste time;
-i "INPUT.mkv": Specify the input file;
-attach "COVER.jpg": Attach a cover art (thumbnail picture, poster, whatever) to the output. The cover art is usually shown in file explorers;
-map_metadata 0: Copy over any and all metadata from input 0, which in the example is just the input;
-map_chapters 0: Copy over chapter info (if present) from input 0;
-metadata title="TITLE": Set the title of the video;
-map 0:0 ...: Map stream 0 of input 0, which means we want the first stream from the input to be written to the output. Since this stream is a video stream, it is the first video stream in the output, hence the stream specifier
:s:v:0. Set its language tag to English;
-map 0:1 ...: Similar to line 8, map the second stream (DTS audio), and set its language and title (for easier identification when choosing from players);
-map 0:2 ...: Similar to line 9, except this stream is a subtitle;
-metadata:s:t:0 ...: Set metadata for the cover art. This is required for mkv container format;
-c:v libx265 ...: Video codec options. It's so long that I've broken it into two lines. This setting is good for high quality bluray video (1080p) with minimal banding in gradient (which x265 sucks at). It is most likely an overkill for DVDs and TV shows and phone videos. This setting is mostly stolen from this Doom9 post;
crf=22:...: Continuation of video codec parameters. See the forum post mentioned above;
-c:a copy: Copy over audio;
-c:s copy: Copy over subtitles;
-t 120: Stop writing to the output after 120 seconds, which gives us a 2-minute clip for previewing trancoding quality;
"OUTPUT.HEVC.DTS.Sample.mkv": Output file name. I tag my file names with the video codec and the primary audio codec.
Whew. This is my first answer so if there is anything I missed please leave a comment. I'm not a video production expert, I'm just a guy who's too lazy to watch a movie by putting the disc into the player.
PS. Maybe this question belongs to somewhere else as it isn't strongly related to Unix & Linux.
--losslessmight in fact enlarge the file, if it decodes the previously lossy codec and then encondes what it did decode losslessly. Quality will stay exactly the same as input.