x265: Is it possible to re-compress HEVC video without quality loss?

Question

My situation:

1. Transcoding 1080p bluray videos to HEVC. Audio untouched;
2. My rig is old (pre-2012) and to speed up the process I'm using --preset ultrafast;
3. From x265 documentation, I understand that video quality is independent of the preset used. And the slower the preset, the better the compression ratio it seems;
4. I'm using FFmpeg.

My Reasoning:

1. I want to start converting to HEVC now because the codec is good enough for me and the space saving is phenomenal;
2. I've seen people reporting a 35GB video compressed to 1GB or so with crf=18 and --preset veryslow. It sounds ludicrous but an experiment of my own shows about 40% additional space saving between presets ultrafast and veryslow.

The Question:

When I get a new rig with a more capable CPU (Skylake or later, I hear), is it possible for me, to "re-compress" my HEVC videos using eg. --preset veryslow, without quality loss due to encoding a second time?

Answer 1

In strict mathematical terms, no.

In terms of maintaining an acceptable image quality, yes. If you use CRF 18 and a preset like ultrafast now, you should be able to get a smaller acceptable file with preset veryslow later on.