[FFmpeg-user] Reencoding from H.264 to H.265

Nicolas George george at nsup.org
Thu Sep 1 20:19:15 EEST 2016

Le quintidi 15 fructidor, an CCXXIV, SDSS a écrit :
> However, in my search I also found out that, according to many, re-encoding
> H.264 videos as H.265 is a waste of time, for the most part, on the grounds
> that H.264 videos are already compressed, and re-encoding them as H.265
> won't achieve much.

It is even worse than that. Unless you work with lossless settings, both
kind of compression are lossy: they damage the image a little, in ways that
are supposed to be barely visible, in order to achieve better compression.
If you pay careful attention (and do not choose too high a quality), you can
see the damage, it is called "artifacts".

The problem is H.264 and H.265 do the work independently. That means that
for H.265, the artifacts caused by H.264 are part of the image: H.265 will
try to preserve them, wasting bits in order to do that or degrading other
aspects of the image.

> Would it make sense to uncompress the H.264 first, and then encode the
> resulting video material with H.265? Is this even possible, in general, with
> ffmpeg, in particular?

You cannot not do that. (Double "not" intentional.) You have to decompress
in order to compress.

(I suppose it could be theoretically possible to translate the H.264
bitstream into a H.265 bitstream, keeping all the quantization intact and
then try to optimize it further, but I do not think any software can do it.)

>			 More to the point, is the H.264 to H.265 conversion a
> rather pointless exercise?

For the same level of quality, I would say yes.

If the source is of very high quality and you want to compress significantly
further, then it makes sense, because the original artifacts will be drowned
by the new ones. And in that case, of course, take the best codec you can at
the time.


  Nicolas George
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