CRF : "Very fast" better than "Slow" ?

[ditche]

Hello, first sorry for my English and if the question has been asked before. :o
MeGUI, x264 2120, I encoded the same chapter of a movie with quality CRF22.
There is something I do not understand. I used all the presets (Ultra Fast> Placebo), and saw that the encoded file with the preset Very Fast is smaller than the encoded file with the preset Slow... :sly:
http://img828.imageshack.us/img828/6971/8012012142810.png (http://imageshack.us/photo/my-images/828/8012012142810.png/)

I encoded another videos : idem.
I thought that size could only decrease when using slow presets ?

What do I mean? :thanks:

[Majin3]

crf does not guarantee constant quality since a program cannot exactly predict how it will look like for a human.
crf only means "similar quality at similar settings with similar source", so comparing file sizes while keeping crf and changing settings won't say much.

[hello_hello]

I thought that size could only decrease when using slow presets ?

Given the rest of their file sizes seem to correspond to the speed it does seem like a bit of an anomaly. I don't have an answer but are you 100% sure you didn't change any other settings when using that speed preset?

Did you encode the video a second time using that preset? It's probably something I'd do just to make sure I didn't do anything silly the first time and I get the same result again.

[Sapo84]

Given the rest of their file sizes seem to correspond to the speed it does seem like a bit of an anomaly. I don't have an answer but are you 100% sure you didn't change any other settings when using that speed preset?
Not every setting just decrease speed/increase quality, there are a bunch of settings that increase quality and filesize (no fast pskip e no dct decimate increase filesize and quality iirc).

Very fast has no psy-rd and no trellis, that may be one of the reasons why the filesize is so low.
Superfast has no mb-tree (huge filesize reduction when active) and that's why the filesize is so different (and I guess the quality is also much higher).

[LoRd_MuldeR]

MeGUI, x264 2120, I encoded the same chapter of a movie with quality CRF22.
There is something I do not understand. I used all the presets (Ultra Fast> Placebo), and saw that the encoded file with the preset Very Fast is smaller than the encoded file with the preset Slow... :sly:

CRF mode is not a perfect "constant quality" mode!

Instead the same CRF value gives the same level of quality (roughly!) for different sources, as long as you don't change other influential settings.

Switching to a another preset (e.g. Very Fast -vs- Placebo) makes quite a difference and you will have to re-adjust your CRF value :eek:

Or in other words: You probably are comparing the file sizes of two encodes that came out at different visual quality. This of course is meaningless!

So if you want to compare the effect of different presets, then use 2-Pass mode. This way you can compare the quality of files with identical size.

Alternatively you could compare the size of files with identical quality, but this is much harder to establish in practice... :(

(Again: Identical CRF value does not imply or guarantee identical visual quality!)

[hello_hello]

I think the main thing he was trying to do was compare size, but it does seem a little odd that if he didn't change any other setting, all speed presets produced relative increase (or decrease) in file size, except for one.

[ditche]

Thank you for your answers. :thanks:
Indeed, I note that the file "Ultra Fast" is much lower quality than the "Placebo", for example. :)
Yes, I found it logical that all these files were the same qualitatively.
To be sure, I will test again with the default settings. But normally, I only changed the CRF field.

[Audionut]

all speed presets produced relative increase (or decrease) in file size, except for one.

Take a look at the differences between ultrafast and superfast.

no-8x8dct
aq-mode 0
b-adapt 0
bframes 0
no-cabac
no-deblock
partitions none
subme 0
no-weightb
weightp 0