Compressibility check values

[Kangiten]

Hello everyone :)

I have a little question about compressibility check values in Gordian Knot. The Xvid guide says that you should choose a bitrate/resolution combination so that you reach a 0.17 bits/pixel ratio.
My question is: what percentage of the value returned by the compressibility check should I configure when said value is less than 0.17?

For instance, I ran a compressibility check to encode "Blood The Last Vampire" and the check returned a value around 0.15. Does it mean I should configure the resolution/bitrate to reach 100% of this value? 80 %?. I did a test on another video (clip of "I have Forgiven Jesus" by Morrissey; the original clip is very slow with only black, white and green colours and the compressibility check returned a value of 0.080! Does it mean I can set an ultra low bitrate for my encode?).

I apologise if this has been answered before, I'm quite new to this.

Thanks for any input.

[skipper152]

Hi Kangiten,

I've been lurking around the boards for a couple of years now and probably do things old fashion (I use the guides from about 1 - 1 1/2 years ago). But this seems to work for me.

After I load the file into GK (the .d2v), crop, etc., I use the resolution slider to obtain the bits/pixel. I try to keep the bits/pixel between .18 and .20.

Next I run a 15% compressions check. After the compression check comes back, I don't use the bits/pixel anymore. I look at the percentage of the compressibility. I think it is in the DivX guide (I used divx 4/5 a while back before switching to xvid) that I read suggests 40-60% as ok. I try to keep it between 50-70%. I adjust the resolution or go to 2 cds if necessary.

After adjusting, you should go back and do the comp test again. I've been too lazy to do this, especially if the comp test comes back around like 45% (its close to the 50%) so I just readjust the resolution and encode.

Most of my encodes come out good me, but there are some I've had to go back and take another look at.

Hope this helps.

[jellysandwich]

Next I run a 15% compressions check. After the compression check comes back, I don't use the bits/pixel anymore. I look at the percentage of the compressibility. I think it is in the DivX guide (I used divx 4/5 a while back before switching to xvid) that I read suggests 40-60% as ok. I try to keep it between 50-70%. I adjust the resolution or go to 2 cds if necessary.



What's the point of using bits/pixel at all then?

js

[skipper152]

What's the point of using bits/pixel at all then?

js

Hi Jellysandwich,

I won't pretend that I know what I am doing. Much of my encoding has been based on my trial and error, what looks good to me, massive amounts of reading posts and following guides.

To answer your question directly, I use the bits/pixel to determine my starting resolution. If I load in a d2v file that starts out at .10 bits/pixel, that, for me is way too low. I need to either decrease the resolution or go for more cd's. This, of course, is before I start the compression test.

I can point you to an (ancient) link: http://forum.doom9.org/showthread.php?p=69151#post69151
(Hope I'm doing this correctly, but the title of the thread is: "Bits/pixel*frame" : With 2 CDs I stay around 0.217! What have I to expect?)

(blasted bosses keep walking in.....)

From what I remember, bits/pixel are important up to the saturation of the codec. Throwing more bitrate (hence higher bits/pixel) at an already saturated codec isn't going to make the encode look any better.

Using the compress test actually allows you to test the codec/movie, B-frames and any filters which you may have used. The bits/pixel for my starting resolution does not take this into account. If the compression value comes up too high, you can increase the resolution, not use B-frames, remove filters (or add sharpening filters, which I believe decrease compression???)

I'm not going to say that the compress test is accurate either! If you do a 5% compression check you may be unlucky to get 5% of the movie that is slow (maybe just talking) so the high action scenes that come after it will not look good.

I've tried using only bits/pixel before (no compression testing) and many of my encodes did not look too good to me. But as I started to use the compression test, I found I liked my movies somewhere between 50-70% compression, not at a specific bits/pixel.

This works for me. Ask someone else and they may not use compress test at all.

Sorry for the long winded post, but I hope that explains my reasoning, however right or wrong it is...

[jellysandwich]

Wow, that was a lot longer than I expected. I was just wondering why you use it at all, since you said that you disregard it after the compression test (and use the % instead - which is what I do).

js

[Kangiten]

Thanks for the replies.

I encoded "Blood" using 70% of the compressibility check value. The bitrate is low (around 800 Kbps) I think but the encode looks great. I guess I'll just stick to that way of doing things since Xvid seems to work great even at low bitrates.

Again, thanks for the opinions and tips :)