Compressability problems

[Jens_Rasmus]

I have run into some problems encoding Mission: Impossiple. It is only 1 hour and 45 minutes, so i thought it would make an excellent 1 CD rip. I am using the Gordian Knot package and DivX 5 pro and followed the guide here on Doom9.org. I used 128 kbit/s MP3 audio and 740 kbit/s video, which should make for quite good quality.
But the quality of the movie was low. When i checked the compression it said 43%, which is much lower than what i usually do.
But when going two CDs the compression tests says 83%, which would be wasting CDs, don't you think?

I can't believe Mission: Impossible can't fit on one CD. Am i doing something wrong here? I thought a bitrate of 740 kbit/s would produce pretty good quality.

Should i try a one CD rip again?
Or go two CDs with AC3?

Any help is greatly appreciated!

[Jens_Rasmus]

I forgot to mention that the bits/(p*f) is 0.254 at a resolution of 800x336. The compressability is still only 53% even at two CDs.

I'm not really sure how i can tell what quality i will achieve after the encoding. Is it the 1740 kbit/s video stream, the 0.254 bits/(p*f) or the compreassability of 53%?

Sorry if i'm being too dumb.

[khp]

Hi Jens

Great to see more danes around here. :)

How well a movie compresses, varies greatly from one movie to another.
You didn't mention what resolution you were using, if you want to fit the movie on one CD I would suggest that you go for a lower res, and of course chop off the black bars, if you haven't already done so.

Of course doing a 2 CD rip will always give you better quality, some people almost never do 1 CD rips, and some people never do 2 CD rips, it's a personal choice.

[jggimi]

800 x 336 ??!!??

The maximum vertical resolution of any dvd is 720 lines. So I'm confused by that. You seem to be resizing larger than the original.

To answer your question ... without knowing anything about this particular dvd, you may be able to get a 1-CD rip with acceptable visual quality.[list=1] Turn on B-Frames (bi-directional encoding)
Ignore b/p*f after running your compressibility check.
Set a resolution that provides a loading factor between 60-80%[/list=1] `

[Jens_Rasmus]

800x336 was the resolution of two CDs if i were targeting a b/p*f og 0.25.

The problem may have been, that i was trying to keep the b/p*f value around 0.17 (for one CD) and 0.25 (for two CDs). I will just disregard this value if i do a compressability test next time.

Thanks for your help!

[Zalbar]

Originally posted by jggimi
800 x 336 ??!!??

The maximum vertical resolution of any dvd is 720 lines. So I'm confused by that. You seem to be resizing larger than the original.



I typically do this to some degree, compressability permitting. The reason being that DVDs are recorded at 720x480 (ntsc) in a distorted aspect ratio. In order to correct this, you have to either sacrifice some resolution in one dimension, OR, size-up the resolution in the other dimension.

Case in point, 16:9 Anamorphic format - "Enhanced for widescreen TVs." The aspect ratio is squashed left to right. If you play this on a standard television, your DVD player will correct it by squashing it top to bottom, tossing out video information and sacrificing resolution. This creates the letterboxed black bar style picture. If, however, you play it on a 16:9 television, it will enlarge the picture left and right in order to retain the full top-to-bottom resolution. The picture is superior as no video is tossed out.

If during compression you were to correct your aspect ratio by scaling up left-to-right while leaving the top-to-bottom at 100%, you would in effect be simulating a 16:9 television rendering of an anamorphic DVD. Yes, it's true you are "creating" data which wasn't there to begin with when scaling above 100%, but you are only doing this in one dimension in order to correct the A/R and allow you to retain 100% of the top-to-bottom resolution of 480. This is ideal, though depending on how much space you have for your 1 or 2 CD rip you may not be able to do it to this extent - but the less you have to shrink either resolution, the better. A few movies which are easy to "max out" compressabilitywise can even be fit on one CD in full anamorphic resolution.

Another, more bit-rate economical alternative is to encode 1:1 at 720x480 (NTSC) or 720x576 (PAL) and use a player which can stretch the aspect ratio for you to 16:9. This is less compatible, because you may need a player that can do this conveniently for you. Also, if you have cropped the black edges of the film, the A/R will still be wrong - especially on a 2.35:1 cinemascope film.

Anyway - I find that stretching the film this way during encode usually does not drive up the bitrate requirements very much.

[theReal]

I make it a two-CD rip with AC3.

I've done Die Hard I recently and it compressed much too good for two CD's (the compress test was about 97% at maximum resolution with 448kbit AC3). I just encoded the movie at 100% quality (quant 2), added the AC3 and got two nice 645MB files from it :)

[grug2k]

Zalbar, I agree and disagree with you.

There is nothing wrong with encoding a movie at the full DVD resolution of 720x576 or (x480 NTSC), and stretching it at playback...but...

When you resize more than 100%, it can...

a) Seriously hurt the compressability of the movie (encoding information that isn't in the source material isn't exactly a good encoding practice...)

and

b) Create artifacts and other problems. Ringing and haloing often occurs.

There is no real excuse to encode > 720. You are wasting bits on information that is simply not there. If you absolutely want the extra resolution, the best option is to stretch it in realtime during playback. All you need is a decent player such as BSPlayer or whatever.