DivX question

[didikai]

How much video quality will I lose if I uncompress a DivX file, and then convert it to DivX again?
(I have a lot of files I converted from DivX but no longer have the original DivX file)

[poisondeathray]

Why would you uncompress? to do editing? maybe there are other options.

It depends on your settings. If you wanted to keep the same bitrate and filesize, you lose quite a bit of quality. To compensate you need a higher bitrate (quantizer), thus ending up with a larger filesize. If you wanted almost exact quality as the original file, it will be about 2x the size.

[jman98]

First of all, you CANNOT uncompress. You can convert Divx to another format which is lossless, but the video information that was thrown away by the Divx encoder to compress the video CANNOT EVER BE RECOVERED. No offense, but you've been a member here for more than 4.5 years and posted over 200 times. I'm really surprised that you don't know this.

The amount of quality loss is impossible to say because when you re-encode to Divx, there will be some quality loss. The closer your bit rate is to the original bit rate, the less quality loss. Converting Divx to a lossless format and then re-encoding gains NOTHING over just re-encoding the current Divx file to a lower bit rate if that's what you want to do. There is NO DIFFERENCE at all between your current Divx file and that file converted losslessly to another format.

[didikai]

No offense, but you've been a member here for more than 4.5 years and posted over 200 times. I'm really surprised that you don't know this.

Perhaps you'd like this forum renamed to Newbie / General questions / Dumb questions by dumb people?

[PuzZLeR]

To add:

DivX -> Uncompressed format (ex:HuffYUV) -> DivX

is lossy in the end, no matter what, even if you want to increase the bitrate by 1,000,000 X for the final DivX file. I believe you want to do some edits like Poisondeathray said and if it's just cuts and joins, there are better options indeed - you can do those losslessly with VirtualDub. Only re-encode if edits like crops/watermarks/etc. are absolutely necessary.

The amount of quality you will lose is not straightforward - it depends on the complexity of the video. However, if you want to lose the most minimum amount of quality, at the lowest bitrate I would recommend using quality based encoding for the final encode with parameter ("quantizer value") of 2.

[jman98]

No offense, but you've been a member here for more than 4.5 years and posted over 200 times. I'm really surprised that you don't know this.

Perhaps you'd like this forum renamed to Newbie / General questions / Dumb questions by dumb people?

I neither called you stupid nor said your question was stupid. I simply expressed surprise that after 200+ posts and 4.5+ years of forum membership that you asked a question like that. To me that suggests a gap in your knowledge that you might wish to address, but if you prefer to seek offense where none was offered, you can do that too.

[PuzZLeR]

Actually it wasn't an intelligent or a stupid question either. It was a question without an answer without the use of physics - only a quantizer's sensitive readings can detect the answer through signal processing since every video's quality losses will vary among clip and among bitrates.

Since there's lots of videos that (may) need to be re-encoded, the one pass will be advantageous as well.

IMO Q=2 is the optimal balance of near losslessness at the minimum bitrate.

[didikai]

No offense, but you've been a member here for more than 4.5 years and posted over 200 times. I'm really surprised that you don't know this.

Perhaps you'd like this forum renamed to Newbie / General questions / Dumb questions by dumb people?

To me that suggests a gap in your knowledge that you might wish to address, but if you prefer to seek offense where none was offered, you can do that too.

When people say 'no offence' they know it will cause some.
There is a huge gap in my video knowledge. That's why I visit the 'Newbie' section. 99% of the time I get help.
Now for anyone else reading this maybe I wasn't clear in my question.

I converted a lot of DivX files with Convertx2DVD, because at that time I didn't know there was such a thing as a Divx standalone player. And I discarded the DivX file. All I have is the DVD which plays on any player.
My question is, if I convert the DVD back to Divx will the quality be crap? Or is there a way to minimise the bad quality

[PuzZLeR]

First of all I'd recommend you keep it at DvD since it's now the incumbent source - all that processing will be a pain and you will certainly lose quality. And DvD video is very, very compatible as you said here:

All I have is the DVD which plays on any player.

Your video is safe in this format.

As well the bigger file size of MPEG-2 isn't a big deal with the cheap price of blank discs today.

But if you do desire the better compression of DivX, which probably would be the only reason you'd need to do this, then to answer your questions:

My question is, if I convert the DVD back to Divx will the quality be crap?

Depends. Depending on the bitrate you use, and the complexity of the video, it will range from nearly identical to crap (but never exactly as good).

Using Q=2 would give you almost the same quality, but it will cost a little file size advantage. It will make your files bigger than what you're used to. Even though they should be smaller than MPEG-2, you'd still defeat the purpose.

Using Q=3 or Q=4 would give you a very acceptable quality compared to the source - but putting them side-by-side would reveal the differences. If you must go back to DivX for smaller file sizes I would go here.

Using Q=5 and up you are verging on crap quality.

There is no "one bitrate". It's the quantizer that determines it.

[DereX888]

No offense, but you've been a member here for more than 4.5 years and posted over 200 times. I'm really surprised that you don't know this.

Perhaps you'd like this forum renamed to Newbie / General questions / Dumb questions by dumb people?

To me that suggests a gap in your knowledge that you might wish to address, but if you prefer to seek offense where none was offered, you can do that too.

When people say 'no offence' they know it will cause some.
There is a huge gap in my video knowledge. That's why I visit the 'Newbie' section. 99% of the time I get help.
Now for anyone else reading this maybe I wasn't clear in my question.

I converted a lot of DivX files with Convertx2DVD, because at that time I didn't know there was such a thing as a Divx standalone player. And I discarded the DivX file. All I have is the DVD which plays on any player.
My question is, if I convert the DVD back to Divx will the quality be crap? Or is there a way to minimise the bad quality

You must understand what you are asking for:
You took heavily compressed low quality "divx" file of ~700MB and simply inflated it like a balloon to the size of 4.34GB, most likely you also have "upsized" it from usual "divx resolutions" like 512x288 or 640x480 (at best) to a DVD resolution at 720x480 or 720x576. You have added great amount of distortion at this moment - added it to the already greatly distorted source divx file. Similar thing happened with the sound - you have just inflated heavily compressed MP3 audio into less compressed MP2 or AC3 sound.
Now you want to add more distortion by "downsampling" the video and audio back to heavily compressed formats like "divx" and "mp3".
It doesn't make sense to most of us, so don't think jman98 was mean to you. I am too puzzled how can you, after being so many years here (unless you didn't read anything else but your own threads only?!) how can you even think the result from such "double distortion adding" processes can ever yield any good quality? (or actually it is "triple distortion" since there was 1st reencoding DVD->divx, 2nd you did from divx-> and 3rd is by going back to DVD->divx).
Understand: at 1st compression large amount of data that the film consisted of was discarded. At 2nd compression you have only inflated the file size, but you didn't "add back"any of the lost data (once it is gone - is gone, and there is no way to reconstruct it back to frames, even if you reencoded your divx to HD-DVD instead of standard DVD; at best, if you did right, you have preserved the greatly reduced quality of divx on your DVD-Video). And now you want to further remove another great amount of data from this already castrated picture... do you see what I'm trying to tell you?
If you had your original source divx file and you could compare it to what will be the result of your divx->dvd->divx reencodings, I am sure you could have seen with your own eyes, without need for any special measurements, how much worse will be your output file compared to the source.

However, the "acceptable quality" policy differs from person to person.
In my opinion you probably already have unacceptable crap quality DVD-Video, and converting it down back to divx again will create complete garbage in my eyes. But that's me. Maybe to you the "quality" of such file will be still acceptable? I don't know. None of us knows, only you can tell what is acceptable and what is unacceptable quality in *your eyes*. Some people watch i.e. widescreen 2.35:1 movies on tiny iPods junk - and they still insist they saw the same movie as I saw on a giant IMAX screen

Do a small test yourself.
Calculate bitrates etc settings, and use them on a minute-long short high-motion scene and create test sample, then judge it for yourself is it acceptable or is it junk.

No offence, but you must understand you're wrong asking actually about very personal preferences.
Its like you'd come here and asked us for advice should your next GF had short or long hair, big or small tits, blondie or a brunette... etc. Each one of us will answer differently.
I know, your dilemma is not *that much* personal, and is more common than about the GFs (obviously i.e. 100kbps divx at even tiniest 320x240 - if it is not a still picture - will look like horrible crap to everyone of us, regardless of preferences) but all I'm saying is that that thin line between "acceptable" and "unacceptable" really varies greatly from person to person, that's all.

[didikai]

You must understand what you are asking for:
You took heavily compressed low quality "divx" file of ~700MB and simply inflated it like a balloon to the size of 4.34GB, most likely you also have "upsized" it from usual "divx resolutions" like 512x288 or 640x480 (at best) to a DVD resolution at 720x480 or 720x576. You have added great amount of distortion at this moment - added it to the already greatly distorted source divx file. Similar thing happened with the sound - you have just inflated heavily compressed MP3 audio into less compressed MP2 or AC3 sound.
Now you want to add more distortion by "downsampling" the video and audio back to heavily compressed formats like "divx" and "mp3".




This is all you needed to say to answer my question.

My life doesn't revolve around bitrates and resolutions. Video is just a small hobby.
I'm not really interested why things do what they do. I just want to use them. (Pilots and passengers)
As for coming here for four years and being expected to know all this stuff... same answer...If I can't figure it out.... I ask someone who knows.
I thought this why this site is called Videohelp.
Nearly every time I have asked for help here I have been helped.
This time I just seem to have found a couple of arrogant pratts.

Till my next post for help

Regards

[jagabo]

I agree with PuzZLeR's quantizer guide. I usually use Q=3 and Xvid. Q=2 for something more critical.