re-encoding woes

[slayer4567]

I recently just re-encoded a mpeg video that was 720x480 to 352x480, mainly to save some size for the dvd I want to create. The problem is, is that the 352x480 video is actually LARGER in size than the original video. Does this sound right? I would think that after re-encoding to a smaller size resolution, that the video would be smaller...

And just so you have more information, I used vob2mpeg to get the video to mpeg from the original DVD, then I demuxed it using PVAStrumento, then used DGIndex to get the d2v file, and finally I used HCGUI to re-encode.

[Baldrick]

you must lower the video bitrate also, just the frame size does not make the file size smaller.

[slayer4567]

Really?? I would think that by losing half of the information that was there would have lowered the file size... and every other time i've done this, the video file size has been smaller....

[Baldrick]

no, you must change the bitrate also.

can't you set the bitrate in hcgui? or output file size?

[KBeee]

No - 240 X 360 @1500kbps will be exactly the same file size as 2400 X 3600 @1500kbps
You're not "losing half the information" - your "information" is kbps X time - frame size has nothing to do with it

[slayer4567]

Yea I can and I always do. I just did a test, and its strange. I looked at 2 videos that I converted yesterday. One of them was able to shrink down to half the size of the original, and their average bitrates are the same. The one that was bigger than its original counterpart has roughly the same average bitrate as its original as well.

Now I'm not doubting you that it could be the bitrate, but why would video1 be about half the size in KB as original1 when the bitrates are the same and video2 is larger in KB as original2 and their bitrates are the same?

[yoda313]

Did you change the audio settings at all? Was it pcm audio and you converted to mp2 or ac3 audio? That would reduce the overall file size.

[slayer4567]

I'm not even looking at the audio part... just the video itself.

[KBeee]

Think of filling a sack with cupfulls of sugar
Your cup holds 50 sugarcubes, or 1000 sugargrains
You put 1 cupful of sugarcubes into the sack every 1 second - after 10 seconds, your sack holds 10 cupfulls of sugar - this is your file size.
You put 1 cupful of sugargrains into the sack every 1 second - after 10 seconds, your sack holds 10 cupfulls of sugar - this is your file size. They are the same i.e. 10 cups of sugar.
It doesn't matter if you used sugarcubes (many pixels), or sugargrains (few pixels), your sack still has 10 cupfulls of sugar after 10 seconds.
You can reduce the size of your sack of sugar by -
1. Putting less sugar into the sack each second (reduce bitrate) or
2. Reducing the length of time you load sugar into your sack (i.e. reduce the playing time)

[slayer4567]

That is a very good analogy... but I thought that by reducing resolution, isn't that like using a smaller sack?

[KBeee]

No - Reducing the size of the sack would be like aiming for a final desired file size
i.e. if you want to end up with a 1kg sack of sugar, instead of a 2kg sack you should put 1/2 cup in each second for whatever-time-it-took-to-fill-the-2kg-sack-with-1-full-cup, or fill it for half the time.
If you want to halve the filesize, you need to halve the bitrate, or halve the playing time. Doing both will 1/4 the filesize.
Remember, all your computer knows is that you are putting (say) 4000 bits of information onto it's hard disk (or DVD or CD) every second (4kbps). After 20 seconds your hard disk has got 4000 X 20 bits of information (say 80000 bits/10000 bytes) stored.
Your 20 second movie is taking up 10KB @ 4kbps.
It'll have 5KB @ 2kbps,
or the same 5KB if it's only 10 seconds instead of 20 seconds.
Why do I suddenly want sugar in my coffee?