Should I reduce the resolution, and how?

[aldaco12]

Hi. My problem is: which is the best way to 'squeeze' a rather large movie (for instance, 2h 30') into a single 700 MB AVI?

That is, one can pray God and let DivX work, but I thought that the concept of 'good bitrate' was resolution-dependent.

My 'rule of thumb' is that, if I have a 640x360 video (typical 16:9 movie) a video bitrate off approx 700 kbps is adequate.

My problem is: what should I do, if my Bitrate Calculator gives me, for my 2h 30' movie, the following video bitrates:

- 515 for audio bitrate = 128 kbps;
- 531 for audio bitrate = 112 kbps;
- 547 for audio bitrate = 96 kbps?

That is, if I assume that:
==========================================================================================
if the bitrate is too low, you should have a video which 'fatigues' DivX a little less, and therefore you should lower its resolution, to compress it effectively
==========================================================================================

The problem is that I observed that the 'effort' is NOT simply proportional to X*Y, and therefore my 'rule of thumb' that 700 kbps is a correct video bitrate for a 640x360, an approximate bitrate of 700*[(452*254)/(640*360)] = 700/2 = 350 kbps is totally inadequate.

Once, I tried to apply the equation:

Bitrate_min = 700*(X*Y)/(640*360)

to a PAL VCD movie (352x288), and under this equation

Bitrate_min = 308 kbps.

I tried to compress a 352x288 movie at about a video bitrate of 350 kbps, and the result was lousy.

Is there a sort of 'rule of thumb' which gives an approximate 'good' bitrate for a X x Y movie?

Thanks very much.

[Awatef]

Hmm... 700kbps seems too low for me for 640x360. I wouldn't go below 850kbps (at least 0.15 bits per pixel).
The better way of evaluating things is doing a compressibility test. For example, encode at quantizer 2 to see how big the movie will get, and try to stay over the third of the size you get when you then encode in 2-pass.
If the size you're targeting is lower than the third, you reduce the resolution proportionally.
For very low bitrates like 500kbps, you may consider using H.264 compression instead of DivX if you don't have player restrictions.

[Your_Idol]

the best way to avoid blocking with a low bitrate is to use some filters. gordian knot has some noise reduction filter settings built-in, you can choose between little, medium or heavy. or use a filter like temporalcleaner. or in the video tab of the divx codec properties you can choose the noise reduction setting, ether light, normal, strong or extreme. or choose a filter in virtualdubmod like temporal smoother.

I would use the virtualdubmod filters.

i find that the gordian knot default noise filters, even in light, seem to lower the color depth significantly and the divx noise reduction can add very noticeable color changes and flickering with avs scripts.

[Blue_MiSfit]

Bitrates are not effectively determined through equations! There is NO correct bitrate!

Repeat it with me:

It
All
Depends
On
The
Source!

Period!

If a movie doesn't look good at the resolution / average bitrate or constant quantizer you chose, change one or the other and re-do it :)

Fitting 2h30m of most movies onto a 700mb CDR is going to be pretty tricky. I would assume that resizing to 640xWhatever would be necessary, as would some de-noising.

AviSynth is your friend. FFT3dfilter or its crackhead cousin fft3dgpu are effective, powerful, and flexible. Filtering through DivX is a simply dreadful idea. VirtualDub has lots of good filters, but they all involve a YV12->RGB->YV12 colorspace conversion which should be avoided. AviSynth lets you keep the video in YV12 always.

Consider doing 2 cds, or switching to DVD5 media. The burners and media are so insanely cheap these days it's not even funny.

I usually shoot for 1/4 DVD-R for shorter movies or more compressible movies, 1/2 DVD-R for longer / less compressible movies. The nice thing about this is that you can usually include the original AC3 track or a gently compressed AAC version of it. It also lets you keep full resolution, which is priority number one in my book. RESIZERS ARE EVIL!!!

If DivX can't do it, try x264. Its a bit more effecient, though it comes with a whole different workflow. MeGUI is the shizzy :D A nice "in the middle approach" would be XviD with a low bitrate matrix like EQM-V3LR or Jawor's 1cd.

Anyway, I'm through ranting. Good luck

~MiSfit

[aldaco12]

But the source is a regular DVD!

My question is: if I wished to put a 2h 30' in a 700 MB AVI, what could I do?

I know that 700 MB per hours/hours an 15' makes an excellent compression, so that you can even avoid resizing the DVD movie.

I thought that, if you wished to put a long movie in 700 MB you could make a decent work by lowering the resolution.

Ot not?

[CWR03]

My question is: if I wished to put a 2h 30' in a 700 MB AVI, what could I do?
Short answer: settle for a poor quality file.
I thought that, if you wished to put a long movie in 700 MB you could make a decent work by lowering the resolution.
When you reduce the resolution you sacrifice detail. If you don't reduce the resolution you'll have pixellation in areas of similar color, especially smoke, fog or fire.
My problem is: which is the best way to 'squeeze' a rather large movie (for instance, 2h 30') into a single 700 MB AVI?
The "best" way is to find a middle ground that suits you.

[Awatef]

Aldaco, yes you lower the resolution to be able to keep a good compression ratio.

[Slitheen]

Why does it have to be 700MB? Use a DVD-R

[Sharktooth]

But the source is a regular DVD!
He said the source, not the media.
The source is the movie itself. He meant the compressibility of the movie.

[Your_Idol]

No really, with any source you can run a filter over it and it will make it compress so much better. You can hardly tell the difference in quality.

I had tried some avisynth filters and had not been happy with the results but after Blue_MiSfit said that virtualdubmod does a YV12->RGB->YV12 colourspace conversion i tried avisynth again.

I got some good results with fft3dfilter(sigma=2, plane=4) but it was too slow and blury. But you should give temportalSoften(3,5,5,18,2) a go.

Why is the divx noise reduction a "dreadful idea"?

[DarkZell666]

Why not resize to something lower than 640*y ?
Try using bilinear resizing (softer picture, more compressible), and resizing to 576*324 ... The blockiness/bluriness tradeoff is really worth it, imho, since you'll get much less blocks for slightly less visual detail.

@Your_Idol: My personnal experience with DivX's noise reduction is negative too, even set to "Light" there was something "artificial" about the picture ... lol.

[aldaco12]

Exactly what I do. But I saw ther the 'required bitrate' is not a linear funcion of X*Y.
For instance, using a bitrate of
B_LOW= (352x288)/(720*576)*B_HI does not give a result comparabile to a compressed 720x576 movie with a bitrate equal to B_HI.

[DarkZell666]

It can't be linear because there is motion involved in the compression process, not just "pictures", and twice less resolution doesn't mean twice less motion either ;)

Everyone has encouraged you to use noise reduction filters because noise (ie: "snow", or "grain" on the picture) generates "fake" motion (every codec mistakes noise for motion at some point), which generates a need for a bigger bitrate to reproduce that noise. So why not get rid of it ? Most DVD's are noisy anyway, even if on your TV they don't look like they are. My favourite is hqdn3d(2,2,2).

Anyway, trying to fit a 2'30 "regular" movie on 700MB is a bit unrealistic from the beginning. Beyond 2 hours you'll need to target for 1GB or 1,4GB (2 CD's), or use a more advanced H264-based codec, such as x264 :)