(For Divx team): Maybe an encoder issue? (Encoding at low bitrates...)

[Fr4nz]

Hi guys.

Watch this sample (it's 1.5 M, you cannot notice this effect in a pic):

http://users.libero.it/i3ltt/Cazzate%20varie/Sample2.avi

As you can see (especially at the upper-left corner), it seems that some blocks move after the others and there's also some sort of orizontal trembling. The effect is very strange and I've never seen it. This doesn't depend from the decoder (in fact nothing changes if I play back the video with 5.0.5 decoder).

Settings were: 2-passes, 840kbps, psycho fast/fast, b-frames, standard/standard.

The effect goes away if I increase the bitrate.

Maybe this depends from XMPEG 5?? (I use it to encode my films) But it seems strange to me that this depends from xmpeg, also because I don't remember this sort of "effect" on previous beta versions of divx 5.1.

Thanks for any answer!

EDIT: Maybe this issue is present also in 5.0.5.

[Sharktooth]

That happens when encoding in pretty high resolutions with low bitrate.
I suggest you to use 512 pixels as horizontal resolution (choose the vertical pixels according to the movie AR) and to not use PVE.
You can still correct some of those artifacts with some good post processing filter (ffdshow's blur -> denoise3D 4-3-4-HQ).

EDIT: And no... thats not an issue...

[Fr4nz]

Pizzaland?? Sei italiano? :D

[Sharktooth]

Eh si:)

[SeeMoreDigital]

840kbps is not that slow for DivX. I've generated many encodes below this bitrate that are rock solid!

To my eyes, your 720x384 encode did not look too bad. Better when pumped to a TV.

There's much to be said about cropping and resizing, as some tools can produce a shaky/wobbly image.

Generate some short test encodes first. Starting with no DivX settings at all, not even BiDi. Then introduce them one at a time!

If the problem persists, try a different encoder.

Cheers

[jggimi]

Bitrate and resolution are tied-at-the-hip for perceived quality. You should not consider one without the other.

You may already be familiar with the forumula bits / pixels X frames. It's a simple method to analyze the ratios between bitrate, resolution, and content length.

It's the method that users of Gordian Knot use to set resolutions and bitrates, and is also used as the comparison values against peak possible bitrates for compressibility testing.

Your sample has 1054kbps, 720x384, 25fps, 300 frames. Your b/p*f value is 0.152. Doom9 says in his DivX 5 guide: It's not a good idea to go much below 0.17 bits/pixel*frameBut that's only a guide to use prior to one's initial compressibility test. The best practice is to select an initial resolution/bitrate using b/p*f guidelines, then select final choices based on compressibility tests, which provide a ratio of maximum bitrate / selected bitrate at the proposed resolution with the chosen filter set (cropping, resizing technique, noise reduction, etc.).

[SeeMoreDigital]

Granted Doom9's DivX 5 guide is useful.

However, the DivX codec is improving all the time and as such I've found that it is sometimes possible to use a lower bits/pixel*frame than the ones calculated.

I found this certainly to be the case if your intension is to generate encodes specifically for use with an Mpeg4/DivX standalone player.

Generally, TV's can get away with less bits per pixel and when you sit down to watch the encodes, you do so at an more suitable distance from the screen!

Cheers