FfmpegX x264 settings

[bvdw]

Hi,

I'm new to ffmegx and I don't understand some settings..
I want to encode my DVD's so i can watch them with VLC (and if possible on my iphone) and in the future with appleTV.
I already figured out how to adjust the bitrate, so my output file is +/- 700 MB. However i don't understand a thing from the option-tab.
I just need to know some good settings, so i still have a good quality (no matter how long it takes)

thx

[Case]

x264 H.264 options:
- CABAC: a form of entropy coding. A way to get smaller files. Requires a considerable amount of processing to decode. "Main Profile" will be used. Not compatible with iPhone/AppleTV/default QuickTime, AFAIK.
- B-frames: Bi-directional predicted frames. A way to get smaller files. The current frame may use references of the previous frame or the next frame, whichever uses the least bytes. Requires more processing to decode (several frames in advance). "Main Profile" will be used. Not compatible with iPhone/AppleTV/default QuickTime, AFAIK.
- Constant bitrate: Each time-interval of encoded material uses the same amount of bytes. Complex scenes may suffer quality while simple scenes use more bytes than needed. Benefit is predictable file size and constant read speed. Default for iPhone preset (not sure if it is needed).
- Two pass encoding: Encodes the clip twice to determine which scenes need the most bytes to describe them. Results in better quality clips.
- i4x4 analysis: Enables i4x4 partition analysis. Not sure exactly what it does, but it is supposed to boost quality. Perhaps it looks at 4x4 pixel blocks instead of 8x8 pixels?
- Trellis (requires CABAC): Enables Trellis quantization. Trellis quantization is an algorithm that can improve data compression in DCT-based encoding methods, such as H.264. Activation should result in smaller output files. Some say the algorithm costs quality, but the file size saving may be used to allow for a higher bitrate, thus improving quality. Not compatible with iPhone/AppleTV/default QuickTime, AFAIK, because of CABAC requirement.
- Decode with Quicktime: Does not use ffmpeg to read the source file, but uses QuickTime instead.

Encoding profile: set the target playback device.

Max. GOP size: Maximum number of frames to form one Group-of-Pictures. This will determine when an i-frame will be used. For MPEG-4, including H.264, i-frames will be used when needed, but the maximum number of frames between two i-frames can not exceed the maximum GOP size as set here. "250" frames is about 10.5 seconds, which seems like plenty. The recommended GOP size is only based on the codec. For a different codec, one would use a different GOP size. Stick to the defaults, unless you know what you are doing.

Adaptive Quantizer: For Constant bitrate, this allows to vary the compression between two quantizer values (Qmin and Qmax) to get the same bitrate for each time-interval. Qmin is lesser compression (and higher quality); Qmax is higher compression (and lower quality). AVC (H.264) uses a different quantizer scale (1-51) than most other codecs (1-31). Changing Qmin and Qmax will have an effect on the bitrate. Lowering Qmin will allow for better quality in some scenes, upping Qmax will allow for worse quality in some scenes, determined by calculations based on the set bitrate. Stick to the defaults, unless you know what you are doing.
If you set Qmax to the same value as Qmin, you will not get constant bitrate, but a sort of constant quality (mathematically determined, not visual!). It will ignore the bitrate setting.
Setting Qmin to a value lower than recommended will result in (much) bigger files without noticeable visual improvement, that is: Qmin should not be lower than 20.
If Constant bitrate is disabled, only Qmin will be used.

AVC level: Definition of maximum values and properties to be used (framerate, frame size, etc). Using a higher level than needed will limit compatibility. Does not affect quality. iPods/iPhones typically will not play H.264 files with AVC level above 3.0, older models even lower.

ME function: Motion estimation method. Reduces file size by looking if a section moves (mostly) unaltered from one frame to the next. More complex function use more time to encode, but will get better results, thus smaller files. The list is from fairly simple and fast (Diamond) to complex and slower (Exhaustive).

[bvdw]

So what would you recommend? Two pass encoding or a constant bitrate?
And which values for qmin and qmax for the best quality?

Does decoding with quicktime give an advantage?


Btw, thx for the explanation!

[Case]

So what would you recommend? Two pass encoding or a constant bitrate?

Two pass encoding will give better quality, at the cost of twice the encoding time. I only use constant bitrate if the task requires it.

And which values for qmin and qmax for the best quality?

The defaults are good. Changing them will affect the output file size, beyond control of the bitrate calculator. Qmin=Qmax=20 will give superior quality, but the output size may be unpredictable. It will use as much bytes as it needs to get the quality that goes with that quantizer value.

Does decoding with quicktime give an advantage?

It can, sometimes. Decoders (may) do some post processing to improve picture quality. ffmpeg handles it slightly different than QuickTime, because it was developed by different programmers. Not sure if it is noticeable, though, unless you know which details to look for. Tweaking other settings have a bigger impact, though.

The better reason to use one instead of the other, is of a different kind. Some codecs in the source file fail with one decoder, while the other handles it well. Sometimes QuickTime decoding handles a/v sync better.
Think of it as a secondary tool in the conversion toolbox, so you won't have to give up if it doesn't work as planned at first.

[jpschuck]

CABAC, B-Frames and Trellis quantization are indeed default Quicktime compatible. ffmpegX doesn't offer any h264/x264 setting that isn't Quicktime compatible.

Given your unspecifical needs (no iPhone, no AppleTV, no iPod), bvdw, you should use these settings as long as encoding speed is not important for you :
- a bitrate producing a 700 MB output
- 2 pass encoding
- Cabac, B-Frames, i4x4, trellis enabled
- Exhaustive ME function
- Max AVC level (5.1)
- no encoding profile
- do not modify GOP and quantizer values
- do not use quicktime unless you're encoutering some A/V problems, or you're switching between PAL/NTSC