X264 encoding (general discussion)

Khannie

I do a lot of x264 encoding and have done for a long time. Used to be for the PSP, now it's for the iPhone.

Do you encode H264 / x264 a lot? If so, what settings do you use?

I used to do 2 pass encodes to get the best quality for the same file size when space was an issue but now that it's less of a concern for me I'm currently using the following settings in mencoder:

nocabac:level_idc=30:bframes=0:global_header:subq=6:frameref=6:partitions=all:trellis=1:chroma_me:me=umh:crf=17

That looks ugly, so the important bits are:

• 1 pass
• CRF of 17 (I was using a bitrate of 1000 but crf of 17 seems to result in similar quality for significantly smaller files...I checked the PSNR)
• threaded
• frameref of 6
• subq of q

Both of the last two are on advice of the x264 documentation. They produce better quality video at the expense of longer encode times.

Bframes are disabled because the iPhone doesn't support them. :mad:

On my quad core (2.7GHz) I get around 50 FPS with that profile and it results in very high quality video.

Voodu Child

I use CRF, though rarely less than 19, I just find there is diminishing returns (in terms of visual quality) beyond that. To be honest I dont have a fixed figure, I tweak the RF depending on the source quality, and whether im going to be watching it on a big screen or a PMP. But with the current x264 build, i'd rarely go above 19 or below 20.

As for encoding settings, there's an old saying - 'Speed, quality, size....pick two'. Well for me, once i started using CRF that is quality taken care of, I get consistent visual results no matter what. And of the other two, speed is more important to me than size, because ive tonnes of storage space, but a relatively slow processor. So im not too bothered these days about using crazy x264 settings. Just a simple Main or High profile with conservative settings is fine for me. Full DVD rip (with full-res anamorphic frame and original AC3 track) rarely goes above 1GB or 1.5GB, looks 95% as good as the source, and is very quick even on my crusty old laptop. Job done.

Khannie

I did a few tests with the CRF and compared the PSNR value (basically how close to the original it was). I felt like the point of diminishing returns v's size was at 17. Between 19 and 17 there was a noticeable difference in PSNR (I can't remember exactly what it was, but it was enough that it should be perceptible).

I've only just switched to CRF there in the last week or so. I was using constant bitrate before that (mostly 2 pass). I'm amazed at the difference in file size for the same length of input video. I'm watching the "world at war" BBC documentary series and I encoded 2 episodes there yesterday. One came in at 719MB and another at 890!! (128 bit aac audio)

I should admit that I'm a quality snob. With the quad core and good support for threading in the x264 library (coupled with the fact that I can kick off an encode on the home machine while I'm in work...I love you linux) I'm not concerned with encode times at all. Even if I only had a single slow core I'd use the same settings.

Voodu Child

Khannie wrote: »

I've only just switched to CRF there in the last week or so. I was using constant bitrate before that (mostly 2 pass). I'm amazed at the difference in file size for the same length of input video. I'm watching the "world at war" BBC documentary series and I encoded 2 episodes there yesterday. One came in at 719MB and another at 890!! (128 bit aac audio)

And even more amazing is that if you were using High-Profile settings (which you can't obviously as you are encoding for the iPod) those encodes would likely be coming in at 500MB or less, and the same (or better) visual quality...there really is a massive difference in efficiency when you enable CABAC, use a few b-frames, etc. It really is a testament to the efficiency of x264, and even that has come on in leaps and bounds over the years.

I should admit that I'm a quality snob.:D

Not with 128kbps AAC you're not :pac:

Khannie

Voodu Child wrote: »

Not with 128kbps AAC you're not :pac:

Haha. I had it at 160 but the train just doesn't justify it. I'm about to invest is some mega earphones so I may go back to 160 again.