bit depth, resolution, and x264

[mindbomb]

I have been wondering this. Popular thinking is that to save bandwidth, when compressing 1080p video, you can downscale to 720p first. However, it has been shown with 10 bit compression, even on 8 bit video, compression is improved.

Does it follow that compression is actually better for 1080p video for analogous reasons?

[Dark Shikari]

This would be true if H.264 was an (near) infinitely scalable format, like most wavelet-based formats. If it was, more resolution would never hurt, because even in the worst case, the encoder could simply remove the extra data.

However, it isn't, and is based heavily around transforms of certain sizes (4x4 and 8x8), so it starts to lose efficiency at very low bitrates for a given resolution.

Bit depth is different -- bit depth is a level of precision, not an amount of pixel data.

[mindbomb]

Ok, thanks.

I was wondering about this especially now, since it appears Apple is offering 1080p video now, and despite having over 225% increase in the amount of pixels, they only opted to increase the bitrate 25% from 720p.

[mandarinka]

That's the problem with services delivering over internet - low bitrates.
Personally I tend to curse silently when reading about blurays being 'obsolete relics of a hopelessly outdated business models of the past' somewhere. People don't appreciate the luxuries of optical media with ample space for high bitrate content. Maybe they will remember later, when the discs are gone from shelves... (And in this case, it won't be vinyl X cdda kind of nostalgia.)

[Blue_MiSfit]

@mandarinka: People like you and I and others on this forum care, and really appreciate the quality allowed by high bitrate formats like BluRay. Most people, however, do not. In fact, the mere placebo effect of "1080p" is enough to make most people happy.

That being said, 1080p video can look pretty damned good at surprisingly low bitrates (5-10mbps) in most scenes. I'm not sure which encoder Apple uses, but it's probably not as good as x264. In any case, the iTunes stuff is usually pretty watchable, so I'd expect this to be a fairly good quality service. If I recall, it's a progressive download system, not a (static or adaptive) streaming system, so you get the advantages of proper 2 pass VBR encoding, hopefully with a reasonably sized buffer and long, adaptive GOP.

From the perspective of a media company supplying them content, the interesting thing about iTunes is their utter obsession with source quality. It's all required to be spotless 1080p ProRes 422 HQ. They're more picky about quality control than anyone else, and are clearly in an "acquire once, and re-encode when necessary" mindset.

Derek

[mandarinka]

Well, it can obviously look surprisingly good at low bitrates as you say, naturally. But extra bitrate on bluray acts as airbags/deformation zones again encoder screwups, incompetence, or just hard to compress scenes/sources (artificial/intentional noise...). And generally, higher bitrates usually help, even if there are diminishing results :)

[mindbomb]

I assumed apple had their own h264 encoder. Doesn't quicktime come with one?

I think they probably also changed the profile from 3.1 high to 4.1 high btw.

[cacepi]

I think they probably also changed the profile from 3.1 high to 4.1 high btw.
Well, they had to since the maximum resolution level 3.1 supports is 1280x720.

[kolak]

@mandarinka: People like you and I and others on this forum care, and really appreciate the quality allowed by high bitrate formats like BluRay. Most people, however, do not. In fact, the mere placebo effect of "1080p" is enough to make most people happy.

That being said, 1080p video can look pretty damned good at surprisingly low bitrates (5-10mbps) in most scenes. I'm not sure which encoder Apple uses, but it's probably not as good as x264. In any case, the iTunes stuff is usually pretty watchable, so I'd expect this to be a fairly good quality service. If I recall, it's a progressive download system, not a (static or adaptive) streaming system, so you get the advantages of proper 2 pass VBR encoding, hopefully with a reasonably sized buffer and long, adaptive GOP.

From the perspective of a media company supplying them content, the interesting thing about iTunes is their utter obsession with source quality. It's all required to be spotless 1080p ProRes 422 HQ. They're more picky about quality control than anyone else, and are clearly in an "acquire once, and re-encode when necessary" mindset.

Derek

Yes and Apple has only few approved encoding facilities in the UK for example. You can't just add content to iTunes- it can be done only by few companies. IF you want became one you have to go through whole process and it's not easy to be approved.
I would say it's done by something like Carbon Coder, which uses Mainconcept engine. It's done on large scale, so source has to be very good. If source is good than most likely encoded file will be ok.