svendson

I have to believe that David is talking about "in practice," b/c the math is not perfect.

Here's a simple example that illustrates what I'm saying:

Have an image with a perfectly uniform shade of grey that falls between two 8-bit values but closer to the higher value. Lets say between 100 and 101 but closer to 101. Well it's going to be encoded as 101.

But let's say you take the same perfectly uniform shade of grey and sample it with 10-bit precision. So it falls between 400 and 404, but lines up perfectly with 403.

There is no way that four 8-bit values of 101 are going to mathematically sum to 403. They are going to sum to 404. And 404 <> 403.

While I'm sure the downrezing helps the bit depth considerably, the math is not perfectly reversible.

A rounding error of slightly less than half a least significant bit on a uniform scene, as per your example, is not a significant or noticeable issue. To see the advantage of four 8 bit samples -> One 10 bit you need something like a very gentle gradient that will generate noticeable banding. Then the four samples will, when averaged, allow 3 extra values between 100 and 101 - 100.25, 100.5, and 100.75, depending on how many of the 4 samples are 100 and how many are 101. Now of course, they won't actually take a decimal value as we are now working in a space with 10 bit resolution, not 8 bit and so the actual integer value will be times 4 and you can in fact resolve anywhere integer value between 400 and 404.

This is in fact perfect math. But, as you point out if the absolute value of a uniform scene is subjected to a quantization error due to 8 bit resolution, the correct absolute value cannot be correctly reconstructed.

Wikipedia has some nice info on over sampling and how you can trade back and forth between sampling rate and bit depth. Lots of info in this with regard to audio recording as well, if my explanation is not adequate

This really is NOT the same as "truncating" to 8 bit and then re-inventing data to get 10 bit.    More data really is there to begin with!

 

David lays the math out neatly:   4,  8-bit values combine to make one 10 bit.  Easy math isn't it?      (256+256+256+256=1024).  In other words you have 4 pixels each with a value between 0 and 255, which gives you 1024 possible combined values.

 

I asked David Newman the same general question about downsampling earlier in the week and got this reply:

 

"Very nicely, overkill even. 4:2:0 maps to an effective 4:4:4 with root-2 the image size. So 2.7k 4:2:0, is a nice 1920x1080 4:4:4 -- notice 2.7K is one of GoPro video modes partly for this reason."

 

 

Ok, on 4k 4:2:0 to 1080p downscale, we get 4:4:4. because at 4x oversampling with 4:2:0 we just manage to get 1 chroma per color channel sample per pixel when we downscale. Everyone seems to agree on this.

 

I get the 4 8 bit -> 1 10 bit part, and agree with it, but once again this is only for luminance, because this is the only channel we actually have oversampled data for! The whole 8 bit to 10 bit thing only works if you have genuinely oversampled data. As per above, we only have one 8 bit chroma sample per pixel per channel after the downscaling. Thus not possible to do any neat math, from a pure mathematics perspective. Thus, yes 4:4:4, no not proper 10 bit.

 

Where it gets murky for me is this root 2 of the image size scaling factor that Mr. Newman has stated. Either he is talking 4:2:0 to 4:2:2 instead of 4:4:4, or there is some other fancy stuff going on based on human perception and not basic math. In any case, still not going to get proper 10 bit chroma at 1080p out of 8 bit 2.7k footage.

My understanding is that the downsampling can yield 10 bit Luma, because there are 4 8 bit Luma samples getting combined into one, not full 10 bit color sampling. Anyone else get that?

As I understand it, the resolution doesn't matter - 4k/1080p, 24p/30p/60p, whatever; 100Mbps is 100Mbps.

The formula is ((Card Size in GB)*953.7*8)/(3600*(Selected bitrate in megabits per second)). This gives recording time in hours, and assumes that SD cards, like hard drives, are not listed in proper gigabytes. I've never bothered looking into that one.

 

64GB card at 100Mbps is then 1.36 hours of recording time.

 

edit: corrected formula to account for camera bit rates actually being in multiples of 1024 and not 1000.

4:2:0 in 4k simply means that the luma resolution is 4k and the chroma resolution is full hd, so it actually will give you 4:4:4 in 1080p. which brings up the question why it cant just record 1080p 4:4:4 natively. or at least 4:2:2...

 

 

I brought this up in another thread, but made the mistake of comparing the decision to Cannikon feature crippling, and my technical questions got lost in fanboy defense, unfortunately.

 

I've since found some interesting discussions on the Black Magic forums, where they talk about this same issue but with regards to 60p, instead of 422 10 bit. The basic issue seems to be the amount of data you are feeding _into_ the encoder, not the data rates coming out of it.

 

Some more looking around, and the issue seems to be with affordable encoder technology lagging the rest of the camera subsystems a little. As noted in this thread, the Odyssey 7 seems to be the only reasonably affordable device that can capture 1080p60, but it does it at 8 bit 422. Blackmagic and a bunch of other recorders do 10 bit 422, but only up to 1080p30. I guess there are must be some high end recorders that can do 10 bit 422 1080p60, but I suspect they must be running some pretty low production volume encoder chips (expensive) or using FPGAs for the encoding (even more expensive).

As such, given the current choice in the low budget segment seems to be high frame rate 420 8 bit, or max 30 fps 422 10 bit, I think Panasonic made the right call for differentiating their product. Certainly, as a rank amateur looking for a hybrid camera to take snowboarding and kiteboarding, the high fps appeals a lot more than 10 bit 422.

The best discussion on this camera, which I am extremely interested in, seems to be happening here so I guess it is time to stop lurking and join in.

 

I think the article has an error re burst modes - according to the Panasonic website, 40 is the number of raw frames it can capture in burst, not the frame rate.

 

8 bit 4:2:0 seems a bit of a bummer on paper, but realistically as a complete noob it probably isn't such a big deal for me. It is the what-seems-to-be-intentional crippling, just like Canon et al, that really turns me off. Downsampled 4k should be able to give 4:4:4 right? Still 8 bit, but if they are giving at least a 50% work around built into the camera, why make the work around necessary at all?? Likewise, isn't 10 bit 4:2:2 about 1.7 times the data rate of 8 bit 4:2:0 for a given compression level? So seems like the data rates, for 1080p at least, are well within the given specs?