Uncompressed HD vs Arriraw

[LucaManciniLuca]

Hello Mr. Deakins,
<p style=”text-align: left;”>I was reading an ASC article on Skyfall and they mentioned:</p>

“<span style=”color: #000000; font-family: Merriweather, sans-serif; font-size: 20px;”>The uncompressed ArriRaw format was also a new development for the Alexa since </span><em style=”border: 0px solid #e5e7eb; box-sizing: border-box; –tw-translate-x: 0; –tw-translate-y: 0; –tw-rotate: 0; –tw-skew-x: 0; –tw-skew-y: 0; –tw-scale-x: 1; –tw-scale-y: 1; –tw-scroll-snap-strictness: proximity; –tw-ring-offset-width: 0px; –tw-ring-offset-color: #fff; –tw-ring-color: rgba(59,130,246,0.5); –tw-ring-offset-shadow: 0 0 #0000; –tw-ring-shadow: 0 0 #0000; –tw-shadow: 0 0 #0000; –tw-shadow-colored: 0 0 #0000; color: #000000; font-family: Merriweather, sans-serif; font-size: 20px;”>In Time.<span style=”color: #000000; font-family: Merriweather, sans-serif; font-size: 20px;”> Deakins notes, “We were told </span><em style=”border: 0px solid #e5e7eb; box-sizing: border-box; –tw-translate-x: 0; –tw-translate-y: 0; –tw-rotate: 0; –tw-skew-x: 0; –tw-skew-y: 0; –tw-scale-x: 1; –tw-scale-y: 1; –tw-scroll-snap-strictness: proximity; –tw-ring-offset-width: 0px; –tw-ring-offset-color: #fff; –tw-ring-color: rgba(59,130,246,0.5); –tw-ring-offset-shadow: 0 0 #0000; –tw-ring-shadow: 0 0 #0000; –tw-shadow: 0 0 #0000; –tw-shadow-colored: 0 0 #0000; color: #000000; font-family: Merriweather, sans-serif; font-size: 20px;”>Skyfall<span style=”color: #000000; font-family: Merriweather, sans-serif; font-size: 20px;”> would get an Imax release, so how our images would look on that giant screen was a big consideration. I shot tests comparing uncompressed HD, which I used on </span><em style=”border: 0px solid #e5e7eb; box-sizing: border-box; –tw-translate-x: 0; –tw-translate-y: 0; –tw-rotate: 0; –tw-skew-x: 0; –tw-skew-y: 0; –tw-scale-x: 1; –tw-scale-y: 1; –tw-scroll-snap-strictness: proximity; –tw-ring-offset-width: 0px; –tw-ring-offset-color: #fff; –tw-ring-color: rgba(59,130,246,0.5); –tw-ring-offset-shadow: 0 0 #0000; –tw-ring-shadow: 0 0 #0000; –tw-shadow: 0 0 #0000; –tw-shadow-colored: 0 0 #0000; color: #000000; font-family: Merriweather, sans-serif; font-size: 20px;”>In Time,<span style=”color: #000000; font-family: Merriweather, sans-serif; font-size: 20px;”> and ArriRaw, and we blew those images up and watched them on an Imax screen at Swiss Cottage. It was quite startling; both images looked pretty damned good, but the ArriRaw had a definite advantage.”</span>

 

What was this particular advantage of Arriraw you saw considering both are lossless formats I believe ??

 

Thanks

 

Topic

[dmullenasc]

In the early days of the Alexa, uncompressed HD out to a Codex was used because Arriraw wasn’t an option yet. “Game of Thrones” also used that format at first. Uncompressed HD is uncompressed, unlike ProRes, but it’s still a debayered RGB signal so color temp and ISO are baked into a Log-C output like with ProRes. And it’s downsampled to HD resolution.

I also think three HD channels uncompressed is actually more data to handle than Arriraw… leaving the data as a single Bayer pattern signal is a form of data compression in that debayering it to RGB triples the amount of data (if keeping the same resolution — in this case, though, the signal is being downsampled to HD.)

Today with the internal ProRes 4444 xq option, or Arriraw, there’s no reason to record uncompressed HD out to a Codex.

[gabj3]

To kind of second what was said above –

A RAW signal is a radiometrically linear readout of  your sensor.

In the case of an Alexa a 16-bit unsigned integer linear readout.

The issue with a linear raw signal is human beings don’t perceive light linearly but rather logarithmically and our image typically resembles this.

 

As we know, a stop is double or half the amount of energy. We perceive each stop logarithmically (each stop has the same amount of apparent energy) where as in an imaging sense we perceive light linearly where the brightest stop is half the amount of total values, the second brightest stop is a quarter the total amount of values – very inefficient as we perceive all stops logarithmically.

 

So we compress the linear signal with a logarithmic container. We take the Y value and give it a linear bias to 256 and then apply a log encode that gives 512 values per stop.

 

This logarithmic compression is the only difference in information than an absolute RAW acquisition format (acquiring a 16-bit linear signal).

Infinityvision.tv
Gabriel Devereux - Engineer

[gabj3]

I should note as well, ARRI has its whole  32 bit word and wrapping but the core information is a 16-bit linear Y values compressed with ARRIs LogC container.

That’s it, that’s ARRIRAW.

Infinityvision.tv
Gabriel Devereux - Engineer

[dmullenasc]

Arriraw is a 12-bit log recording but it’s not Log-C, there’s no color space or gamma applied to the image, it’s just data that is converted from 16-bit linear to 12-bit log for storage.

[gabj3]

Hi David,

100% no gamut is applied and the log-c container is applied to the scene linear RGB values.

From my understanding (which could be entirely wrong). Log-C defines the equation that encodes a linear signal to a qazilog container for Arri cameras.

Log-C doesn’t inherently dictate the gamut but just dictates ‘luminance values’. Of course, all of this happens prior to debayer or any kind of chromatic related OETF.

Infinityvision.tv
Gabriel Devereux - Engineer

[dmullenasc]

Log-C gamma is meant to emulate film negative scans but I think it’s applied to Arriraw conversions / debayering to RGB for color-correction, it’s not the 12-bit log storage format of Arriraw. ARRI ‘s own website is not completely clear on this but I believe we’re talking about two different things, the Log-C used for debayered images and the mathematical log used for data storage in Arriraw.

[dmullenasc]

<p style=”text-align: left;”>I could be wrong, of course!</p>

[dmullenasc]

I got this info from someone at ARRI:

There are two distinct types of logarithmic data:

Users see LogC3 (pre-ALEXA 35) or LogC4 (ALEXA 35 and later) RGB data.

Writers at the device driver level see another logarithmic format, extremely early in the imaging pipeline, that is used to store the image while it is still a photomosaic, i.e. has not yet been debayered into RGB.

For pre-ALEXA 35 cameras, there is a ‘hidden’ form for 12-bit photomosaic data, and then for each exposure index there is a particular variant of the LogC3 curve (‘gamma’ if you will, though not in the mathematical sense). The LogC3 differences are slight enough that most people don’t even know they exist.

With the ALEXA 35 there is the same ‘hidden’ form, this time extended for 13-bit photomosaic data, and then there is one (and only one) LogC4 curve. 

SMPTE RDD 55, which documents ARRIRAW in MXF, lays out the details of both 12- and 13-bit low-level bitstreams.

[Author]

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