In Part One we looked at how we can improve results from 8-bit video, regardless of where it came from. In this part we’ll look at higher bit depth videos, namely 10-bit Rec. 709, 12-bit or 16-bit RAW.

The difference between 10-bit and 16-bit

10-bit images can have 1024 shades per channel, which translates into a billion colors. Kodak engineers found out long ago that you could put all the information from 35mm film into 10-bit log DPX or Cineon. The key word is log.

In the real world, color (light) behaves in a linear fashion, and so does a lot of the math. VFX and color correction applications have both kinds of math, log and linear. Sometimes, one is more helpful than the other. It is impossible for anyone to say that linear or log is a winner in absolute terms. That shows ignorance.

All right, what is the linear-equivalent of 10-bit log? Hard to say, but if I had to guess based on my experiences I’d say 16-bit. With 16-bit, you have 65,536 shades per channel, or trillions of colors. What’s the point of a camera having 16-bit colors?

Cameras are not eyes, nor are they theoretical color spaces. They are physical things that react uniquely to light. Each sensor has its own color gamut (which dictates its color space). The camera manufacturer decides to either:

  • Map that into a standard color space like Rec. 709 (which happens on most consumer and prosumer cameras), or
  • Give it to us RAW.

RAW files don’t have a color space attached to them, though their gamut is restricted to the sensor’s. You could theoretically map these RAW files to any color space, but in practicality, some RAW formats don’t work well with certain color spaces! E.g., this is why Red cameras have RedColor color spaces, or Sony F65 RAW has its own gamut mapped, etc. Similarly, it’s why the Arri Alexa is more ‘friendly’ towards log (Log C) – they come from film so they prefer to not rock the boat. Okay, all this doesn’t concern us much. We can’t dictate terms with camera manufacturers, just live with what they offer.

In practice, you normally record 10-bit Rec. 709. Sometimes, 10-bit DPX (linear or log) files are created in post or by an external recorder. Or, you record 12-bit (BMCC, Red, Alexa, etc.) or 16-bit (Sony F5, F55, F65, etc.) RAW. The future points towards 16-bit sensors, so we’ll look at 16-bit images. If the tests are helpful to 16-bit linear images, then anything else will benefit, too.

Therefore, our test subject is a 16-bit uncompressed TIFF image gradient, in Adobe RGB (1998). I’m going with 1920×1080. Here’s the image (it’s only Rec. 709 JPEG, the Adobe RGB TIFF is 12.5 MB):

Red Gradient Rec 709

Why Adobe RGB? Well, camera sensor gamuts are higher than sRGB or Rec. 709, but aren’t close to ProPhoto RGB or DCI X’Y’Z’ by any stretch of the imagination. Adobe RGB is close though, which is why it is included in DSLRs for stills. It’s an educated guess, and could be totally wrong. If you want to do similar tests, but know your camera exactly, you should specifically test that color space.

Please don’t blindly assume I’m right.

Just for fun, here’s the difference between Adobe RGB (smaller gamut) and ProPhoto RGB (larger grey gamut):Adobe RGB vs ProPhoto RGB

The tests

The tests are similar to what we did in Part One. One major difference is that the H.264 Quicktime video will be rendered out at 1,000 kbps (about 1 Mbps). We’ll be looking at:

  • Bit depth – 8, 16 and 32-bit float
  • Working Color Space – Rec. 709, Adobe RGB and ProPhoto RGB (All in linear space)

That’s nine combinations. RAW footage is sometimes brought down to Rec. 709. Sometimes, you have the luxury of working in the camera’s native space. And sometimes, you trade up to ACES. The output color space is Rec. 709.

Very important: A note on linear-mode: Adobe After Effects does not recommend linear mode in 8-bit depth. I’ve only used it to keep the math constant. In the real world, don’t use linear mode with 8-bit!

We cannot see the full effect of a 16-bit image on an 8-bit or 10-bit monitor, so you’ll have to judge the results from the 8-bit videos and stills exported from Adobe After Effects.

A note on chroma subsampling: There will obviously be some variances between going from RGB (TIFF) to H.264 4:2:0. Unlike the 8-bit case, here, the differences might not be negligible. There’s simply more color to lose. Whether or not you’d notice it, is a different matter!

Observations and results

What happens in Adobe RGB color space? Here are the results (Click to enlarge):

Adobe RGB Comparison

As you can see, going to a higher bit depth certainly helps. What about Rec. 709? Here are the results:

Rec. 709 Comparison

The results are similar. Let’s look at ProPhoto RGB:

PP RGB Comparison

As we saw in Part One, ProPhoto RGB and high gamut color spaces really need 32-bit float to come into their own. Except for that, there is no difference between 16-bit and 32-bit visually. It remains to be seen how they perform on compression.

Differences after H.264 compression

Compression always works better with higher resolution material, because the artifacts tend to be smaller and less visible. These images are from 1080p H.264 material with a data rate of 1 Mbps. The smaller view does not hide anything. (First rows – 8-bit and 16-bit, second row – 32-bit. Click to enlarge):

Adobe RGB (Frame 1)

MOVAdobef1

Adobe RGB (Frame 49)

MOVAdobef49

ProPhoto RGB (Frame 1)

MOVPPf1

ProPhoto RGB (Frame 49)

MOVPPf49

Rec. 709 (Frame 1)

MOVRecf1

Rec. 709 (Frame 49)
MOVRecf49

We can discount ProPhoto RGB, obviously. It has to be in 32-bit.

But what about Adobe RGB vs Rec. 709? You can clearly see that Adobe RGB has better colors than Rec. 709, though compression is equally fair to them both. The surprise is ProPhoto RGB at 32-bit, which looks the best.

The difference between 16-bit and 32-bit is negligible, which is similar to what we saw in Part One. My analysis:

  • When you’re shooting in greater than 8-bit, always go to an equivalent or higher bit depth, but 16-bit should be perfect for 90% of any workflow.
  • Never go to a lower color space for post production. Maintain the highest color space until the last moment. If you’re shooting RAW, work natively in the manufacturer’s specified color space or go to ACES or ProPhoto RGB, etc.

Takeaways

What do we learn from this and Part One? I could summarize my findings into two rules, if image quality is your main concern:

  • Always work in 16-bit or higher.
  • Never lower your color space for post production (e.g., moving from RAW or camera gamut to Rec. 709 is not a good idea).

Enough said. What do you think?

5 replies on “Should you raise Bit Depth and Color Space before Color Grading? Part Two: RAW Video”

  1. Hi Sareesh,

    Excellent methodology and some very useful results, but primarily for http://www.adobe.com/products/catalog/software.html applications. Working with images in different colour spaces has been around for many years, but only recently has it become possible for video. As you know, none of the  http://www.adobe.com/products/catalog/software.htmlRGB / ProPhoto conversions are (sadly) possible in FCPX, Avid or Resolve. If you out an ARGB or ProPhoto file into the first two, it will work, but colour will be wrong as sRGB / 709 is assumed.

    Firstly, it should be said that none of this should be tried without a hardware calibrated monitor. Calibration is now cheap and ensures that what the person grading sees is what others will see as a final result.

    Secondly, there is one small, but critical piece of information missing from the above testing, something that you are obviously familiar with, but few others will be: assign versus convert.

    When working with images or video, NEVER, ever ,ASSIGN a colour space to a file, always CONVERT between spaces. Assign is like sticking the wrong label on a tin of paint and will result in inaccurate colour at best, possibly something worse.

    Keep up the good work!

    Nick

  2. Hi Sareesh,

    thank you, I keep folowing you for a while now to get a better understanding for ACES. This article cleared some things on my mind, thank you! I don’t have a lot of experience with ACES but I’m digging into it. More from day to day.
    (Background: I’m currently working on an ACES comparison for my bachelor thesis (workflow comparison of a music video shot with an ARRI ALEXA XT Plus 2.8k, a BMPC 4k and a C500 4k). The results of working with different raw files are quite interesting and we were able to put them together quite well. I’m currently searching for reliable sources of color gamuts and found the one below. They just look a lot nicer than the usual ones i can find online, though I’m not quite sure if I can trust that source in terms of accurate representation. Do you know a reliable one which shows ACES, Rec. 709, sRGB, Adobe RGB and DCI P3 in comparison or each of them in single?

    Thanks Raph from Cologne

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