CAMERA



Autor: Uli Mors


Working with the S-Log color space (4 / 4)

Look Up Tables for S-LOG during Grading.

Colorgrading, contrast correction, color correction - I differentiate between "technical" and "artistic" grading:

In technical grading, the first step is to create a high-contrast image with correct colors from the low-contrast and unsaturated log images. In artistic grading, contrasts and colours are changed according to individual wishes and artistic demands - and they can be very different. In the TV sector, the goal is initially Rec709 (ITU Recommendation BT.709) for HD - in the future this colour standard will still change (BT.2020). 

The image, which at first is still far too low in contrast, must gain contrast and saturation and obtain the gamma curve typical of TV. On average, the usual plug-ins for contrast, brightness and saturation seem to be suitable - but the first "LOG trap" is already lurking here.

Contrast enhancement - recipes in NLEs

The log recording contains - as we could see in the past issues - for example in S-Log2 and S-Log3 up to 14 f-stops contrast range. Usually in the editing program a usual contrast effect raises the image parts above a certain mean value and lowers the shadows above this mean value. This mean value can be observed and determined with a waveform monitor or a histogram.

Example: The software Sony CATALYST BROWSE sets this average value at about 40 % brightness. This is where the normally exposed grey card is at S-Log3. With S-Log2, however, the correctly exposed gray card is somewhat lower, namely at 32%, i.e. below the contrast working point. This example alone already shows that the contrast control will not produce the same result with both variants or completely different log formats.

The same applies to deep shadows. The usual contrast effects of editing programs were designed for editing with Rec709. The log formats, however, contain the darkest image information that the sensor could see in the shadows. With the S-Log formats, the maximum underexposure is 8 f-stops. To draw these shadows "only" darker with a contrast control can end in a nasty surprise.



The contrast enhancement only compresses the shadows, but noise also remains visible in the image.


A comparison: If one exaggerates the use of the contrast control, Sony Catalyst Browse lets the shadows, casually formulated, drown mercilessly. DaVinci Resolve, on the other hand, has a contrast control with a "more beautiful" characteristic, which, with a start operating point of 50 %, initially only compresses the shadows when used excessively and only very late below 0. 

The idea behind it: the shadows are compressed increasingly despite boosting contrast and thus saves shadow details for a long time. The visible noise of log formats, however, remains too.

After all, the "Pivot" slider can be used to move the contrast center in Resolve. Unfortunately, this option is not available in all contrast/brightness/saturation plug-ins. And even if they were: Working with these parameters requires a lot of sensitivity and is - at least for technical grading - a blind flight.

Other approach: Look Up Tables (LUTs)

Fortunately, most camera manufacturers have already thought about how to create a correct Rec709 image with little effort: With Look-up tables, in short: LUTs. They can function very differently and serve different purposes. Literally translated, a look-up table is a Lookup table: For each red, green or blue value that comes from the sensor, a table is displayed to determine which new value is to replace it.

At 10 bit this would be 1,024 new table values for red, 1,024 for green and 1,024 for blue. A simple table would therefore have a total of 3× 1,024 = 3,072 values. Actually, we are talking about a new contrast curve for each basic color. These R/G/B curves are called "one-dimensional" or 1D LUTs - they resolve the colors well (10 bit), are therefore very accurate, but can't handle extensive color changes (for example mixed colors).

In other words: A 1D- LUT cannot turn red into green.


Three-dimensional Look-up Tables (3D LUTs) use all combinations of R, G, and B. 

That would be at 10 bit: 1,024 × 1,024 × 1,024 = 1,073,741,824 combinations that would find new values in an equally large list. Such 3D LUTs are possible with today's image processors are not common - maintaining 1 billion values and even replacing them in real time is (still?) too time-consuming. Instead, the respective RGB combinations are divided into 65, 33 or 17 segments, the reference values are defined in a list, and interpolated or averaged from segment to segment. 

With 33 × 33 × 33 values "only" about 36,000 entries in a list are necessary. This makes a possible kriktic at the 3D LUTs clear: The coarser the segments are, the less accurate the image information becomes. Thanks to modern interpolation and sophisticated algorithms, the use of LUTs is now common both in monitoring and in color grading - and simplifies the workflow, especially for the flat, colorless LOG- and formats enormous. Purists who want to get the most out of their footage and absolutely avoid LUT artifacts will either do without LUTs or resort to finer segmented LUTs.

Example: SONY 1D-LUTs

Sony offers 1D-LUTs as image correction in the current cameras. FS7 and F5/F55 have LUTs for the viewfinder and/or SDI and HDMI outputs in Cine EI mode:

- REC709(800%)
- Hypergamma 7: HG8009G40
- Hypergamma 8: HG8009G33
- S-Log

Nice detail: The LUT "Rec709(800%)" has a smooth highlight rolloff (therefore "800%"):

A higher-contrast everyday LUT than the 1D-LUT: "Rec709(800%)" is not available in the Sony cameras for S-Log. This makes it well suited for exposure and focus searches. The other HG-LUTs simulate the use of the hypergamma curves from the custom mode.

The following ones are visually more interesting:

SONY 3D LUTs

Sony established four 3D Luts (with 65 segments!) a few years ago for the F55/F5 under the creative name "Look Profiles", which can also be found in the FS7 today:

- LC709 ("LowContrast Rec709")
- LC709TypeA ("LowContrast Type A")
- S-Log2 709 (REC709 color matching with S-Log2 curve attached)
- CINE+709 (high contrast look)

In my opinion only the first and second versions are suitable for shooting and monitoring. "LC" generates a somewhat lower contrast REC709 image from the 14-aperture image ("Low Contrast"), which still contains a lot of information in the shadows ("black stretch") and provides a clear, but very pleasant highlight compression.

There are some rumours about the "Type A" version of it. It's a fact that Art Adams developed a LUT a few years ago that was based on the color/brightness behavior of an ARRI Alexa. Among other things, the colors desaturate with increasing brightness, which takes the "squeaky" oversaturation of typical REC709 broadcast cameras from the shots or the monitor image.

These 3D LUTs are not only available in the cameras, but also as online downloads and as presets in Catalyst Browse, for example.

The easiest way to get the correct image during post-processing:

Load the clips into Sony Catalyst Browse (free for PC and MAC) and automatically select the LUT parameters from the shoot - simply via VIEW - ADJUST COLOR - RESET. The Catalyst Browse software takes the LUT selection, exposure index and white balance used during shooting from the clip's metadata - and sets the display to it. 


S-Log3 becomes "Rec709(800%)" with a soft High light-Rolloff.

If a color RESET is performed, the software gets the exposure index, white balance and the LUT used in the camera from the meta data and "graded" the picture.
This way I get the same image as seen in the viewfinder/monitor. Alternatively you can select the Look-Profiles or other LUTs manually. Below you can change all color and contrast parameters similar to a grading software.


Free viewer/transcoding software with LUTs and gradation options for SONY recordings: Catalyst Browse


LUTs for the editing software?

If you are afraid that your editing or post-processing program does not have the correct LUTs, you can export the "pure" Look-Up-Tables as 33 LUTs yourself after resetting or manually selecting the LUTs in Catalyst Browse:

TOOLS -> EXPORT COLOR SETTINGS

In the following selection you can define whether only the basic parameters (Exposure Index, White Balance, Tint and LUT) should be included in the new LUT or also the creative parameters (RGB curves, Saturation, Lift Gamma Gain etc.).

A word of warning:

The Lumetri panel in Adobe Premiere offers two "SLog SL Profiles" in the preset list but they are NOT identical to the camera LUTs - probably they were not created directly for the CINE EI mode and SGamut3.cine and deliver a much different image impression than the camera's own LUTs. Furthermore, Lumetri does not react to the camera's parameter data yet - intentional overexposure has to be compensated manually at the moment.

Another word of warning:

If possible, use the "Browse" function in Premiere CC to select LUTs. Our attempt to modify the preset list failed mercilessly when we tried to export a project via MEDIA ENCODER. Apparently, the Media Encoder does not take the (modified) preset list, but looks in its own preset list after the same entry position - where then another LUT lies. During the manual "Browse" search for our LUTs, this path was then also used in the Media Encoder.
and the export was correct afterwards.

Using LUTs exported in Catalyst Browse (see above) also works in other editing and grading programs and in field monitors/recorders.

Conclusion

You can get LUTs with the name "Slog-to-Rec709" from countless sources! If they are not well documented, you can only speculate about the color space they come from, the sources they are designed for, and their characteristics. Hence my urgent note: Use LUTs whose origin you know. Catalyst Browse and the export of standard LUTs is a good starting point for material with Sony SLog.

Related Articles: