What is a 3D LUT
By Chris Heinonen on
What is a 3D LUT
If you’re looking to get a new LG OLED or LG Nano LCD in 2018 or later, you’ve probably seen the announcements or read the specs for them. One feature on them you probably didn’t see, or maybe didn’t know much about, is the presence of a 3D LUT, or Cube LUT (Look-Up Table) with direct access. While many TVs have had 3D LUTs in their video pipeline for a while, the LG models are the first to offer direct access for AutoCal. This allows a level of control, and performance, that has never been available on a TV before. So what is a cube LUT, and how does it improve your image quality?
The key to understanding the benefits of a cube LUT for color gamut adjustment, over a conventional CMS requires and understanding of how a TV CMS works. The way your TV views color is like a cube. There are 8 points on the cube which represent white, black, the primary colors (Red, Green, and Blue) and the secondary colors (Cyan, Magenta, and Yellow). Inside the cube, those 8 points combine to mix all the resulting colors that a TV can display.
Currently, on a TV, you’re likely to set the white and black points of the cube using the contrast and brightness controls. The white balance controls allow you adjust the points between white and black, making your grayscale accurate to the D65 reference that HDTV, 4K, and HDR use. To get accurate colors, most TVs now let you adjust the 6 color points in three axes, allowing you to adjust the Hue, Saturation, and Luminance of each of the six colors. Some TVs don’t allow this, but most higher-end TVs let you calibrate these to get them exact. The limitation of this technique is that you are only making adjustments to the edges of the color gamut with a tradition CMS. That means you have no control over the whole inside of the color volume.
So once you have your grayscale correct and color points correct, what happens when you need a color that isn’t one of those exact points? At that point, the TV uses those 8 points to calculate the millions of other colors you see. The LUT cube is usually a certain size, perhaps 3x3x3 or 9x9x9, which have 27 or 729 points respectively. Those points are calculated and stored, and then colors at those points are, as the name suggests, looked up and adjusted. Colors that fall between those points are ‘interpolated’ or calculated. In practice, this can work well, but not always, and the larger the LUT cube size, the less interpolation necessary.
Some TVs are very good at doing the calculations between points. Once you set those 8 primary points to be correct, they can do the calculations and get the other colors to be spot-on. However many TVs can’t do this as well. If you adjust the main color points too much, it can create errors in the calculations that propagate down as it tries to do the math. The TV might not have enough storage or space to properly calculate these other data points. You’ll often see TVs where charts look perfect, but the content looks bad as these math errors become apparent. On some TVs instead of adjusting the corners of the cube, you do better by adjusting points inside of the cube, but there is no way to know this without testing. Every TV panel is different as well, so calculations that are accurate for one TV will be incorrect on another one, just from variance.
Enter the Cube LUT
On the 2018 OLED models and high-end SK LCD models from LG, they are introducing a cube LUT that calibration software will be able to directly address. With 33x33x33 or 17x17x17 cube LUTs, instead of adjusting 8 points, you will now be able to directly control and address 35,937 or 4,913 individual points. This means that the TV, instead of having to do all its color calculations from 8 reference points, it has thousands to use. By calibrating these points, we now have thousands of reference points instead of a handful, enabling the display to do less math and increasing the accuracy of results when it does calculate colors.
Directly adjusting these LUTs is something that you cannot do yourself because of the huge number of data points that need adjusting. CalMAN software will automate this process, taking multiple reads and adjusting the many points available. CalMAN has been able to do much of this for years, but it required niche products that cost thousands of dollars. Since CalMAN now has direct access to the TV hardware, it also means that it will do all the other TV adjustments as well to dial in an ideal image. The process of calibrating is automated, and can be much faster than a traditional manual calibration, but will require the appropriate hardware and software. Even without doing a LUT calibration there are benefits for everyone with this new architecture.
CalMAN will offer you multiple options for calibration of the 3D LUT. You can choose to calibrate 100 points (Lightning LUT), between 100 and 10,000 points (iRP LUT), or have CalMAN create a LUT based on the readings of the primary color (Matrix LUT). The variable point option can take several hours of measurements, depending on your selection, and is likely to be used by the videophile that owns their own calibration equipment, or studios and production houses that use an LG OLED to master content. The fastest, creating a LUT based on primary color measurements, is the simplest, and is based on the measured response of the specific panel you’re calibrating and should be more accurate than it came from the factory which would have been based on an average of a number of panels. We don’t know how much better or worse each version will be, but hopefully we can test them once the LG models ship.
This new larger LUT architecture provides benefits even if you aren’t doing a calibration. Video data is stored in gamma space, which allocates additional data for shadow areas and less data for brighter highlight areas, which fits with how the human visual system works. Traditional color management systems require a conversion to linear space, where shadows and highlights have the same amount of data allocated to them, which leads to issues like posterization in shadows. The new LG models can remain entirely in gamma space, allowing for better color management without the added possibility of artifacts that are inherent in other TV CMS systems.
The LG video processing pipeline has a 1D LUT and a 3D LUT to use in calibration
LG also has provided full hardware access to a separate 10-bit 1D LUT just for the gamma and grayscale that occurs just before display on the panel, after the 3D color LUT in the internal processing. This allows storage of 1024 different correction points, or the same number of points that are possible in HDR10 content. Compared to previous 10-step or 20-step grayscale and gamma controls, this allows a level of control that has never been available and can be optimized for the display. Previous LCD concerns like shadow crush should be a thing of the past when this is calibrated. It’s unlikely that every point will be directly measured and adjusted, as that would be overkill, but having all those points available should allow for effectively perfect grayscale and gamma. The 1D LUT is calibrated fist then the 3D LUT afterwards. Using a 1D LUT + 3D LUT is better than using a 3D LUT on its own, since the grayscale and gamma is already calibrated, the 3D LUT doesn’t have to try to fix the grayscale and the color volume.
The auto-calibration in CalMAN will handle most of the settings for you. Controls like Brightness, Contrast, and White Balance will not need any adjustments at all since the 1D LUT calibration will handle that. When you adjusted those controls in the past, you were actually adjusting the 1D LUT indirectly, and now with direct LUT access that is simply no longer necessary. The only control you need to adjust is the OLED Light/Backlight level, since panels behave slightly differently based on how bright they are set, and different room environments might call for a different peak luminance target.
These new LG TVs are also the first to let you calibrate HDR10 and HLG in gamma space now, something proposed by Tyler Pruitt of Portrait Displays at the recent SMPTE conference. Calibrating HDR has always been a challenge since a TV can respond differently to test patterns than to content, but now this is a solution to that issue. This gamma space calibration method was developed by LG, Dolby, and CalMAN for calibration of LG’s 2017 OLED models Dolby Vision mode, and a similar process has now been implemented for HDR10 and HLG.
Dolby Vision calibration also gets an update – In 2017, a user configuration file generated by CalMan needed to be loaded into the TV via USB; this is now sent to the TV over the same IP interface as the calibration commands, making the process far more seamless.
All HDR picture modes offer automated grayscale and color calibration, and CalMAN recommends that for best HDR calibration accuracy, you only want to take measurements for 5 seconds at a time, and then let the panel rest for 5 seconds to avoid heat build-up at high luminance affecting results.
One other thing worth mentioning is that LG is allowing calibration of a host of different picture modes, rather than just their Cinema mode for example. Of particular note are the low latency game modes for SDR, HDR and Dolby Vision which now have the same calibration capabilities as the other modes.
Despite the improvements that LG has made in reducing latency in their game modes in recent years, LG has gotten some flack for the out of the box picture setup and the lack of an ability to make any significant changes. This ability to calibrate the game modes will be a very welcome addition for gamers, as well as game developers.
Cube CMS Results
While we haven’t used the LG cube calibration yet, we have tested cube calibration external processors from Lumagen in the past. In our testing with those, they took whatever display we had on hand and reduced the error levels down to invisible levels. dE2000 error levels would average below 1.0, making them invisible to the naked eye even when paused, and the only flaws it couldn’t correct were limitations of the TV itself. It might have taken us a few hours to do the readings and the calculations, but the resulting image could be described as perfect and was always worth the time to us.
Whether the LG TVs are as accurate as the Lumagen is yet to be seen, but it seems unlikely they would introduce this new feature without making sure it offers performance improvements over what you can do now. The ability to do this sort of calibration would push the LG OLED TVs to the top of my list if it works, as the performance of the OLED models from different companies is so close it is features like these that help to separate them. That said, for most people, this will mean hiring a calibrator to come in and do this for them. But as we mentioned earlier, even without hiring a calibrator the new LUT and new internal processing architecture will lead to a better image than before even without hiring a calibrator.
We are happy to see a consumer TV enter the world of Cube Calibration, and believe this means everyone can look forward to more accurate images going forward. As soon as we can test the effectiveness of the LG cube we will report on that as well.