How to use Livegrade’s exposure assist

With huge, high-resolution HDR TVs in living rooms and increasingly detailed cinema projections at the movies, it is now more important than ever to be highly precise when setting the exposure of a digital image. At the same time, the number of different log and display formats is increasing, making this task more and more complex.

The 1D LUT-based exposure assist mechanism in Livegrade enables you to precisely analyze images, giving you the freedom to adapt to the individual needs of a production.

In this article, we start with a brief introduction to false color in general before taking an in-depth look at Livegrade’s exposure assist tool, which supports the use of false color schemes or the EL Zone system. In the end, we will show you how to build your own false color LUTs to adapt to the special needs of a production.

What is “false color”?

The most common use case of false color in general is to show parts of the image that are not visible to the human eye by changing the colors of the original image.

In the film industry, false color helps to check the exposure of a scene by indicating the luminance values of an image. First, the colored image converts to a black-and-white representation in order to translate RGB to luminance values. Ranges of these resulting code values are then colored to indicate parts of the image that are crucial, such as highlights, blacks, or skin tones.

To a certain extent, this builds on the zone system (formulated by Ansel Adams in 1948 in his book The Negative: Exposure and Development) which was invented for black-and-white analog photography. 

In this system, individual luminance areas of a set are identified as certain zones (Zone 0 to X), which enables the photographer to anticipate how individual parts of the image will look in the final print.

The benefit of false color in digital cinematography is very similar: it gives you a chance to quickly check parts of the image, to make sure that everything is exposed properly for the upcoming post-production pipeline.

False color on different log formats

Today, false color is widely used and implemented by many manufacturers for in-camera use. The following example shows some of the most common log formats and the ranges used in the camera to generate a false color image.

What we see is that different camera manufacturers are using different scales to interpret the luminance values of log-encoded footage. From a technical perspective, this makes absolute sense: Every log format distributes luminance values differently and must go hand in hand with many other camera-specific attributes, where the camera sensor is probably one of the most important ones in this regard.

From a user’s perspective, this fact can be challenging sometimes, as different ranges on different cameras mean different things.

In addition, false color mode implementation and range of functions differ from camera to camera. While some cameras offer the option of changing ranges manually (e.g. in the Sony Venice 2), other cameras include the set ISO value in the analysis of the image (e.g. ALEXA 35).

To accommodate these facts, Livegrade introduced a LUT-based mechanism and provides LUTs for the most common log formats. As an alternative Livegrade also offers the possibility to work with the EL Zone by Ed Lachman (ASC).

The EL Zone system

EL Zone is a stop-based exposure tool that displays false colors for full stop increments above and below 18% grey. It ensures consistency and precision across different camera sensors and aligns with the language of a DP’s spot meter. Thinking in stops often feels intuitive, reflecting how DPs and gaffers set light and balance sources on set. As with false color, this analysis is based on the original image, which is available in a different log format depending on the camera.

Custom false color LUTs

You can also create and add your own false color LUTs in Livegrade. The idea behind it: giving you full control over the false color pipeline with whichever false color LUT you choose to work with.

But before we get into more detail about setting up false color in Livegrade, let’s take a look at what actually happens when you activate the tool.

How does Livegrade’s exposure assist work?

When you hit Exposure Assist, a black-and-white image is generated to translate RGB into luminance values first. 

Then, a 1D LUT is applied to the black-and-white image, which converts certain luminance ranges into certain colors. Depending on the exposure assist LUT, the luminance values will be colored.

Why 1D false color LUTs instead of 3D LUTs?

Since we are examining the luminance values of an image with false color, the use of 1D LUTs is particularly suitable. With 1D LUTs, clear input-to-output relationships are established, so that a 10bit 1D LUT allows us to clearly distinguish between 1024 individual luminance values.

Common 17-, 33- or 65-point 3D LUTs, which are ideal for complex operations like gamut conversions, channel mixing, or the application of looks, are only suitable for indexing luminance values to a limited extent.

The reason for this is that common 3D LUTs do not contain input-to-output relationships for every possible R-G-B combination over the entire luminance range, as this would make the file size of a single LUT too large. Instead, 17, 33, or 65 input-to-output relationships are stored in it.

Therefore, 3D LUTs must always be interpolated. When using them for false color, this leads to blurred separation of individual luminance ranges, as the following figure shows.

False color image on a 3D LUT box

Livegrade applies the 1D exposure assist LUT internally and outputs an exact false-color image on the viewer and connected output devices.

Nevertheless, the look — including false color — is sent to an attached LUT box as a 3D LUT. Therefore, the false-color image that the LUT box outputs via its SDI out will differ slightly from the one seen in Livegrade. As already pointed out above, this is because the transmitted 3D LUT, which is applied in the LUT box, cannot distinguish brightness ranges as clearly as the original 1D LUT can.

If you need the most accurate false-color image possible, use the “Force Display Mode To” feature for Livegade outputs. It enables exposure assist on a single output without affecting the source slot, so your LUT box continues to receive the graded image. At the same time, you view a false-color image on one of your monitors.

Setting up Livegrade’s exposure assist

In order to set up exposure assist work for your production, we should now take a look at the exposure assist settings (Livegrade > Settings > Grading) and show how you can use them to improve your workflow. By selecting an option from the dropdown menu, you choose the LUT to apply as the default when enabling Exposure Assist.

It’s up to you to decide which color scheme you want to use: The False Color section includes many manufacturer LUTs. By applying them you will to get the same result in Livegrade as it would be the case in the internal false color processing of the respective camera. EL Zone offers the option to work with a cross-camera color scheme.

In both cases, the underlying log format is crucial for accurate analysis. If you’re working with a mixed camera setup, you can also select an exposure assist LUT in your slot settings that differs from the default setting used here.

Enabling the exposure assist tool

The exposure assist tool can be enabled at a slot level or on a dedicated output, giving you the flexibility in how you monitor exposure across your setup.

On a slot: Enabling Exposure Assist on a slot lets you check the exposure for that individual source. Note that this also affects the look sent to any attached LUT box.

On an output: Using the Force Display Mode To option on an output, you can enable Exposure Assist for all tiles currently displayed on that specific output. This approach lets you monitor exposure on a dedicated output without affecting the source slots or any other outputs in your setup.

Example: Setting up Livegrade’s exposure assist for ALEXA 35

Let’s take a look at an example now. The task is to set Livegrade so that it behaves exactly as if you were activating false color on the ALEXA 35 with EI800.

To do so, open the Grading section at Livegrade > Settings. There, select the following: ARRI > LogC4 > LogC4 EI 800. This LUT creates the same false color image as the ALEXA 35 would create with EI 800.

From now on, every time you hit Exposure Assist Livegrade will behave exactly like an ALEXA 35 would when activating false color in the camera.

Keep in mind: ARRI cameras take the exposure index setting into account when displaying false color in the camera. So, if you change the exposure index from EI 800 to a different value, you should change the false color LUT accordingly, to get the same behavior as the camera.

Creating a customized false color LUT

Now that you have learned how the false color mode in Livegrade works, let’s see how you can modify it to the individual needs of a production.

We already mentioned that the resulting false color image is controlled by the selected LUT. So you can choose from different manufacturer LUTs that we deliver out-of-the-box. This way you get the same false color image as the corresponding camera. On the other hand, you can also add your own false color user LUTs and thus define your own false color scheme.

For this, we recommend using the Pomfort False Color LUT Generator, which helps you with a simple user interface and generates 10bit 1D *.cubes that you can insert into Livegrade.

Let’s go through it using an example:

• After the camera tests and look creation, the DP has determined that it is particularly important for the exposure of the material not to expose skin tones too dark.
• During the shoot, a signal color should be used to check whether there are any skin tones in the image that are already too dark.
• The film is shot on an ALEXA 35 EI800.

To solve this task, you could create a false color user LUT which, in addition to the standard scheme of the ALEXA 35, colors the luminance area cyan, indicating not enough exposure for skin tones.  

To do so, you could proceed as follows:

• Specify with the colorist and the DP at what IRE a face would be “too dark”. For the example, let’s assume that in LogC4 everything below 15% would be too dark for the exposure of skin tones.
• Open the False Color LUT Generator and select the ARRI LogC4 EI800 preset.
• Rename it to LogC4_EI800_13-15_cyan
• Add a new area that colors everything from 13 to 15 percent cyan.

• Click on Download to save the false color LUT as a *.cube file on your computer and name it LogC4_EI800_13-15_cyan.cube
• Open Livegrade, navigate to your grading settings, and select User LUTs > Show in Finder
• Copy your saved LogC4_EI800_13-15_cyan.cube LUT into this folder
• Click on Refresh
• Select LogC4_EI800_13-15_cyan

From now on, your LUT will always be loaded as soon as you click on Exposure Assist. 

During the shoot, you can react after checking false color by increasing the exposure on the camera so that there is no cyan shown in areas that cover skin tones.

Conclusion

Using false color or EL Zone as a tool for judging exposure is widely used in the film industry. However, many manufacturers have implemented it differently. For the DIT whose task is, among other things, to check the exposure of an image, it is therefore essential to know what actually happens when exposure assist is activated.

The LUT-based exposure assist mode in Livegrade makes the mechanism transparent and gives the live grading DIT comprehensive control over what happens to the image. Finally, Livegrade users are even free to create custom user LUTs and adapt to the individual needs of a production.