Calibrate color matching

When adding a virtual set extension to an XR setup, matching the colors of the LED walls to the AR graphics manually is tedious. A variety of components in the video chain have an impact on the output colors

• the XR render machine

• the Display Processor for the LED wall

• the LED wall itself

• the on set lighting

• the camera

• the AR render machine

The Color matching panel does the whole calibration for you. Just follow the steps listed below.

Example calibration done with 9 samples

	LED wall	Set extension
No color matching Notice the visible seam in the middle
Color match AR to LED wall This can be useful if you don’t want to change the look of the LED wall, but instead have the graphics adapt. Notice that the AR now inherits the blue shift from the wall.
	LED wall	Set extension
Calibrate LED wall Notice how the LED wall is color corrected, but the AR still looks the same and is not gamut clamping.
Both This is the best possible calibration which will color correct the wall and gamut clamp the AR. Notice the loss of intensity in the white color chart and show in the AR.
	LED wall	Set extension

Choosing color profiles for XR

The most important concept to understand: there is no connection between how you color manage the LED wall and how you color manage what the camera captures. From the camera's perspective, the LED wall is the real world - a physical object being filmed, just as if someone had painted the backdrop. You do not color manage the real world from the camera's point of view. The colorimetry of the wall and the colorimetry of the camera are completely distinct, and the profiles are chosen independently:

Output to the LED wall (XR machines)

• Choose a format that takes full advantage of what the wall can do. Modern LED walls are HDR capable (often several thousand nits peak), so this typically means PQ or HLG - both use the Rec.2020 gamut

• PQ is an absolute mapping: a given code value always corresponds to a specific amount of light (nits), which makes it possible to specify exactly how much light comes off the wall

• HLG is a relative mapping: the maximum value maps to whatever maximum brightness the LED processor is configured for (typically 1000 nits)

• Using as much of the wall's dynamic range as possible also lets you expose the camera lower, so the wall's darkest values can read as proper black through the camera even if they look grey in person

Camera input and set extension (AR machines)

• The camera input profile is simply whatever the camera actually delivers (e.g. Camera Rec.709, Rec.2100-HLG, a log format). The wall's output format plays no role in this choice

• The AR set extension uses the camera's profile - never the wall's. The set extension has to match what the camera captured, not what was sent to the wall

Display referred content (aux inputs) shown on the wall

When you bring a display referred source (e.g. a Rec.709 video feed or graphic) onto a Rec.2020/HDR wall, set its input profile to what the content actually is (e.g. Camera Rec.709), so it is linearized into the same scene referred world as the rest of the graphics. Remember that the wall is then filmed by a camera - content that looks correct in person will not automatically look correct through the camera. Color match the AR to the wall through the calibration below, and adjust the content as needed for the camera view.

Calibration

Prepare the following

For the calibration to be as accurate as possible make the following preparations to your

LED wall

• Make sure the LED wall itself is properly calibrated before Pixotope enters the picture. Panels are normally factory calibrated (measured with a spectroradiometer), and the studio should verify the wall and processor are set up correctly - a grossly miscalibrated wall (e.g. with a strong color cast) is not something Pixotope's color matching is meant to fix

• This should be part of the pre-install checklist / scope of work: the customer calibrates their studio (lighting, camera, wall) without Pixotope in the loop first, then Pixotope is fitted in and matched on top

Lighting

• Set up production ready stage lighting

• Set up production ready ambient lighting

Camera(s)

• Disable automatic white balance

• Disable dynamic aperture/exposure

• Set accurate black/white balance

Frame

• Aim the camera at the center of the LED wall

• Zoom in so the camera frames only the LED wall

• Lock off the camera head

Select machines

1. Select the AR machine and its camera input

   • This camera input is used in the calibration process filming the LED wall

2. Optionally: Filter XR which XR machines to display the calibration pattern on (a series of flashing lights) during the calibration process.

Configure process - Check phase offset

1. Optionally select Save debug images

   • Saves all the frames seen by the AR machine during phase offset detection. This allows to manually inspect if the phase offset is wrong

   • Images are saved on the AR machine into [Pixotope installation\Services\VideoIO]

2. Click "Check phase offset"

3. Black and white colors are projected to check if the camera and the LED wall are properly synced

4. In case a phase offset is detected

   • Adjust the Phase offset on your LED processor and re-check

Configure process - Run calibration

1. Specify which calibration steps you would like to run (see below)

2. Set the sample number of the profile

   • Larger values improve the result, but will increase the time for analysis. Values between 7 and 15 usually give good results.

3. Optionally enable Include HDR range samples

   • Enable when your camera feed uses an HDR transfer function (e.g. PQ or HLG), so the color matching profile captures the full HDR brightness range

   • This makes the calibration generate test patches with linear values above 1.0. Without this, the calibration never measures how the wall + camera system responds to brightness above SDR levels - sampling continues upwards until the values start clipping, which reveals the maximum brightness the wall and camera can reproduce together

4. Start the calibration

Calibration steps

During color analysis

Possible warnings during color analysis

After color analysis

Color grading

Using the color grading panel in combination with color matching will deliver varied results. Here are our recommended use cases.

Vignetting

Using the Effects panel might be needed to accommodate possible vignetting effect depending on the lens characteristics. Vignette intensity should be increased in oder to extend the darkness effect on edges when appropriate.

Profiles

This advanced tab allows to

• manually assign the generated

  • AR profiles

  • LED profiles (AKA XR profiles)

    • optionally limit the profile to a specific mappings

• show a Chromaticity diagram for any selected profile

• unassign all AR & LED profiles selected

Chromaticity diagrams

The purpose of this diagram is to visualize color accuracy. In addition, it can help detect if something went wrong during a long calibration process (eg. LED wall occluded by something onset).

How to read this chart:

• Yellow triangle is the area corresponding to Rec.2020 color gamut

• Blue triangle is the area corresponding to Rec.709 color gamut

• Blue square is the theoretical position of the color patch in the Chromaticity diagram

• Green dot is the position of the color patch in the Chromacity diagram after it is displayed on the LED screen, and was captured by the camera routed to the AR machine

Ideally the green dots will be in the center of the blue squares, but this is not always the case due to imperfect color accuracy of the LED screen. Pure black level is rarely encountered, and LED walls have gamut limitations that prevent the color patch to be shown on gamut boundaries (the blue triangle).

AR profile validation

The state of the validation text changes depending on the type of AR profile selected, and the corresponding LED profiles assigned in the table below.

If you have created an AR profile where only Color match AR to LED wall was enabled → Do not assign any LED profiles below

If you have created an AR profile where both Color match AR to LED wall & Calibrate LED wall were enabled → Assign the corresponding LED profiles to all outputs below

The validation text will keep track of which AR profile you have active, and update based on its specific criteria.

Unassign all will clear both the AR and LED profiles selection.

Profiles storage

The profiles can be found on all connected machines:

...\Pixotope\[Version Number]\Services\VideoIO\ocio-configs\Pixotope XR Calibration

• AR and LED/XR profiles can be found in the above path as .cube files

• Chromacity diagrams can be found in the above path as .png images