High Dynamic Range FAQ

High Dynamic Range (HDR), the media-buzz-word of 2017, is on every content creator’s mind. While the advances of HDR in post, distribution and displays are fantastic and important technologies, they are also very new. This, added by rapidly evolving and competing HDR standards, leads to some confusion in the market. This collection of frequently asked questions is designed to provide information on the subject and clear up some of the confusion. Version: January 20, 2017. Use command + F to open all answers.

 

Please note:
ARRI provides HDR look files for ALEXA SXT, ALEXA Mini and AMIRA cameras for on-set viewing as well as 3D LUTs for HDR post-production for images captured with ARRI cameras. You can find those files here.

 

1. Introduction
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What is HDR?

HDR is short for High Dynamic Range, a term that describes images with deeper blacks and brighter whites, leading to a higher contrast ratio than was available up until now. In the last years, the term HDR has mainly been used to describe new display technologies which have a greater dynamic range and therefore can display images with a higher dynamic range. While a higher contrast in images is great on its own, it also makes the image look sharper, increases color saturation, provides more depth and allows for a larger range of looks.

HDR is considered an important new technological step in image display since it provides an immediately visible clear improvement in image quality, even to the untrained eye and independently of image resolution, screen size or viewing distance. In addition, the increase in data rate for an HDR signal is minimal, thus creating great visual impact for small increase in cost. 

 

What is SDR?

Standard Dynamic Range (SDR) compared to HDR is the regular contrast ratio we are used to from TV sets and in the cinema.

Is motion picture HDR the same as still photography HDR?

In still photography the term HDR is used in a different context to describe a technique to capture more dynamic range by stacking several exposures done with different shutter speeds. The result (rendered for a Standard Dynamic Range display) creates a “hyper-realistic” looking image. Recent smart phone cameras often use this method to capture HDR images.

The term HDR used in a motion picture context focuses on the capture and displaying of HDR images. These are images with more detail in the highlights and shadows shown on an HDR display. ARRI digital cameras like the ALEXA 65, ALEXA, ALEXA Mini or AMIRA capture the highest dynamic range of any camera, and thus have been capturing HDR images since they were introduced in 2010. In contrast to still photography HDR, ARRI digital cameras capture a high dynamic range image at one point in time, and thus avoid any motion artifacts that would result from capturing different exposures at different times. 

What is HDR acquisition?

HDR acquisition is capturing images with a high dynamic range for distribution and display on a HDR display. When you’re on a job capturing footage for a show designated to have HDR deliverables, make sure you are using a sensor/image-processing and post pipeline that is capable of capturing sufficient latitude for a HDR display. The richness and fidelity and therefore the grading possibilities of an HDR image are defined in the acquisition phase.

What is an HDR display?

An HDR display is a display that can produce a higher than “standard” contrast ratio. In the past, TV sets have had a maximum brightness of 100 nits, and cinema displays up to 48 nits. How much brighter an HDR display has to be to be considered an HDR display is not clearly defined. The only cinema HDR projection so far is based on laser technology and has a brightness of 108 nits. Recent HDR consumer TV sets are based on active backlight or OLED technology and have a maximum brightness of 500 to 1000 nits. 

What HDR standards exist?

There is one general HDR standard (Rec 2100) and a number of different distribution standards, of which three are the most popular (for a detailed description of all standards see our "HDR Standards" section below):

  • Dolby Vision
  • HDR10
  • Hybrid Log Gamma (HLG)

These are some terms which are currently used in conjunction with these HDR implementations. They are explained in all detail in the sections "HDR Standards" and "HDR Terms ". 

  • EOTF
  • PQ
  • SMPTE ST 2084
  • Rec 2020
  • Rec 2100
2. Acquisition
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Why should I shoot for HDR? Where are the benefits?

Generally HDR is being described as “more lifelike images with deeper blacks and brighter whites”. Think of looking out of your window: HDR aims to recreate the richness of colors and color-intensities that we know from real life. This ultimately leads to a more immersive viewing experience and adds a groundbreaking depth to 2D imagery.

More and more projects require an HDR delivery either for TV or cinema. Content to be advertised as “UHD Premium” (see UHD Alliance) needs to be mastered in HDR. 

Some Studios master all their UHD delivery’s in HDR only (for example Fox Studio).

How do I record HDR with ARRI cameras?

While HDR displays are just now becoming available, ARRI digital cameras have always captured a high dynamic range far exceeding the capabilities of displays. So ARRI digital cameras have been capturing HDR since they were introduced in 2010. All ProRes clips encoded in ALEXA Wide Gamut/Log C or in ARRIRAW are ideal for HDR post and delivery. For on set monitoring see “Monitoring HDR”.

Are there motion artifacts with HDR?

No. ARRI digital cameras capture all image data at once and therefore have no need for mixing different exposures as is commonly use in still photography. 

Are ARRI cameras HDR compatible?

ARRI’s digital cameras capture a high dynamic range and are therefore ideal for creating HDR imagery since their introduction in 2010. A majority of the current HDR productions are shot using ARRI digital cameras.

What are the advantages using an ARRI camera for HDR capture?

ARRI digital cameras capture the highest dynamic range of any digital camera to date. This high dynamic range provides a soft and gentle transition from almost to fully overexposed and thus avoids hard clipping in highlights. The low noise floor of ARRI digital cameras is great for VFX and necessary for HDR grading, as a higher dynamic range will show more details in the dark part of the image. For all these reasons ARRI digital cameras are best for capturing HDR content. 

What is the bit depth of ARRI’s color processing and how are images stored?

Images in the camera are processed in 16 bit linear and then encoded in 12 bit log on the recording medium. 

Why does ARRI store 16 bit linear images in a 12 bit log format?

16 bit linear encoding is neither efficient nor necessary for recording images. Images can be converted from 16 bit linear to 12 bit log without any loss of information or quality, but with a great savings of data rate and data volume. 

ARRI’s ALEV III sensor captures 16 bit image data; our image processing chain packages that data in ARRIRAW or ProRes images using a 12 bit log scale. This assigns a fairly equal number of code values to each stop, which is more efficient than 16 bit linear and better represents how the human eye perceives light.

Should one monitor HDR on set?

When shooting for an HDR deliverable, it is a good idea to monitor HDR on set so issues with the HDR images (that may not be of concern in SDR) can be identified early on. HDR monitoring has to be done with an HDR monitor, as it is impossible to see the effect of HDR on an SDR monitor. 

How do you monitor HDR on set with ARRI cameras?

Use an HDR monitor on set to judge your images in HDR. ARRI provides look files and 3D -LUTs that convert thea Log C image into ann HDR image using the SMPTE ST T-2084 („PQ“) encoding. These look files can be used in the following cameras: ALEXA SXT, ALEXA Mini and AMIRA and ALEXA Mini. You can find those files here. The monitor gets connected via a BNC cable to the SDI output of the camera. The monitor has to be set up for SMPTE ST 2084 (PQ) encoding. 

When using other ALEXA models please use a LUT box to do the conversion from Log C to PQ encoding. Some HDR monitors allow to do this conversion using a 3D -LUT.

How do I expose/light for HDR?

In general you don’t need to work differently than for SDR. Here are some issues to pay attention to:

  • It is critical to properly expose when shooting for HDR. Clipped highlights can look very distracting in HDR, as there will be no detail in a large region of the image.
  • View you camera image on an HDR monitor on set to judge the ratio between background highlights and practical lights in frame and your actors faces. In HDR this ratio is different and can draw the viewers attention away from the actors.
  • Because the dynamic range of a HDR master is larger than an SDR master, the possibility to push your image in post is reduced. The clipping of highlight or shadow detail may become more visible in a HDR image.
  • Higher contrast makes noise more visible. Try to avoid noise as much as possible. 
  • Higher contrast may reveal details in the shadows that were supposed to stay hidden. 
  • Higher contrast accentuates backlights, edge lights, windows and glare on costumes, jewelry, etc. 
  • Higher contrast makes items look sharper. Check make up, clothing and set design. Also check on backdrops. A backdrop that was a little blown out in SDR may not be blown out in HDR and thus look fake. 
  • Higher contrast increases motion judder, which is a strobing effect caused by high contrast edges moving across the screen. Panning speeds may have to be adjusted and movement in front of large bright areas has to be carefully monitored. 
Are there any pitfalls when shooting for HDR with no HDR monitor on set?

Any very bright areas in the frame could potentially be an issue, like practicals, shiny highlights, backlights, edge lights, windows and glare on costumes or from jewelry. In an HDR image these may become too bright and distracting when displayed in HDR compared to SDR. The best way to avoid this is to monitor in HDR. 

Are there any special requirements to a lens when shooting for HDR?

When you want to capture a scene having a large contrast, stray light or flare is your enemy. Therefore the quality of the lens coating is very important. Telephoto lenses with many elements are most likely to show flaring. External filters in front of the lens can also increase flare. 

3. Post Production
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What HDR Standards exist for mastering?

Currently, there are three distribution standards for HDR video to the home: 

  • HDR10
  • Dolby Vision
  • Hybrid Log Gamma 

And one standard for distribution to the cinema:

  • Dolby Vision Cinema
How does an HDR sample workflow look like?

A HDR workflow is not much different from current workflows. The ALEXA Wide Gamut/Log C (or ARRIRAW) footage is loaded into a color correction system. A 3D-LUT is then used to convert the images for the monitor output. This 3D-LUT needs to be changed for different kinds of HDR. ARRI provides such 3D LUTs but every production is free to design their own look and to use it in-camera as well as in the mastering process.

Should I edit in HDR?

Since it is advantageous for the editor to see how much information is visible in the image, it is best to edit in HDR. This can be done by creating editing files with the SMPTE 2084 (PQ) EOTF, and using an HDR monitor in editing. Please be aware that the computer monitor connected to the editor will then not display the image properly (since it is an SDR monitor showing an HDR image), and should be ignored. 

Can I generate an HDR master in an ACES workflow?

Yes, the ACES system includes transforms (ODTs, output display transforms) for HDR output based on the reference rendering transform. For more information on ACES, please see the ARRI ACES landing page

Should I grade HDR or SDR first?

There is no consensus in the industry yet, which color grading strategy is better. Both approaches are being used today by HDR productions. The advantage of first grading for SDR is that most displays currently are still SDR, so you are grading first for the largest audience, and then making an HDR trim pass to also have an HDR deliverable. The advantage of first grading for HDR is that we have often found that this in turn also makes for a better SDR grade. However, the client who has seen the HDR will not be happy with the SDR. 

When using the 3D LUTs from ARRI for HDR and SDR grading and performing all grading decisions in Log C space (the 3D LUT is used as the last grading step) projects can be easily switched between SDR and HDR by exchanging the 3D LUT and vice versa. A trim pass is always needed after switching the 3D LUT to keep your creative intent for the new format. 

Do I need special software to edit/post produce a HDR movie?

You do not need any special software. Any system that can work with Log C data and 3D LUTs can be used to generate an HDR master. You may need special software, however, to create specific distribution formats.

Is it possible to up-convert an SDR master to HDR?

Yes, some color grading tools offer such features. The quality most likely will not be as good as working from Log C original files (or film scans). But since there is a lot of existing SDR contents, and a great demand for HDR content, this will often be done. 

Is it possible to down-convert an HDR master to SDR?

Yes, but since color changes when changing image brightness, you have to do a trim pass in color correction. 

4. Display/Distribution
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How can we see HDR images at home?

Two general requirements must be met to experience HDR at home: 

a) your TV set must be HDR compatible. At CES 2016 many HDR televisions were introduced which are compatible with one of the three main HDR distribution standards (HDR10, Dolby Vision Home, Hybrid Log Gamma).

b) the content must be delivered in HDR. Major streaming services offer more and more content in HDR as well as a lot of new UHD BluRay discs are being mastered in HDR.

What formats are being used to distribute HDR content to the home?

The HEVC (H.265) codec is being used for streaming and providing HDR content to the consumer. 

Who is broadcasting HDR today?

Several streaming providers are currently providing HDR content, including Amazon, Netflix, Vudu and others. To our knowledge there is no HDR broadcast channel to date. 

How can we see HDR images in the cinema?

Make sure to find a “Dolby Vision Cinema” – Dolby plans to have 200 Dolby Vision screens by 2024 around the world. Every “Dolby Vision Cinema” needs a special “Dolby-certified” room design and it has to follow minimum requirements of Dolby. Dolby is using two Laser projectors to achieve an HDR projection in cinemas. Laser projectors offer higher contrast, more brightness and a wider color range as can be achieved with traditional xenon lamp projectors. 

5. Standards
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What is Dolby Vision?

Dolby Vision is a “high quality, high dynamic range and wide color gamut system for delivery of entertainment content” (from the Dolby Vision White Paper).

Dolby Vision exists in two versions today: 

  • 1. Dolby Vision Home for TV distribution
  • 2. Dolby Vision Cinema for cinema distribution.

Dolby Vision uses the SMPTE 2084 (PQ) EOTF for distribution and Dolby Vision Home uses additional dynamic metadata (SMPTE ST 2094) to be compatible with SDR displays.

In terms of brightness, Dolby Vision is specified from 0 to 10,000 nits. Today’s display technology (2017) is capable of 0,005 to 4,000 nits (Dolby Vision Mastering Display). Standard Dynamic Range in comparison is specified from 0,05 to 100 nits. 

What is HDR10?

The HDR10 format is specified by the CTA (Consumer Technology Association) and is incorporated into the UHD Premium standard. HDR10 uses static metadata (SMPTE ST 2086) to describe the mastering display and mastering color space. The format is currently not backwards compatible with SDR TVs. 

What is Hybrid Log Gamma (HLG)?

The Hybrid Log Gamma curve is an HDR standard defined by NHK and BBC. It is specified in ITU Rec BT 2100. The HLG specification offers a degree of compatibility with legacy displays by more closely matching the previously established television transfer curves (ITU Rec 1886). The HLG signal is similar to about 0.6 signal to Rec 1886 and then changes to log encoding for the highlights. This makes it backwards compatible with SDR displays without the need of metadata.

Below is a comparison of an SDR signal encoded to a maximum brightness of 100 nits and an HLG signal encoded to a maximum brightness of 1000 nits. Note that the vertical axis is logarithmically scaled.

6. Terms
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What is the UHD Alliance?

The UHD Alliance is a consortium of manufacturers, studios and content distributors defining standards and shaping the ecosystem around new viewing experiences. The UHD Alliance has defined the “UHD Premium” (HDR10) standard in early 2016. ARRI is a member of the UHD Alliance. For more information about the members of the UHD Alliance please use this link.

What is UHD Premium?

The UHD Alliance defined an ULTRA HD PREMIUM certification and logo for consumer devices and content with the following minimum specifications:

 

  • Resolution: 3840x2160 (4K UHD)
  • Color bit depth: 10 bits distribution and 10 bits for playback displays
  • Color representation: BT.2020
  • Mastering display: SMPTE ST 2084 (minimum 100% P3 Color Space and min. luminance 1000 nits and black level less than 0.03 nits. )
  • Content Transfer Function: SMPTE ST 2084 (PQ)
  • Playback display: SMPTE ST 2084 (minimum of 90% of P3 color space, a minimum luminance of 1000 nits and a black level of less than 0.05 nits or peak luminance more than 540 nits and black level less than 0.0005 nits.)
What does the term “Nit” stand for?

Nit is another name for candela per square meter (cd/m²), the unit for luminance. Nit comes from the Latin word for "to shine": nitere. 

As a reference: the peak luminance of a studio monitor is between 100 and 120 nits, a modern TV or desktop monitor may go up to 200 or even 300 nits. Special displays for outdoors may have a much higher luminance. One should note HDR means high-dynamic range - not super-bright. The important feature is a high ratio between maximum luminance and black level. 

Is there an HDMI standard for HDR?

HDMI is an interface for transferring UHD-1 and UHD-2 content to consumer displays. For delivering HDR content with static metadata (SMPTE ST 2086), the system must comply with the HDMI 2.0a standard. 

What is wide color gamut (WCG)?

WCG is a generic term for color gamuts larger than Rec. 709 (e.g. Rec. 2020, P3, and others).

What is Rec 2020?

Rec 2020 is a color space that is larger than Rec 709, which is the current industry standard for HD. Rec 2020 promises more brilliant images when new display technology is able to display these colors. Traditional Rec 709 displays (like TVs or ordinary computer displays) cannot display Rec 2020.



While Rec 709 (short for Recommendation ITU-R BT.709) is the encoding color space for HDTV, Rec 2020 is the encoding color space for UHD. The primary colors lie on the spectral locus and it’s thus possible to have more saturated colors in the images.

Rec 2020 is an encoding standard. A TV or display may not support the full gamut. Nevertheless, it will display correctly the colors within its physical limitations. To get the “premium” logo of the UHD Alliance, for example, a TV needs to support a minimum of 90% of the P3 gamut.

Will Rec 2020 replace DCI P3?

DCI P3 is foremost a color space for digital projectors which covers around 72.9% of Rec 2020. As display and laser technology evolves, Rec 2020 may take the lead not only in UHD TVs. Digital Cinema Packages (DCPs) use neither DCI P3 nor REC 2020 color space – DCPs are encoded in XYZ color space. For HDR DCPs a metadata flag within the DCP container marks the footage “HDR on”. 

What is an OETF?

The OETF (Opto-Electronic Transfer Function) is a method to convert light into a digital signal. It defines how a camera converts light into a signal that can be recorded. 

What is an EOTF?

The EOTF (Electro-Optical Transfer Function) is a method to convert a digital signal back into visible light. It is used by displays to convert a recorded or transmitted signal into light, and thus an important part of any HDR specification. The EOTF for HD distribution has often been called “gamma curve”, since the EOTF of CRT displays is a power function. ITU Rec 1886 states that the reference EOTF should be a power function with an exponent of 2.4. For HDR distribution two new EOTF curves were introduced: PQ and Hybrid Log Gamma (HLG). 

What is PQ?

The Perceptual Quantizer (PQ) transfer curve was defined by Dolby Laboratories and has been standardized as SMPTE ST 2084. The PQ curve is a new EOTF for HDR distribution. 

What is SMPTE ST 2084?

SMPTE ST 2084 is a new EOTF standard for HDR displays. It’s also known as the PQ curve. The PQ EOTF achieves a very wide range of brightness levels for a given bit depth using a non-linear transfer function that is finely tuned to match the human visual system. The transfer curve encodes absolute luminance values up to 10,000 nits. This is a radical change from previous EOTF definitions. 

What is ITU R-BT 2100?

This ITU standard defines two different EOTF’s for HDR displays: the PQ transfer function and the HLG transfer function. It’s expected that the PQ curve is rather used for dramatic content productions while the HLG curve is used in broadcast applications. The standard further defines the viewing conditions and monitor parameters for critical viewing of HDR content.

Parameter

Values

Background and Surround

Neutral grey at D65

Brightness of background

5 cd/m2

Brightness of surround

≤ 5 cd/m2

Ambient lighting

Avoid light falling on the screen

Viewing distance 

for 1920 x 1080 format: 3.2 picture heights
for 3840 x 2160 format: 1.6 to 3.2 picture heights
for 7680 x 4320 format: 0.8 to 3.2 picture heights

Peak luminance of display 

≥ 1 000 cd/m2

Minimum luminance of display (black level)   

≤ 0.005 cd/m2