Common question regarding High Dynamic Range
Common question regarding High Dynamic Range
Standard Dynamic Range (SDR) compared to HDR is the regular contrast ratio we are used to from TV sets and in the cinema.
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.
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.
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.
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):
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 ".
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).
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”.
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.
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.
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.
Images in the camera are processed in 16 bit linear and then encoded in 12 bit log on the recording medium.
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 3 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.
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.
Use an HDR monitor on set to judge your images in HDR. ARRI provides look files and 3D LUTs that convert the Log C image into HDR using the SMPTE ST-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.
In general you don’t need to work differently than for SDR. Here are some issues to pay attention to:
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.
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.
Currently, there are three distribution standards for HDR video to the home:
And one standard for distribution to the cinema:
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.
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.
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.
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.
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.
Yes, but since color changes when changing image brightness, you have to do a trim pass in color correction.
Two general requirements must be met to experience HDR at home:
a) your TV set must be HDR compatible. The first displays compatible with one of the three main HDR distribution standards (HDR10, Dolby Vision Home, Hybrid Log Gamma) have been introduced on CES in 2016.
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.
The HEVC (H.265) codec is being used for streaming and providing HDR content to the consumer.
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.
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.
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.
The UHD Alliance defined an ULTRA HD PREMIUM certification and logo for consumer devices and content with the following minimum specifications:
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.
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.
WCG is a generic term for color gamuts larger than Rec 709 (e.g. Rec 2020, DCI-P3 and others).
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.
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”.
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.
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.
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.
Background and Surround
Neutral grey at D65
Brightness of background
Brightness of surround
≤ 5 cd/m2
Avoid light falling on the screen
for 1920 x 1080 format: 3.2 picture heights
Peak luminance of display
≥ 1 000 cd/m2
Minimum luminance of display (black level)
≤ 0.005 cd/m2