Topics covered in this article:
- Source video considerations
- Systems which support Dolby Vision color correction
- Reference display requirements
- PQ encoding (SMPTE ST2084)
Before creating Dolby Vision content, there are a few requirements that have to be taken into consideration that are reasonably different from traditional SDR content creation workflows.
In general, the ideal source material for Dolby Vision would be that which best retains the native color gamut and original dynamic range of the content at the time of its origination.
More details about recommended source material for creating high-quality Dolby Vision content can be obtained from the article FAQ: Dolby Vision Recommended Source Formats (force.com).
Color Correction System
Dolby Vision is currently supported on all leading color correction systems, like:
- Blackmagic Resolve
- FilmLight Baselight
- Digital Vision Nucoda
- Autodesk Flame
- SGO Mistika
These systems are able to work in PQ, create/modify Dolby Vision metadata, and finally output a finished Dolby Vision master.
[refer to the article FAQ: Dolby Vision compatible Color Grading & Packaging Systems (force.com) for more information about supported systems and partner solutions]
HDR Reference Display
Dolby does not publish a list of monitors that can be used for creating Dolby Vision content but instead provides a list of recommendations to take into consideration when evaluating HDR reference displays for Dolby Vision content creation. The recommended guidelines closely follow the EBU Tech 3320 specification for a grade 1 HDR mastering monitor. The suggested guidelines are:
- At least 1,000 nits of peak brightness
- At least a 200,000:1 contrast ratio
- A minimum black level performance of 0.005 nits
- At least 99% of the P3 Color Gamut
- D65 Whitepoint
Other aspects of the content creation pipeline like VFX creation, packaging, and QC may have less stringent guidelines. For more information about reference displays for creating high-quality Dolby Vision content refer to the article FAQ: What monitor should I use for creating Dolby Vision? (force.com) on our content support web portal.
When Dolby first embarked on its quest to improve the imaging experience in the entertainment space, we decided that the next generation imaging technology should be modeled on the human vision system. The human eye is sensitive to a much larger range of luminance and color than what was possible with the SDR television standard. After considerable study of the sensitivity, tolerances, and comfort levels of real-world human subjects and situations, it became necessary to find a way to encode, distribute, and reproduce images with a dynamic range of 0 to 10,000 nits (candela/m2 or nits). Since existing Gamma-based encoding (EOTF) would not be able to accommodate the entire range efficiently, a new logarithmic EOTF known as “Perceptual Quantizer” or “PQ” was developed by Dolby as a fundamental step towards making HDR imaging a reality. Using PQ, the entire range from 0 to 10,000 nits (candela/m2 or cd/m2) can be captured in a 12-bit image file, while preserving all the sensitivity and detail that a human eye can see. PQ is an open standard (SMPTE – ST2084) that is available today to anyone wishing to develop tools, technology, and devices for HDR imaging.
Additional information about PQ can be found on this page: https://en.wikipedia.org/wiki/High-dynamic-range_video
Therefore, the PQ container holds information pertaining to a dynamic range of 0 to 10,000 nits (cd/m2). The luminance levels across this range are distributed logarithmically and are quite unlike the linear distribution seen in Gamma-based encoding.
A few points to note:
- Unlike in SDR workflows, when working in HDR, the colorist must monitor the image using scopes and waveforms as their HDR reference display may be limited in capability to a certain peak luminance and unable to display the entire image. For example, when working with a 1000 nit/P3/PQ monitor, the colorist can inadvertently push the image they are grading beyond 1000 nits, and all the way up to 10,000 nits, without visualizing it correctly on their monitor. Similarly, they could also push the colors in the image to beyond P3 and extend the color volume into Rec2020 and be unable to see the resulting image correctly on their monitor.
- The logarithmic nature of the PQ scale results in the halfway mark (50%) being around 100 nits and the 75% mark being about 1000 nits. So, is it important to note that the maximum brightness/white that could be achieved in SDR falls at about 50% of the PQ scale.
- When creating text and graphics, it is important to prepare them for an HDR/PQ finish further downstream in the workflow as they can produce unexpected results. For example, white text/graphics created at code value 1023 (10bit) will appear as 100% white which is around 100 nits on a calibrated SDR display. This same text/graphic will again present itself at 100% white in PQ which would translate to 10,000nits on a PQ display.
Understanding Color in HDR
Besides increased luminance and contrast, high dynamic range imaging also provides highly enhanced color that allow colorists and creatives to use colors that could not be seen on television in the past. This incredible increase in visible color is brought about by two aspects of the way color is captured, encoded, and presented in HDR, namely color gamut and color volume.
The colorful horseshoe shape in the CIE Color chart below represents the entire range of colors that are visible to the average human being. The three triangles inside represent the different color gamuts that are used in the entertainment ecosystem for color reproduction in cinema and television today.
[For more information on the CIE 1931 color chart, refer to CIE 1931 color space - Wikipedia]
Rec709 is the color gamut specified for SDR television and is represented by the smallest triangle in the diagram above. The triangle represents the colors that can be reproduced on SDR televisions. DCI-P3 or P3 was chosen as the color gamut for Digital Cinema presentation when cinemas around the world adopted digital cinema projectors in the early 2000s. Digital Cinema projectors are therefore expected to be capable of reproducing all the colors in the triangle that represents the P3 color gamut. Rec2020 was recommended as the color gamut for television by the International Telecommunication Union (ITU) in 2012. Although it is a clearly defined broadcast standard, it is still quite theoretical in that there are not many display devices today that can accurately represent all the colors in the Rec2020 color gamut.
P3D65 (P3 color gamut with a D65 white point) is therefore recommended for HDR content creation and it is supported by Dolby as well as many movie and television studios around the world. Some studios today do request for HDR Master deliverables in Rec2020 in which case the post facility has two options namely,
1. Work/create in P3D65 and convert the final renders into Rec2020 for delivery which is essentially delivering P3D65 images in a Rec2020 image container.
2. Work/create in Rec2020 with the working color space limited to P3D65, and finally deliver in Rec2020.
The difference between Rec709 and P3D65 or Rec2020 may not seem significant especially when comparing the different color gamuts in a two dimensional, graphical form.
The difference becomes more obvious when the 2D color gamut shapes are visualized in 3D with the available luminance on the third axis. For example, in SDR the Rec709 gamut is limited to a maximum luminance of 100nits (Gamma) and is therefore limited to the colors that exist within the smaller shape in the figure on the left (above). This shape can be called the SDR 'color volume' to illustrate the volume of color that can be reproduced in SDR today.
If hypothetically, the same Rec709 gamut had a dynamic range of 0-10000 nits (PQ), the color volume would increase dramatically and a lot more colors would become visible. In PQ, with a P3D65 or Rec2020 color gamut, the color volume is significantly larger than what is possible with Rec709 and Gamma. This extended color volume results in a dramatically improved color palette and visibly vibrant colors in HDR.
Another interesting aspect of the HDR (or PQ) color volume is the way that colors like blue are able to retain saturation and brightness without compromise. In SDR, when colorists try to create a brighter blue in the image (like a bright blue sky), they have to make a choice between a brighter, less saturated blue and a darker, more saturated one. This can be explained from the diagram above where the maximum pure blue that can be achieved in the SDR can be seen to be approximately 10nits. When the colorist tries to increase the brightness in the blue, red and green will have to be added to achieve more luminance and this results in a drop in the saturation of the blue.
In HDR on the other hand, it can be seen from the diagram that it is possible have pure blue up to around 110nits which is much higher than what is possible in SDR. This allows the colorist to create vibrant blues in skies and water unlike ever before. Hence, increased color volume plays a far more important role in color reproduction in HDR, than color gamut.
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