Autor: Tobias Wiedmer

HDR in shooting and grading - 1/ 3

In-depth and best practice for HDR.

In the course of a workshop for cameramen, which we organized together with MBF and Delight Rental Studios, we at Cine Chromatix have intensively dealt with the topic HDR and wanted to test together with colorists and DoPs practical examples, which light setups and settings work for HDR, where difficulties lie, how to make compromises to SDR, and of course enjoy the new possibilities and strengths of HDR. 

We had two Eizo 3145 Prominence HDR displays both on set and in the Grading Suite and for comparison several Sony SDR Class 1 monitors on which we could watch SDR and LogC. Unfortunately it is quite difficult to show HDR without an HDR monitor, so I can only simulate the effect for this article. The graphics for the part "HDR in theory" are based on a template and courtesy of Kevin Shaw, C.S.I. (Colorist, and John Hoare (DoP, The workshop was held by Tobias Wiedmer, C.S.I. (Lead Colorist, Cine Chromatix) and Luisa Hermanns (Senior Colorist, Cine Chromatix) and organized by Simon Sturzenegger (Project Manager, Cine Chromatix) and Michael Carstens (MBF Berlin). Dirk Steiner-Sennheisser was responsible for the lighting.

Regarding HDR, many people think of overlaying differently exposed images.

HDR in theory

In the case of HDR monitoring and grading, however, this is not the point. It is an advantage to capture as much drawing as possible from the original scenery with the camera. HDR means "High Dynamic Range", and this means primarily the area between the darkest black and the brightest white. In contrast to this, the dynamic range used so far is called the "Standard Dynamic Range", or SDR for short. The brightness is usually given in the unit Nits (Latin "nitere": seem). 1 Nit corresponds to 1 cd/m².

Monitor brightness and image brightness

Compared to an SDR Class 1 monitor, an HDR monitor has a lower black level and a brighter white level.
According to current standards, the white level can be up to 10,000 Nits and the black level close to 0 Nits. A current SDR Class 1 reference monitor, on the other hand, is calibrated to 100 Nits, with a maximum black level of 0.05 Nits. The fact that the monitor is brighter does not mean, however, that the image content must also be brighter. 

If you were to correct everything proportionally brighter in grading without changing the ratios, the image would in all probability no longer be perceived as aesthetically pleasing. You get used to the brightness and lose the advantage of HDR. An image that does not contain any special lights should look similar to an SDR image. The potential of HDR is therefore only used if particularly bright lights (or particularly deep blacks) are to be displayed, which should stand out strongly from the rest of the image content.

With the HDR standard a larger color space comes at the same time. The color space known from HD(TV) Rec. 7O9 (small triangle) is extended to Rec. 2O2O (large triangle). In practice, however, only laser projectors achieve this rich coloration so far, so that this color space is used as a container, but practically the P3 color space (middle triangle) is used. This is already used for cinema grading and mastering, except that instead of the greenish DCI or the more neutral D6O white point one works with the D65 white point known from Rec. 7O9. For the new HDR brightness potential, it wouldn't necessarily be necessary to increase the gamut, but as a trend-setting step, this has been sensibly merged with the new standard.

HDR standards

HDR is divided into two basic standards. One is the SMPTE ST-2084 standard based on the PQ-EOTF (Perceptual Quantizer Electro Optical Transfer Function), developed by Dolby. The second is the BBC-developed HLG (Hybrid Log Gamma) standard ARIB STD-B67. The HLG curve corresponds over large parts to that of Rec. 709, except that it has a logarithmic curve at the upper end. Thus a wrongly displayed image (HLG in SDR or vice versa) still looks reasonably correct. In Germany, ZDF has opted for this standard. The other common HDR standards are based on the PQ curve.

The graphic shows that the exposure zones from the SDR range are not simply enlarged. Rather, HDR adds additional zones that do not affect the original zones. The different brightness levels can be used to tell variations in brightness levels. With SDR, the difference between a white sheet of paper in the sun and an explosion is very small. Both are approximately 1OO% white. With HDR, on the other hand, the different brightness levels can differ considerably.

Top: SDR, Middle: SDR Streched, Bottom: HDR

This curve is very different from the usual Gamma 2.4 curve. Due to the steeper slope, logarithmic camera material looks almost normal. The big advantage of PQ is that it corresponds to the visual habit of the eye and thus creates a very natural image. The feeling of looking into reality has been exaggerated. Examples of the use of the PQ curve are Netflix and Amazon Prime Video. One of the PQ standards is HDR10. 

It's an open standard that doesn't allow for preference metadata. Dolby Vision is a proprietary Dolby standard that allows for preference metadata. This allows Trim Passes to be created to other monitor brightness levels (e.g. 4,000 Nits to 1,000 Nits or to SDR 100 Nits) without having to export different video masters. The use is associated with high costs (hardware and license fees from Dolby) and has not yet established itself in Germany.

HDR10+ combines the two previous standards, i.e. it is free and you can use metadata.

Enlarged by the larger color space and the greater brightness of the individual colors the ink volume is practically the same. For example, with a 1.OOO-Nits monitor the red channel at 22O Nits brightness and thus be brighter than SDR white with 1OO Nits.

With HDR, it is now possible for the first time to create a bright sky simultaneously with a rich Blue to provide. With SDR it would lose its blue saturation as soon as it becomes too bright, because then R,G and B tend to converge and thus become more neutral.

Requirements for Consumer HDR Monitors

According to the standard, HDR monitors must be able to display the image at least in UHD resolution. This fact is actually independent of HDR. One could theoretically also do HDR mastering in 720 x 576 pixels (SD). However, this should be considered sensibly. However, the monitors must be able to display at least 10 bits. Because the main part of the image is played in the lower half of the code values, a quantization of 8 bits would not be sufficient. One would often see unattractive banding artifacts depending on the image content.

For the sake of future security, the monitors should also be able to display at least 90% of the P3 color space. Different display types are distinguished in the classification of the required brightness of black and white value.
For OLED displays that can display a very low black level (less than 0.0005 Nits), a minimum brightness of 540 Nits is sufficient. For LED displays that can only achieve a very low black level with tricks, a maximum black level of 0.05 Nits is tolerated. To compensate for this in the overall contrast, the white value should be at least 1,000 Nits.

Professional HDR Reference Monitors

The market for professional reference monitors that are of good enough quality is still manageable. The best known monitor, which is also commercially available, is the Sony BVM X300 - an OLED display with very deep black and a white brightness of 1,000 Nits. However, this brightness is only achieved in a very small part of the image (which is sufficient in most real situations). If you wanted to show a full white image, the brightness would only be about 300 Nits, because the monitor automatically reduces the brightness of the image above a certain amount of white. 

This spring Eizo launched its CG3145 Prominence HDR monitor - you can read the test on page 36 of this DP issue.
This works with LCD technology, so that it can also display a completely white image across the entire screen with 1,000 Nits. At the same time, it creates a low black level that is almost at OLED level. Other important displays are Dolby's non-commercial reference monitors: the Pulsar with 4,000 Nits and the Maui with 2,000 Nits.

Read part 2 soon on DPP Film Tech App.

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