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Autor: Tobias Wiedmer


HDR in grading and shooting - 3/ 3

In-Depth and Best Practice for HDR.

Back-light

Back-light has a slightly stronger effect in HDR due to the higher brightness. DoPs could work with slightly lower intensity or higher diffusion. If you attenuate the edges on the set too much for HDR, they might be too weak for SDR grading. In this case it is better to do the attenuation not in the light setting, but in the HDR grading. This can be done either with log grading (DaVinci), grading regions (Nucoda), base grade (Baselight) or luminance keyers. The Luma curve is also suitable for this in a certain way. One could achieve this, for example, with a luma curve that is roughly shaped as follows:



Fill-light

For HDR it is important that you can see details in the dark areas. It can make sense if the DoP works with more fill light on the set. With camera LUT and grading you can get back to the desired density in the shadows. The advantage of this is that you can also keep the noise level lower. Image noise often has a stronger effect with HDR, as the increased brilliance in the image subjectively gives a higher impression of sharpness. You can help yourself with Denoising.

Practicals

Due to the natural brightness distribution of the PQ curve, Practicals are displayed with their natural brightness in HDR. In HDR it looks like it did on the set without any adjustment. The lamp will draw a lot of attention away from the actor and onto itself. The DoP can consider whether to dim the Practical more on the set and possibly take the opportunity to illuminate the scene very naturally and if necessary extend the Practical with other lamps. As an alternative, one could master the grading of the matter. The advantage of the latter solution would be that the DoP could shine relatively naturally. In grading you could fight it with the mentioned tools, possibly with shapes (Power Windows), to limit the correction to the highlights.

This decision becomes more extreme with very bright Practicals like candles. It's important to remember that the bright flame could quickly lie in the white clipping. Especially in the case of the double wick candles that are usual in film, large burnt-out areas can occur with longer focal lengths. The picture on the right shows a natural candlelight scene compared to a candlelight mood extended with an LED panel. If one has clipped areas in the picture, it can look very irritating for the eye if they are too bright. One should then take special care that these surfaces do not distract from the actual action, i.e. reduce the lights and make the burnt out area appear more pleasant with a slight glow. In the case of a candle, it could also be gently coloured orange.


Picture left: Normal exposure, flame in clipping, picture right: flame Glow and heat in grading


Picture left: Underexposure without white clipping, picture right: underexposure and extra lamp
 
Jewellery, costume and mask

When choosing make-up, jewellery and glitter in costume, you should keep in mind that these elements can flash more during HDR evaluation and thus attract a lot of attention. However, this often looks particularly nice and contributes to the visually realistic impression of HDR. With the mask, you have to work a bit more precisely for HDR. Glossy spots that aren't powdered well enough are much more noticeable here. For grading, this usually means beautiful glitter effects. If these are not desired, you should press the lights again.

Presence of the background

Because you can see more in the picture and discover more, you can of course be distracted more easily. The background of movie scenes can capture more attention. In addition to working with a shallower depth of field, you can also try to solve this problem in grading. It helps if you reduce the contrast in the background.

White clipping / overexposure

In HDR it is hard to hide things like a bright lamp (see pictures below) due to the high dynamic range. What remains inconspicuous in SDR due to the compression of the different brightness levels, appears relentlessly in HDR. One possibility in this case is to reduce the white value and the contrast in the bright area. One could also call this variant "SDR-ization". This hides the lamp a bit more. 


Imagine that an actor with his face is placed in the foreground relatively shady and illuminated with little fill light, behind him there is a practical, e.g. like in the photo.


But for this trick to work, you have to get the viewer used to the reduced white value so that he doesn't perceive it as a bright grey. This is only possible if there are no other parts in the picture that are brighter. In the same way, in the settings before and after, no light can be brighter than in this setting. So you have created a construction site in which you would have to make compromises. With other burnt out areas it can help if you soften the transitions to clipping slightly with a little glow. Often you can keep a brighter light level.

Color casts in the lights

Due to the larger colour space and the greater primary colour intensity, false colours have a much stronger effect in the light. If you work with mixed light, you should judge this more precisely. With LED lights, especially in combination with natural light or conventional film lamps in daylight or artificial light, you should thoroughly test whether there are differences in colour temperature beforehand.

In the example above there is an LED light source on the left that is too green compared to the HMI light source on the right. Since we can't show HDR here, I want to show the difference more clearly in the right picture, at least with a little more color saturation.

Lens Flares in Post

If you want to add Lens Flares to your visual design, you should also make sure that they are not too bright.
SDR, left green LED, right HMI ... More saturation (HDR simulation)

It is often not possible to set the clipping value directly. If you only reduce the brightness, the lens flare quickly becomes unnatural. A Luma curve behind it, as demonstrated before, can help.

Source material in 8 bit and / or Rec709

Sometimes you are forced to go ways that should actually be avoided:
On average one finds old archive material or already SDR-colored material in sometimes worse quality. What was annoying in SDR becomes even worse in HDR.

Here the artefacts are even more noticeable. A natural differentiation of the lights can no longer be found. If necessary one can arrange artificially certain areas in the different lights with keyers and shapes. But you shouldn't spread the material too much if the source is only 8 bit, otherwise you might get unattractive banding. The best way is to "SDR-ize" the material and, if necessary, to keep the shots before and after in the white values pressed down as well, in order to accustom the viewer to a lower level.

"The customer wants everything to be much brighter."

This is primarily a matter of client management. But what helps are the following arguments. If you were to set your diffuse white from 100 Nits to 800 Nits, for example, you would not only dazzle your viewers with a very bright picture. Moreover, you would no longer have the leeway in the lights and would take this advantage. In addition, the effect of brightness would also wear off quickly, as the eyes get used to it. Another point that speaks against it is the different display technologies. OLEDs, for example, reduce the overall brightness of larger bright areas. LCDs can sometimes display large bright areas better. If you leave the main part of the grading in the range below 200 Nits, it doesn't make a big difference. If, however, one were to grade much brighter overall, this could lead to incalculable effects for the end consumer. Sometimes the matching won't be right anymore. Therefore it is not advisable.

Creation of an SDR version of a HDR10 grading

Since very few viewers can watch HDR, an SDR version will still be necessary in the next few years. Thanks to Color Managed Workflow you can get into the rough color space by changing the target transform from HDR10 to Rec709. The advantage would be that the higher contrast and colour intensity make for more accurate matching. If you have a solid HDR master, you can easily turn it into a solid SDR master. In this case even a simple change grading on the finished HDR10 render would be possible. 



Finding the right input light for the adjustment isn't trivial and probably won't work equally for all shots and scenes. Depending on how strongly the lights predominate in the scenes, the highlight tools have to be adjusted accordingly. For scenes that look more subdued in SDR, you can get a bit closer to the HDR feeling by increasing the contrast in the lights.

In the darker image areas you have to check if you don't need more brightening here or there. Also the drawing in bright image areas like windows may have to be recovered separately in the grading. A disadvantage that sounds unimportant at first and that can only be understood when you have seen it directly: When you switch from HDR to SDR, you have the feeling that the lights are strangely compressed.

You then need a little time to get used to what you have known for decades as the only form of presentation. This shows how natural HDR is to the eye.

Creating an HDR version of an SDR grading

This way you have to work with a Color Managed Workflow. If you don't have it, then you should better grad completely new. This is not assumed here. For the SDR grading, the Target Transform is set to the new target color space. Now one can try with creative tools to reflect the SDR look in HDR and to decide where one unfolds the higher potential. Especially here, the above mentioned problems may appear surprisingly, which have to be analyzed and repaired on a shot-by-shot basis. The matching should also be carefully checked again, as the greater colour intensity means that the lights in particular can fall apart more - something that was sometimes more inconspicuous in SDR.

Due to the strong lights in backlit shots, you sometimes have to color the reflective shots brighter. You also have to look out for shots in which you have darkened bright windows in the SDR grading with the help of Luma keyers and blurred their mat a bit so that the keyer doesn't flicker. These blurred mattes can suddenly become visible in HDR. The strength of the highlights as well as the above mentioned problems and compromises in lighting should be well looked at.

So it can happen that hidden lamps or objects in the clipping are suddenly visible in the picture. You should also pay attention to how brightly you color the different highlights. You should keep in mind that a cloudy sky in HDR should not be as bright as an explosion. This creates variability.


SDR, left green LED, right HMI ...



More saturation (HDR simulation)


Conclusion

HDR is not just brighter. It is not painful to look at. It is independent of resolution and theoretically also of color space. It comes closer to reality. The lights and shadows are more impressive. The colors are richer. That doesn't mean that you only see bright, contrasting and very colorful pictures now (hopefully). Rather, it offers the potential to take advantage of this freedom whenever necessary. And often not to be used, in the sense of the stories told in the films. In the next few years we will have the situation that we will have to give both HDR and SDR to the streaming services and stations. I.e. the camera work must work for both standards. To what extent one has to compromise here and there everyone has to decide for themselves, and it will probably be a learning process. 

In any case, it is important to know what the stumbling blocks are in order to make the right decisions.

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