This document published by the HDR4EU project, a group of representatives from ARRI (camera manufacturer), Filmlight (post-production software developer), Smoke & Mirrors (post-production company), Brainstorm (broadcast software developer), Barco (display manufacturer), and Universitat Pompeu Fabra (computer graphics and image processing academic group) who got together to come up with guidelines for HDR production, has some useful insights concerning the production, post-production and delivery of HDR content. A few of their observations, in no particular order:
- The most important general recommendation that can be offered for shooting HDR content is to monitor the images at the dynamic range they will be distributed. While this may seem obvious, there always exists the temptation to use SDR monitoring equipment to more or less imagine how the images will translate in HDR. This approach can lead to unhappy surprises in the color grading process. To name a few, noise will increase visually as the dynamic range of images increases, motion judder will become more noticeable, and any over-exposed or sensor-clipped regions (e.g. practical lights) in the image become far more distracting in HDR…
- While exploring the effects of varying global contrast and the average light level (ALL) of images, one will find that the eye easily grows accustomed to any level of constant brightness, as long as there is time to adapt to it. By this token, any images which do not use highlights sparingly risk looking more or less like SDR as the viewer will quickly adapt to the new contrast range.
- By increasing the global contrast of an image, one also increases the local contrast, or sharpness. Large increases in sharpness can result in unpleasant artifacts (blocky clipped highlights which are particularly obtrusive in the case of practical lights, noise, unnatural or overdetailed textures [e.g. skin, hair], etc.).
- The average light level of an HDR image should be roughly the same as its SDR equivalent, just expanding the highlights and the shadows. Otherwise, if the average light level is scaled proportionally with the peak highlight, the HDR effect will be lost completely. The viewer will adapt to the image and it will appear roughly the same as it would on an SDR display.
- Since the light levels in HDR images are distributed differently than SDR images, it stands to reason that the lift, gamma, gain controls which have been traditionally used in motion picture color correction will not react in the same way between formats. With SDR images, these controls could be used to roughly adjust highlight, mid-tone, and shadow regions of images. In essence, the feeling reported by many colorists in using them for HDR grading is that they are too ham-fisted, and do not allow for the same range-specific correction.
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