Apple Log vs SDR & HLG

When we decided to develop applications for filmmaking and for cinematographers, we asked ourselves which profile would give us a stable, measurable and reliable contract, that is, a stable relationship between the light in the scene and the value in the file. We therefore developed a series of proprietary software tools and tests (with charts and patches of known luminance, constant and measured exposure, just as you would for camera tests before a film) drawn from our collaborators’ experience in cinematography.

What did the results of these tests tell us?

In SDR with locked exposure and global tone mapping, the value of a fixed patch shifted by about half a stop as the rest of the scene changed. No static calibration can compensate for it, because it isn’t an offset: it depends on the framing. In HLG, the device’s ISP renders adaptively in HDR too — same problem.

The only stable contract we can obtain on these devices is with Apple Log. We would have liked to be able to work with SDR or HLG as well, but the tests revealed behaviours that are scene-adaptive rather than scene-agnostic. What does that mean? In SDR and HLG, Apple processes the signal upstream, before delivering the file, to optimise the video output: scenes that differ in contrast or saturation could lead the device to alter the signal in a completely different and inconsistent way. All of this translates into unpredictability in behaviour and in the final delivery. In Apple Log, the curve that governs the signal is published and fixed. This adaptive behaviour works very well for the ordinary user who just wants to get the most out of their device with ease. For the professional user who wants stable and predictable behaviour, it is unreliable.

For this reason we decided to adopt Apple Log as the video contract for many of our software products. At launch, Apple published the correspondence between the log curve and the code values, so from its official white papers we know that 18% middle grey corresponds to a code value of ≈0.48826. Apple Log 2 confirms our decision: Apple kept the log curve identical and only changed the gamut (moving from Rec. 2020 to AppleWideGamut)

What changes for the end user? Anyone using our apps, now or in future, has our guarantee that those apps were built on stable, sound and solid data. On the other hand, only those who own a Pro device with Apple Log will be able to use them. Other users will not be able to use some of our apps. We prefer to guarantee reliable operation to fewer users rather than serve a wider market without guarantees. Each of our apps and software products honestly states its minimum requirements and limitations.