There’s a function labelled “Use White Clip” in certain DLP Cinema projectors that can cause clipping and tinting of D60 whites when viewing DCI X’Y’Z’ images such as DCPs produced in ACES workflows.
I first noticed this behavior several years ago during a private DCP screening booked in a nearby commercial cinema. The content had been mastered on a projector calibrated to P3D60, which of course falls within the minimum gamut of a correctly-calibrated D-cinema projector. Needless to say I was surprised and disturbed to see that D60 approaching 48 nits in DCI X’Y’Z’ space appeared to be out-of-gamut in projection. Since that first discovery I have taken every possible opportunity to inspect the running projector configuration at other commercial cinemas and festival venues. In the majority of auditoriums running projectors from the same manufacturer I found that the “Use White Clip” setting was enabled.
At my post facility we use the same series of projectors for our grading theaters. As a rule we disable any unclear or unnecessary processing features as part of our deployment and calibration of any displays, so we have always operated with the “Use White Clip” option disabled. The manufacturer’s documentation is vague regarding the feature. Based on my minimal experimentation, my best guess is that “Use White Clip” increases the maximum light output achievable for DCI calibration white (.314,.351) by removing the headroom that would normally be available if the projector were set up so that X’,Y’,Z’ = 4095,4095,4095 = 52.37Y,0.333x,0.333y. Instead, it might be that .314,.351 (or possibly the “Virtual White” defined in RP 431-2) is mapped to the brightest possible point of the projector’s measured native gamut (aka MCGD) to increase contrast and maximize light on screen. If I’m correct in my assumption, this feature violates the minimum white gamut and the results would vary with each unit’s measured native primaries and white point. Again, I really don’t know why this feature exists and what it really does to the running calibration, but I can see that it causes more or less clipping depending on the chromaticity of X’Y’Z’ signals approaching 48 nits. DCI calibration white never appears to clip with this feature enabled.
I contacted the manufacturer hoping to learn more, and was told that this setting is enabled by default for factory presets configured for X’Y’Z’ (e.g. IMB inputs), and that it’s meant to avoid unnecessary loss of brightness and contrast that can occur with X’Y’Z’ sources (paraphrasing here). They couldn’t confirm whether this feature is specific to the manufacturer, or if is part of the TI light engine processing common to all DLP Cinema projectors.
Has anybody else encountered this in the wild? And can anyone shed more light on why this feature exists and why it causes clipping of correctly-prepared DCPs?