Convert P3 images to DPX for Film Recording

 

What are the hurdles associated with P3 to DPX Conversion ?

In a typical DI environment the goal is to display on a digital screen an accurate emulation (reproduction) of a potential side-by-side viewing with a film print. This is usually accomplished via a custom 3DLut (three dimensional look up table) used in the color correction software system (or uploaded into the digital projector hardware).

The SMPTE standard for a film projector defines the luminance of the open gate white point to be 16 ft. lamberts (approx. 55 candelas / m^2). When a reel of film is loaded onto the projector the maximum luminance is a combination of the open gate luminance and the attenuation caused by the native film base (minimum density of the film). Print film from different manufacturers or any specific emulsion lot may have variability due to processing lab consistency and which lab was used. For example, if the minimum density of the film base was 0.06 0.08 0.12 in RGB Status-A density (Status-A density is the densitometer metric used for film that will be viewed by an observer), and the visual weighting of the RGB is 30% R 60% G and 10% B then the visual density is 0.078. Since density is - Log (transmittance), then transmittance is 10^(- density) or in this case 10^(-0.78) = 0.8356 . Therefore, the maximum luminance in this system from the film projector would be 16 ft. L. * 0.8356 = 13.37 ft. L. In practice this luminance is rarely achieved and to ensure an exact luminance match of the digital projector to film the 3DLut will limit the luminance further, typically to the 10 - 12 ft. L. range. This is due to the fact that scene white is not at the minimum density of the print film but a bit higher to accommodate specular highlights etc.

In practice there are two normal setups for a P3 compliant digital projector. The setup for a Digital Cinema projector in XYZ color space and for RGB P3 color space is: [ see: Digital Projector Standards Setup]

From the setup information, the maximum luminance for a P3 image at the P3 white point is 15.276 ft. L. This is considerably greater than the luminance attained in a film projector with film. Any color in P3 color space that has a luminance greater than the film projector maximum luminance is outside of the gamut of the film system. Outside of putting a larger bulb in the film projector these colors need to be remapped into the film color gamut by either decreasing the luminance or saturation or both.

Digital display devices are almost entirely RGB devices while film is a CMY [cyan, magenta and yellow] system. RGB devices do best with R, G or B saturated colors [to get to C, M or Y needs two color channels]. Film does best with cyan, magenta and yellow colors [to get to R, G or B needs two film layers].

There are colors in the film system that are outside of the P3 RGB color space, but in the conversion of P3 images to film space for recording that is not an issue.

The main problem is to suitably remap any P3 RGB colors that are out of the film color display gamut. [gamut remapping]

There are many techniques in the field, but the best remapping will depend on the source color space and the destination color space.

Another problem with converting P3 data to film data is the mathematical technique. A 3DLut is a viable approach if there are enough nodes in the 3DLut, otherwise, unacceptable interpolation errors are introduced. Suitable pre-1DLuts and/or larger 3DLuts minimize this problem.

 

MJB Consulting can provide advice and assistance in P3 to recorder format film data.