Some best common practice DCP color space workflows part 2

Recommended links :

Cinematic Color Motion-Picture Color Management

Real-Time Rendering – 2011 Color and Imaging Conference, Part II: Courses A

Arri Alexa – Working with HD – Color Grading

How to make 3D LUTs in Nuke

Updated:

Summary:

In commercial cinemas you may see your movie projected in different environments with too much or too little light.

Dark scenes without contrast can look grey in many cinemas, especially 2D movies projected on 3D silver screen with too much light.

If the movie is graded on a monitor with too much light, the movie will lack contrast when projected with less light in the cinema.

To avoid a video look, it is better to use conventional 35 mm film grading techniques.

Many Hollywood movies uses a Orange and Blue color scheme to get a picture with contrast.
See Why Every Movie Looks Sort of Orange and Blue

See also the videos at:http://prolost.com/blog/magicbullet12

Check that your software uses the correct conversion to the DCP gamma encoded DCDM X’Y’Z’ color space.

Otherwise white would look pink.

Some people uses DCI white in mastering, but there is no point in using DCI White because it is greenish and DCDM X’Y’Z’ support other standard color temperatures like D60 or D65.

It is more important to get the contrast right than using P3 colors, you are allowed to use Rec.709 colors.

Workflow: Converting directly from Rec. 709 video to DCDM X’Y’Z’

I have done this color space workflow using Fraunhofer Easydcp Creator on film festival video material intended for cinema projection.

-Some of the other DCP software I have tested have picture artefacts.

-It is possible to retain the D65 white point of REC709 with white at 14 fL.
Use Rec 709 transform with no chromatic adaption.

-The REC709 colorspace fits inside the minimum color gamut of DCDM X’Y’Z’ which currently is DCI XENON P3/Virtual White (SMPTE RP 431-2:2011).

 

Workflow: Converting from source material to P3 and from P3 to DCDM X’Y’Z’
For a cinema look you could use the same workflow as high budget movies where they use a film stock 3D LUT that converts the source material to P3 with a film stock emulation, and then convert P3 to DCDM X’Y’Z’.

Update
Easydcp Creator 2.2 includes a color transformation from P3 DCI WHITE to DCDM X’Y’Z and let’s you make custom transformations.
See this post

You could also go from LOG to P3 without baking in a film stock emulation.
Example: Alexa 3D LUT generator has both LOGC to REC709 and LOGC to P3 3D LUTS.

This way you could first look at the material on REC709 monitors
and then later get the more saturated colors and 12 bit grey scale range (in 2.6 gamma) that is possible when grading on a P3 grading projector.

Testing DCP software

To do a test for picture artefacts you could make a test image (with 8-bit values)like this in After Effects:
-make a solid layer with the color 2/255, (black is rarely 0)
-then add a circle with white at 255/255. Blending mode: Add.
-then add some noise. 1 percent.
-Run this through your DCP software with a rec.709 setting

you should end up with the blacks and noise a little bit higher (around 0-3/255)

and D65 white at around 242,247,255 in 8 bit. (12 bit = 3883, 3960, 4092)

When I tested some software solutions they raised the blacks to around 10/255.

These blacks will now look grey and the barely visible noise will be very visible noise.

It will also look this way in the cinema.

The reason some people do not notice the elevated blacks could be because of high light levels in the movie theatre.

Another software solution clipped some of the channels so white would be tinted. Others had the wrong white values.

Some of the others just used the max white value (255,255,255) which would make white go out of gamut and you would get tinted white. And if the projector is set up to map illegal values to the projector’s internal color space you would end up with a different tinted white. Usually this is seen as a red cast.

Another software solution crushed the blacks and removed the noise. To use this solution you could raise the blacks to a level where they are not crushed.

Before converting your video material to DCP you should check if the software is usable for your purpose.

Most software have a trial version and you can use the trial version of Easydcp player or use the trial version of the Easydcp plugin that is integrated in Resolve 10 to check the result.

Different p3 color spaces

4 different P3 color spaces are mentioned in SMPTE RP 431-2:2011 D-Cinema Quality Reference Projector and Environment:

P3 D55
P3 D61
P3 D65
(P3 DCI WHITE)

These four will fit inside the color space of a P3/Virtual White digital cinema projector. Others white points will also fit, but these are the four mentioned.

If you want to move from REC 709 to P3, you could use P3 D65 to keep the 14 fL D65 white point.

You can use any artistic white you want in a movie.

You don’t have to use a chromatic adaption to the DCI WHITE white point.

But it is common to use the maximum RGB value for R, G, and B as white (Max 12 bit values are 4095, 4095, 4095) and calibrate it to the white illumination target (14 fL).

In P3 D55 the values 4095, 4095, 4095 will correspond to the D55 WHITE values 3893, 3960, 3838 in DCDM X’Y’Z.

In P3 DCI WHITE the values 4095, 4095, 4095 will correspond to the DCI WHITE values 3794, 3960, 3890 in DCDM X’Y’Z.

In P3 D65 the values 4095, 4095, 4095 will correspond to the D65 WHITE values 3883, 3960, 4092 in DCDM X’Y’Z.

In DCDM X’Y’Z the D55 WHITE, DCI WHITE and D65 WHITE have the same luminance (Y) level for all three white points. But the X and Z values are higher/lower for each of the three white points.

White (Y=3960) is 14 fL / 48 cdm2 for all three white points and is also the maximum allowed Y value.

Y values higher than 3960 are illegal and out of gamut, but the projector may have a setting enabled that fits illegal values inside the projector gamut.

Virtual White is described in SMPTE RP 431-2:2011
D-Cinema Quality  Reference Projector and Environment
which is based on the SMPTE Digital Cinema White Gamut Practices Study Group Report. It is also a PCF (projector configuration file) and a color calibration target on digital cinema projectors that can be used to calibrate the projector to comply with SMPTE RP 431-2:2011 instead of SMPTE RP 431-2:2007.

Going from different P3 color spaces to DCDM X´Y´Z´ is described in
SMPTE EG 432-1-10

 

Converting to DCDM X’Y’Z using 3×3 matrix linear algebra

Also see this post: How to calculate RP-177 3×3 matrices in Matlab

RP-177 math is implemented in this free python package:
Colour Science for Python

If you want use the 3X3 matrix linear algebra used in SMPTE EG 432-1-10  to convert to DCDM X’Y’Z, you should grade the material in a digital cinema reference environment.

http://www.dcimovies.com/archives/spec_v1_1/DCI_DCinema_System_Spec_v1_1.pdf

SMPTE RP 431-2:2011 D-Cinema Quality  Reference Projector and Environment:

 

The mathematical transform from reference projector Rec.709 D65 WHITE to reference projector X´Y´Z D65 WHITE is available here:
SMPTE Digital Cinema White Gamut Practices Study Group Report

page 42

 

This transform is not automatically correct if you convert from monitor graded REC709 to DCDM X’Y’Z’ because:

-DCDM X’Y’Z’ is a display referred gamma encoded color space that is meant to be seen on a digital cinema projector with white at 14fL/48 cd/m2 in a dark environment ( and that meets all the applicable SMPTE Standards and Recommended Practices.)
-Monitor Rec 709 material is meant to be seen on a monitor with white at around 80-120 cdm2 in a dim environment.

Only when you grade the Rec 709 material in a cinema with 14 fL/48 cd/m2 will the mathematical transform be correct.

The example math uses:
-12 bit = 4095,
-maximum luminance = 48,
-the normalizing constant = 52.37,
-2.6 gamma
DCDCM X´Y´Z´ = 2.6 gamma encoded CIE XYZ 1931 with DCI primaries (currently P3) with 48 cdm2 maximum luminance and a normalizing constant of 52.37.

For more info see this post: How to make 3D LUTs in Nuke

More sources on color science and color space conversions:

Digital Color Imaging Handbook
google search: Digital Color Imaging Handbook

 

Digital cinema colors vs 35 mm colors

When using some film emulation 3D LUTs you can get a desaturated effect on video material because the colors that are not possible to show on 35 mm film can be mapped to 35 mm allowed values. But 35 mm can show very saturated dark colors.

Digital cinema can show bright colors that is impossible on film.

Digital cinema uses an additive color system while 35 mm film uses a subtractive color system.

-In an additive color system if you combine all the colors you will get white.

-In a subtractive color system if you combine all the colors you will get black.

-Some colors are not very bright on 35 mm film, because you have to combine layers of color to make them, and these layers stops light.

-The Xenon P3 color space was made for grading film and have more saturated primaries than REC709.

-P3 can show more of the saturated 35 mm colors, but it is also a additive color system that can show the brighter colors that was impossible with film.

-There are some films using brighter colors for dramatic effect. This is perhaps because the digital cinema version of the film is now usually the master that other versions will be made from. The 35 mm print would look different than the digital cinema version on these films.

 

REC2020. The future color space for digital cinema?

http://en.wikipedia.org/wiki/Rec._2020

Today the reference projector for DCI XYZ  uses the Xenon projector P3 primaries. It is the current minimum Color Space Gamut.

The TV standard REC2020 have been standardized and there is a reference projector that can show the REC2002 laser primaries. This Super Hi-Vision color space can show 99 percent of the colors humans are able to see.

The DCI P3 RGB white point is not meant to be used as a mastering white point because it is greenish. You are supposed to use something like P3 D60, which is similar to the white Xenon light of a 35 mm projector.

The DCI X`Y`Z`standards can also be upgraded to REC2020 primaries in the future. If the minimum Color space gamut changes from Xenon P3 to RGB Laser REC 2020, the reference projector standard will change and all post production labs need to move to RGB Laser projectors.
The Xenon projectors still in use in cinemas will need some kind of mapping of REC2020 to P3, but would be able to use the same DCPs as the Laser projectors. Most likely there will be two different DCPs, one for Xenon and one for RGB Laser. The contrast ratio possibly with RGB laser projector is much greater.

 

 

 

5 Replies to “Some best common practice DCP color space workflows part 2”

  1. Hello,

    I follow your blog regularly. thanks for all the info that you have here. Helps us a lot.

    Regarding the above post, I have a doubt.

    If my film was graded in REC709 Full Range, with a peak white at 100 cdm2, and I use a LUT to go from REC709 to DCI-P3, what should I expect for the peak white levels ?

    The theater expects a peak white of 14fL, which is around 48cdm2. Will the LUT change the peak white, or rather the grading has to be done at REC709 viewed at peak white of 48cdm2, given that it will be a darkened room ?

    Thank you for your time.

  2. The lamp should calibrated to 14/48cdm2 fL white illumination in the cinema. It used to be a problem that cinemas had dim lamps, now higher values like 100 cdm2 can be seen on 3D/2D screens. 100 cdm2 in a dark room is very bright and black will be more grey at those levels. If you grade on a 100 cdm2 monitor you should remember that white will not be as bright and 0 black in cinema is not 0 black like on a plasma screen. 0 black will always look a bit grey without some brighter areas in the picture that creates some contrast.

  3. So how about if we grade with 48cdm2 on a an lcd tv, in a darkened room ? Would that simulate a theatre ?

    Basically, is there a way we can simulate all properties of a theatre on an lcd/plasma screen in a room ? I know the diferrences between them as in: theatre being fully dark, peak white at 48 cdm2, gamma 2.6, and a white point of DCI white.

    Is it possible that the plasma/lcd be calibrated to these specs and veiwed in a fully dark room ? Would that be a WSIWYG config, leaving just the gamut differences of DCI-P3 and REC709 ?

    thanks,

  4. Computer monitors and cinema screens are different. The rods and cones in the eye will react different to a monitor and a cinema screen. But you could compare the monitor and the cinema and the monitor will get you some idea of how it will look in the cinema. A Plasma screen on eco mode or a monitor adjusted to 48 cdm2 will still have blacker blacks than a cinema theatre. You could try to simulate the greyer blacks of a cinema with the brightness control. You should use 2.6 gamma in a dark room and 2.4 in a dim room. A smaller cinema screen or monitor will have more colorful colors than a larger cinema screen even if the colors are calibrated the same. You could try to compensate for this.

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