All Things Color for Film and Digital Cinema
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Reference images

China Girls 2

Back in 2005 Julie Buck and Karin Segal put on an exhibition in New York called “Girls on Film”. 

Julie was manager of the Harvard Film Archive and had the task of sorting and preserving the Film Archive Collection. The exhibition was a tribute to the many anonymous women who worked in the film industry.


China Girls .

 

China_girl7

China_girl7

For a bit of fun and curiosity I will update this post over time with images of China girls. China Girls are used to adjust and check printer density.

They provide subjective and objective colour and gray scale patches.

These patches can be read on Densitometers to confirm the process. They are a standard laboratory test film usually incoporating a face and a greyscale; used for printer line-up.

 

 

 

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Density and the Film Lab – Density Pt.4

 

Lab Aim Density / China Girl

Lab Aim Density / China Girl

 

China Girls are used to adjust and check printer density.

They provide subjective and objective colour and gray scale patches.

These patches can be read on Densitometers to confirm the process. They are a standard laboratory test film usually incoporating a face and a greyscale; used for printer line-up.

See the post on China Girls for a look at some of the older examples of these majestic images.


Protected: Densitometry Pt2 “the 21 step test strip”

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The Macbeth Chart

The Macbeth chart is a color evaluation chart that represents and reflects the colors of nature under any illumination. It is used as a color rendition and reproduction target for both stills and motion picture imaging labs. Using the  given set of digital scientific values for each patch an operator can evaluate a given image exposed with the card and correct for color inconsistencies inherent in Digital imaging devices, film stocks, lens coatings, old lamps, printing stocks, printing paper, monitoring devices,….ect.

Below is the Macbeth Color Chart and its relative CIE and SMPTE-C RGB values.

Macbeth Color Chart

Macbeth Color Chart

 

 

 

 

 

 

 

 

 

No Name CIE_x CIE_y CIE_Y Hue Value Chroma RGB_TRIPLET
1 dark skin 0.400 0.350 10.1 3 YR 3.7 3.2 116 80 67
2 light skin 0.377 0.345 35.8 2.2 YR 6.47 4.1 196 149 129
3 blue sky 0.247 0.251 19.3 4.3 PB 4.95 5.5 91 122 155
4 foliage 0.337 0.422 13.3 6.7 GY 4.2 4.1 88 109 67
5 blue flower 0.265 0.240 24.3 9.7 PB 5.47 6.7 128 127 175
6 bluish green 0.261 0.343 43.1 2.5 BG 7 6 90 190 169
7 orange 0.506 0.407 30.1 5 YR 6 11 221 119 44
8 purplish blue 0.211 0.175 12.0 7.5 PB 4 10.7 71 90 164
9 moderate red 0.453 0.306 19.8 2.5 R 5 10 198 81 98
10 purple 0.285 0.202 6.6 5 P 3 7 93 60 107
11 yellow green 0.380 0.489 44.3 5 GY 7.1 9.1 158 88 64
12 orange yellow 0.473 0.438 43.1 10 YR 7 10.5 234 161 49
13 blue 0.187 0.129 6.1 7.5 PB 2.9 12.7 47 59 151
14 green 0.305 0.478 23.4 0.25 G 5.4 8.65 65 150 71
15 red 0.539 0.313 12.0 5 R 4 12 181 40 59
16 yellow 0.448 0.470 59.1 5 Y 8 11.1 241 200 38
17 magenta 0.364 0.233 19.8 2.5 RP 5 12 190 78 146
18 cyan 0.196 0.252 19.8 5 B 5 8 0 134 164
19 white 0.310 0.316 90.0 N 9.5 0 242 242 236
20 neutral 8 0.310 0.316 59.1 N 8 0 200 200 199
21 neutral 6.5 0.310 0.316 36.2 N 6.5 0 159 160 159
22 neutral 5 0.310 0.316 19.8 N 5 0 122 121 119
23 neutral 3.5 0.310 0.316 9.0 N 3.5 0 84 84 84
24 black 0.310 0.316 3.1 N 2 0 53 53 53 

Densitometry for Lookup Tables Pt.1

A densitometer is a device that measures the optical density of an image on any printed medium, be that celluloid print film, paper, photographic film negative or any other medium. If the image is printed on paper we could read its optical reflectance, if printed on 35mm print film we would be reading its optical transmission qualities.

As human vision is measured in logarithmic values so too are the results of the densitometer. When taking readings of a Motion Picture print over a densitomter we are reading the prints transmissive qualities of light. In Status A densitometry (more on that in the next post) the lighter the patch the more it transmits light. The denser the image the less transmission of light. Density is defined as  

Optical Density = log10(1/transmittance)

I have a X-Rite310 densitometer which is common to most Film Labs around the world. It reads, for any given reference, the Red, Green and Blue optical transmission values which is imperative in creating RGB Color profiles or 3D lookup tables (3D LUT’s) for film.

Fig:A a 4x4x4 example target

Fig:A 4x4x4 example unity target or Identity LUT

 

Printing a set of specific color and grey scale patches to the desired medium, like the ones in figure A, an operator can read and translate the optical transmission characteristics of each individual patch and create a ‘lookup table’ . This profile can then be used to visualize on a monitor what the image will look like when printed to film. It is also profiling the characteristics of the imaging device which recorded the patch to film. In our case an Arri Laser film recorder. The more colors sampled the more accurate the representation will be. (The example used is extremely ‘low res’ and is only used for demonstration purposes.) To achieve an accurate representation we tend to use a ‘matrix’ of 4096 color patches!

 Lets look at those color patches from Fig:A mapped into an RGB cube to see just how these work. (See Fig:B)

Fig:B Representation of the 4x4x4 Color Cube

Fig:B Representation of the 4x4x4 Color Cube

 

If you compare the image in Fig:A and the Color Lattice to the left you can see each of the 64 patches are represented in a 3 dimensional cube which we can call the RGB Color Cube. The extreme corners represent %100 code values for all the primary colors of the cube, and Black. If you were to only look at the points down the diagonal axis from Black to White you would be looking at a grey scale.   From this would could create an identity LUT which would read something like Fig:C in ASCII text. Being an identity LUt it would have no optical effect to the original image.

 

 

 

ASCII 3DLut 4x4x4 identity matrix

ASCII LUT

         Starting at Black and reading the 4 points across to red expressing them as RGB Triplets. (I am assuming a an 8 bit scale which doesnt divide very well in 3 from 256 code values so excuse the rounded math)

 

        We then move up one row towards Green and read out the next row of 4 points.

 

 

         Until we read the top at which point we move in one row towards Blue and then begin reading the rows on that plane in the same way. Eventually after 64 entries of RGB triplets we have successfully mapped the cube.

 

 

 

In the photographic desktop publishing world images like the one in FIg.D are used. These are called Targets. The most used one is the Kodak IT8 Target.

kodak_IT8_target

Fig:D Kodak IT8 Target

 

 

 

 

 

 

 

Fig:B 4x4x4 identity LUT

Fig:E 4x4x4 identity LUT

Due to the different optical characteristics of different papers, films and imaging devices the original image will never look the same when printed, unless, through software we mimic those qualities on the monitor we are referencing the digital image on.

When printed to the desired medium and each of the patches are measured on a densitometer we remake the cube and will notice that the points are slightly deformed taking on and showing us the optical properties of the medium.We then transpose those results to represent the films qualities in to a new ASCii LUT

Fig:C transformed LUT

Fig:F transformed LUT

 

We then transpose those results to represent the films optical qualities in to a new ASCii LUT that can be fed into the monitoring device.

 

 

What I have just described is the most rudimentary basics of color management and 3D color Lookup’s. The Following Posts will go deeper into real world color calibration also touching on, using Marcie,monitor characteristics, viewing environments, film projection characteristics, different recording and printing stocks.

 

Adrian.

 

Image References:

Nvidia.com – Graphics GPU technology examples

RPIImaging.com  - IT8 printing Targets.

cube-lattice