Conversion of 10 bit Log Film to 8 bit Linear or Video Data

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1.0 Film, Computer, and Video Images

1.1 Film Images

The characteristic curves for a typical motion picture color negative film are shown as Figure 1. The three curves represent the red, green, and blue color records, which are reproduced by cyan, magenta and yellow dyes, respectively. The three curves are offset vertically because the base density of the negative film is orange in color (it contains more yellow and magenta dye than cyan dye).

Figure 1

The next issue was calibration range. In order to provide as much creative freedom in the digital world as is possible in optical printing, it is necessary to digitize the full range of the original negative film. This allows the digital data to be "printed" up or down without compromise in post production. It also allows color grading decisions to be made in context with elements of the final composite. The full dynamic range of a motion picture negative film can be captured in a 2.0 density range.

It is important to note that the original film captures more than what is displayed when it is printed and projected. This extra range or extended headroom can be used to capture specular highlights. Over-exposure can also be used creatively by a cinematographer to alter the look of the image, increasing the contrast, cleaning out the shadow and boosting the color saturation.

1.2 Computer Generated Images

CGI images are typically displayed on CRT graphics display monitors with the gamma set to 1.7 (default). This compensates for the native gamma of the monitor (2.5), producing a displayed image with a gamma of 1.5. For comparison, note that the midscale gamma of a projected film print is also about 1.5.

Video Images

	V' = 1.099 * V^0.45 - 0.099 	for 1 >= V >= 0.018
	V' = 4.5 * V			for 0.018 >= V >= 0

2.0 The 10 bit Digital Negative

2.1 Full Latitude

2.2 10 bits

2.3 Printing Density

2.4 Normal Digital Negative

Figure 9. Cineon Digital Negative (Normal Exposure)

8 bit Linear Representations

3.1 8 bit Linear

3.2 8 bit Video

3.3 Keeping the most important 8 bits

For a normally exposed Cineon Digital Negative, a 90% white card will be recorded at a code value of 685. The Dmin of the film is set to 95, representing the blackest black that can be recorded (approximately a 1% black card). In converting from 10 bits to 8 bits, the range of code values from 95 to 685 are mapped to an output range of 0 to 255. In between these end points, it is necessary to implement the gamma correction that is described herein.

3.4 Densities Above Reference White

3.5 Implementation of a Soft-Clip

4.0 User Specified Variables

4.1 Reference White

4.2 Reference Black

4.3 Display gamma

4.4 Softclip

5.0 Computing a Look-up Table for conversion to 8 bit Linear data

5.1 Determine the breakpoint for softclip

Breakpoint = Refwhite - Softclip

The default value for Refwhite is 685, and the default value for softclip is 0 (softclip turned off).

5.2 Clamp black to 0

The default value for RefBlack is 95.

5.3 Compute a LUT between RefBlack and Breakpoint

where,

Dispgamma = 1.7 for linear data, and

Gain = 225 / (1-10 ^ (Refblack-Refwhite) * 0.002/0.6) ^ (Dispgamma/1.7))

Offset = Gain - 225

5.4 Compute a Softclip above Breakpoint

where,

Kneesoft = 10 ^ ((Breakpoint-Refwhite) * 0.002/0.6) ^ (Dispgamma/1.7) * Gain - offset

Kneegain = (255 - Kneeoffset)/( (5*Softclip)^(Softclip/100)).

5.5 Clip white to 225

6.0 Computing a Look-up Table for Conversion to 8 bit Video Data

6.1 Determine the breakpoint for softclip

Breakpoint = Refwhite - Softclip

The default value for Refwhite is 685, and the default value for softclip is 0 (softclip turned off).

6.2 Clamp back to 0

The default value for RefBlack is 95.

6.3 Compute a LUT between Refblack and Breakpoint

where,

Dispgamma = 1.0 for video data, and

Gain = 255 / (1 - 10 ^((Refblack-Refwhite) * 0.002/0.6) ^ (Dispgamma/1.7))

Offset = Gain - 255.

6.4 Compute softclip above Breakpoint

where,

Kneeoffset = 10 ^ ((Breakpoint-Refwhite) * 0.002/0.6) ^ (Dispgam/1.7) * Gain - offset

Kneegain = (255 - kneeoffset)/( (5*Softclip)^(Softclip/100))

6.5 Clip to 255

7.0 Computing a look-up table for converting 8 bit data to 10 bit film data

OUT = 685 + log10 {(( IN + Offset)/Gain) ^ (1.7/Dispgamma)} / (0.002/0.6)

where

• Offset and Gain are calculated as in section 5.3, using a Refwhite = 685 and Refblack = 95.

This look-up table will create a normal negative density range that is a reasonable match for the displayed image when printed at standard printer lights.

It is not necessary to use the inverse LUT that was used to convert from 10 bits to 8 bits. Rather, it is better to use a standard LUT that reproduces tha displayed contrast range and highlight compression. In other words, "what you see on the monitor is what you get on film", within the limits of an uncalibrated display.

8.0 Limitations of working with 8 bit data

8.1 Limiting the Dynamic Range

8.2 Limiting the Artistic Flexibility

With an 8 bit representation, it is necessary to make some color correction decisions at the time of scanning. The scanner operator will decide which part of the scene to translate to 8 bits, cutting off the extended highlights or shadows which he decides are unimportant.

8.3 Introducing Contouring Artifacts

For 8 bit video images (that have already had a 1.7 gamma correction applied), contouring is unlikely. However, for 8 bit linear images, contouring may be visible in the blacks when displayed through the gamma 1.7 look-up table.

Contouring is the most visible in gradually shaded uniform areas, like the natural darkening of the sky on the horizon.

Appendix A. Lookup Table for Converting 10 bit Density to 8 bit Linear (for Display on a Graphics Display Monitor with Gamma = 1.7)