DCI and OTHER Film Formats

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DCI and OTHER Film FormatsBy Peter R Swinson (November 2005)

This document attempts to explain some of the isssues facing compliance with the

DCI specification for producing Digital Cinema Distribution Masters (DCDM) when

the source material is of an aspect ratio different to the two 2K and 4K DCDM

containers.

Background

Early Digital Projection of varying Aspect Ratios.

Digital Cinema Projectors comprise image display devices of a fixed aspect ratio.

Early Digital Cinema projectors utilised non linear (Anamorphic) optics as a means to

provide conversion from one aspect ratio to another, the projectors also generated

images using less than the full surface area of the image display devices to provideother aspect ratios, and, on occasions, the combination of both.

Film Aspect Ratios

Since the inception of Film many aspect ratios have been used.

For Feature Films, the film image width has remained nominally constant at what is

known as the academy width and the aspect ratio has been determined by the height

of the image on the film. One additional aspect is obtained using the same academy

width but also using a 2:1 non-linear anamorphic lens to compress the horizontal

image 2:1 while filming. A complimentary expanding 2:1 anamorphic lens is used in

the cinema. This anamorphic usage ensures that the films full image height and width

is used to retain quality. This is known generally as Cinemascope with an aspect ratioof 2.39:1. For those who carry out film area measurements against aspect ratios, this

number is arrived at by using a slightly greater frame height than regular 35mm film.

To accommodate any aspect ratio of film in the cinema, tradition has it, that where

possible, the desired film image height always fills the cinema screen height and the

aspect differences are accommodated by a combination of different magnification

projector lenses and curtain masking horizontally.

Such combinations ensure the maximum use of film resolution onto the screen.

Regardless of the film shoot and displayed aspect ratio, it is common for the actual

film image to always be shot and printed full film frame height; even for 1:85:1

projection. The parts of the image not designed to be shown are masked in the film

projector. See Figure 4. This is a dilemma for archivists of the future as we shall see.

DCI Aspect Ratio recommendations

The DCI DCDM recommendation offers two native aspect ratios 1.85:1 and 2.39:1.

This paper raises the issues of how to accommodate other aspect ratios, both film and

TV, into these two standardised DCDM ratios, especially when considering creating

Digital Source Masters (DSM).


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Some basics of the DCI standard.

1) Single Projector lenses. No non linear or variable magnification lenses.

2) DCI does not concern itself with DSM standards.

3) DCI specifies two projection resolutions, 2K and 4K.

4) DCIs DCDM container provides for 1.85:1 and 2.39:14) DCI members suggest that other aspect ratios be accommodated by reducing

displayed width. (EDCF DCI meeting IBC 2005).

Concerns

1) Is the industry going to produce (DSM)s with different base resolutions to DCDMs.

2) DSMs influence the Digital Intermediate (DI) business.

3) If mastering for DCDM from non DCDM projection aspects, how are they best

fitted to the DCDM standard container.

4) While unsqueezed anamorphic camera aspect is 2:39, the projected ratio is 2.35

to avoid splices showing on cut material.5) What quality issues are there regarding 3 above.

6) Can we agree a standard for ALL existing and potential future formats. (For

example; Hollywood creatives, at the inception of Digital Cinema desired a 2:1

ratio, could this be a new standard with Digital capture!)

Common Film & TV Aspect Ratios

Film

(Camera apertures tend to be larger than projection apertures, this documentrefers generally to Film Projection Apertures except for anamorphic)

I have not included 8mm, S8mm, and 65/70mm in this document.

S35mm/35mm

1.33:1 Full aperture (S35mm). Not theatrical prints but common as DI master

1.37:1 Academy Aperture SMPTE 195 Style C

1.66:1 Widescreen Aperture SMPTE 195 Style B Used Outside USA

1:75:1 Widescreen Aperture SMPTE 195 Style B (Little used)

1.85:1 Widescreen Aperture SMPTE 195 Style B USA Hollywood format

2.39:1 Anamorphic Aperture SMPTE 195 Style A USA et al, Cinemascope

S16mm

1.66:1 Camera aperture SMPTE 201M. Does not exist as prints.16mm

1.33:1 Regular Aperture SMPTE 233

TV

*(The BBCs 1.55:1 aspect ratio is included to indicate that even in TV there are non

conventional ratios! However I have not included it in terms of image conversion

calculations )

SDTV 1.33:1 (4:3) Regular TV aspect ratio

*1.55:1 (14:9) A BBC standard. A compromise for widescreen on 4:3 TV.

1.78:1 (16:9) Widescreen TV aspect ratio, adopted from proposed HD ratio.HDTV 1.78:1 (16:9)


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Understanding Film Digital Scanning

Where do the terms 2K and 4K originated in terms of converting film images to

Digital Files for D.I.

In the 1990s it became practical to scan motion picture film at its native resolution.Not the print, but the original camera negative; or at least a 2nd generation Inter-

Positive. Early adopters such as Kodaks Cineon and Quantels Domino systems set

some common parameters.

Kodak identified that original 35mm camera negative could capture resolutions with

detail smaller than 12um in the films emulsion.. To capture this detail would require

sampling the film with pixel sizes as small as 6um.

These sampling sizes have translated into a 35mm academy film width sampling of

3656 pixels and for Super35mm sampling width 4096 pixels. This is where 4K is

derived from. D.I. scanning these days is scanned at 4K, or 2K where the relative

horizontal pixel counts are 1828 and 2048 pixels, more than adequate for 2nd

generation Inter-Positives but, in the authors opinion, inadequate for original camera

negative where the detail is high.

In practice a 4K scan of 4096 horizontal samples and a 2K of 2048 horizontal

samples, allows slight over-scan of the Super 35mm aperture area.

The vertical scan depends on the composed aspect ratio. However it is common to

scan the full frame height even of widescreen material as this allows some

repositioning choices in D.I. environments.

Anamorphic cinemascope needs special consideration. The film material largelydefines the resolution limit, not the image aspect ratio. Therefore where the resultant

film image is un-squeezed 2:1 for projection the horizontal resolution is spread over

twice the normal area, thereby halving the resolution per unit screen area when

compared to non-anamorphic projection. There is however some benefit from

increasing the horizontal scan sampling to 4K for anamorphic shot material as this

when unsqueezed provides about 2K samples over the same area as unsqueezed

material. Anamorphic projection of cinemascope is normally vertically cropped to

2.35:1, but for digitisation purposes a full 2.39:1 vertical film resolution can be

maintained so long as the source film has not been spliced. (There is no unused

interframe bar using this format).

How will it all fit together ?

Having defined the sources of moving images, how they are typically scanned and

knowing the DCDM aspect ratios, how can we best generate DCDMs and how will

this influence the creation of source DSMs?

If we follow the DCIs recommendation then we simply ensure that all aspects are

made to fit the DCDM container. However to make DSMs to these values would in

some instances diminish the DSM quality for archive purposes.


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Figures 1, 2, 3 and 4 show 35mm Film and TV Format aspect ratios and how each

will best fit the DCDM container. Only the 2K container has been considered as has

only 2K scanning of 35mm film. The 4K scenario is a simple ratio to both the

scanning and container, double everything, use pixel numbers twice the values shown.

For best quality DSMs the pixel resolutions shown on the source images in figures 1,

2 and 3 should be considered, or indeed it could be argued that the DSMs shouldalways, from camera negative, be 4K scanned; even if the DCDM version is only at a

nominal 2K. Moreover for wide-screen formats, as Figure 5 shows, it should be

considered for Digital Archive scans to scan the entire film frame rather than just the

composed wide-screen area.

As can be seen from the figures the quality losses when matching to the DCDM

container vary dependent upon the source material aspect ratio. It can be argued that

for regular 35mm the 1.6% loss for 1.66:1 widescreen is minor. However the 19%

loss of quality for Academy and nearly 30% loss for Super35mm full aperture is

considerable.

There is now a new tendency to shoot Super35mm in widescreen aspects. As shown

in figure 2, these will also suffer varying degrees of losses if a DCDM is regarded

also as the archive master.

Cinemascope losses, as shown in figure 3, are interesting. If the source is

Cinemascope letterboxed on the film then no losses will occur. However if the image

is Anamorphic Cinemascope, a 44% loss in vertical resolution is likely !

For TV, both HD and SD, as as shown in figure 4, there are no losses using the

DCDM container. It will be interesting to see whether SD TV is actually uprezzed

by 2:1 to fit the container, or whether a smaller, less memory hungry container is

devised for SD TV; if there is indeed a market for SD in Digital Cinema. I guess there

will be, unless the upcoming generation can all afford HD cameras.

It can be seen that all formats/aspect ratios, except Cinemascope, will fit into the

DCDM 1.85:1 container. Bearing in mind the losses indicated, it will be wise to not

generate DSMs that directly equate in terms of pixels to DCDMs, in all instances.

Also archivists should still consider whole frame scans for DSMs of all widescreen

formats, else the additional height information may be lost forever!

It could be argued that if the DCDM 4K standard is adopted then the losses are

avoided, however applying the 2:1 improvement ratio still leaves some scenarios with

less than the 4K resolution, that would be desirable for DSMs.

Another area of interest is the retention of image graduation, often mistakenly

confused with dynamic range. While any bit depth can provide any dynamic range

only large bit depth can encompass subtle shading changes within the overall range.

The Digital Intermediate industry has today settled on 10 log RGB as providing a

range similar to original camera negative film. This, many agree, is similar to 14 to 16

bit linear. The DCDM specification seems to sit at 12 bit XYZ color space with a


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gammad transfer characteristic. Whether this can encompass the entire DSM

dynanmic range I leave to others to establish!

I have not include 16mm, S16mm, 65/70mm or 720P HD in this document.

Peter Swinson. Peter Swinson Associates Ltd

SourcesDCI Digital Cinema System Specification V1

The Focal Encyclopedia of Film & Television Techniques (1973 edition)

SMPTE 96M (Motion Picture scanned areas for Television)

SMPTE 195 (Motion Picture 35mm Projectable Print aperture Areas) SMPTE 233

(Motion Picture 16mm Projectable Print aperture Areas) MPTE 201M (Motion

Picture S16mm Camera aperture Areas)

SMPTE RP187 (TV Aspect Ratios Defines 4:3 and 16:9 TV Aspect Ratios)


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DCI and OTHER Film Formats