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Topic: DCI image container size

Antti Nayha
Master Film Handler

Posts: 268
From: Helsinki, Finland
Registered: Oct 2008

posted 08-01-2017 06:47 AM

I've recently encountered a case where an unusual container size (such as 2880x2160, sometimes used for 4:3 content) prevents DCP playback on certain equipment. That got me wondering what the current specs say about this.

Now, I have previously believed that such container resolutions are indeed allowed. Up to version 1.1, the DCI specification said at section 3.2.1.2:

quote:
The number of active pixels (e.g., the pixels of the image structure that are intended to be displayed) shall extend to the maximum in either the horizontal or vertical direction of the defined level of operation as shown in Table 1. For example, a 4K image file with a 2.39:1 aspect ratio would require an image pixel array of 4096x1716, therefore filling the horizontal resolution of the 4K container.

Furthermore, section 3.2.1.8 has a table listing 4096x1716 and 3996x2160 (among others) only as "example aspect ratios". This seems to imply that any container size is OK as long as EITHER:
1) the horizontal resolution is either 4096 or 2048, OR
2) the vertical resolution is either 2160 or 1080.

Now, the current DCI spec omits the above details completely and simply points to SMPTE at section 3.2.1:

quote:
The SMPTE published standard "SMPTE 428-1: D-Cinema Distribution Master - Image Characteristics" shall be utilized.

Since I'm sure that some of you fine colleagues here have access to the SMPTE spec: what exactly does it say about allowed container sizes?

(Not that I would personally create DCP's with a container size other than 2.39:1, 1.85:1 or full container... it's just that I need to know if I can truthfully blame the equipment for not accepting such DCP's or not.)

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Carsten Kurz
Film God

Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009

posted 08-01-2017 07:34 AM

The allowed DCP container sizes have always been only flat, scope, and full, both in their 2k and 4k variants. All other formats need to be letter boxed or pillarboxed (padded with black) to accomplish this. This is backed up by SMPTE documents. SMPTE ST 429-2:2013 lists only the mentioned container dimensions, and demands padding/letterboxing/pillarboxing. This also means that the common 'ad' 1080p container at 1920/1080 is not allowed per spec. It should be padded to 1998*1080.
ST 428-1-2006 is more open, but is 7 years older, AND, is referring to the DCDM:
'The number of pixels shall extend to the maximum in either one or both of the horizontal and vertical directions of the defined operational level.'
This is the same passage from earlier DCI spec versions, and that passage from the DCI spec you quoted also deals with the DCDM (TIFF) master. Yes, I admit, there is still some ambiguity left, but then, SMPTE ST 429-2:2013 simply is a later revision.

Some DCP authoring solutions either did not check this, and some allowed different container types based on earlier assumptions that 'every container is allowed, as long as either dimension fills one of the full container dimensions' - e.g. 1920*1080, or 2048*1024. However, these were assumptions based on equipment capabilities/realities, not standards (or, if at all, based on ST 428-1-2006).

Barco's Alchemy actually has issues with non-standard containers. So far, this is the only machine I know of with that problem (and there are workarounds), but, this also brought me to rethink DCP-o-matics container choices.

I recently agreed with Carl Hetherington to remove all non-standard container formats from DCP-o-matic (another main intention was also to make the 'scale to' and 'container type' choices better specifiable, and thus use different names/labels). As such, the recent test versions only allow flat, scope, full, and 2k vs. 4k for container types. As previously, DCP-o-matic will automatically pad other aspect ratio footage with black (or crop, it you want to). Usually it hurts me if software updates at some point offers less options than available before, but in this case, I think it is right to do this. If we want to encourage the use of open source software, it shouldn't allow to do things that are against standard definitions. There is one little drawback of this move when it comes to use the new export function (non-standard ar footage will be exported with letterboxing/pillarboxing as well), but it seems this will need to be dealt with separately.

- Carsten

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Mattias Mattsson
Film Handler

Posts: 90
From: Göteborg, Sweden
Registered: May 2007

posted 08-03-2017 10:58 AM

quote: Carsten Kurz
The allowed DCP container sizes have always been only flat, scope, and full, both in their 2k and 4k variants. All other formats need to be letter boxed or pillarboxed (padded with black) to accomplish this.

The first version of 429-2 was published in 2009 (and 428-1 in 2006), whereas DCI 1.0 was published in 2005. As Antti notes above, the DCI have gradually removed sections in the spec that are instead stipulated by SMPTE documents.

So I don't really agree with the statement that Flat, Scope and Full have "always" been the only allowed DCI container sizes. Certainly not before 2006.

I agree with the conclusion though, that no other container should be used for new DCP:s. But Postels law apply; "Be conservative in what you do, be liberal in what you accept from others."

I have personally experienced an IMS2000 not being able to play a DCP in a 1800x1080 container. (Played, but distorted picture)

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 08-03-2017 11:49 AM

That may not be the fault of the server. I suspect, if you set up an appropriate input file in the projector that the image would have been correct. However, if you have a 1998x1080 input file (flat) and you hit it with a 1800x1080 signal, it might try to expand the 1800 to 1998 (in the projector, not the IMS).

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Mattias Mattsson
Film Handler

Posts: 90
From: Göteborg, Sweden
Registered: May 2007

posted 08-03-2017 12:02 PM

Hmm.. that's an interesting theory and I didn't try to match the INPUT file with the container in this case.

But I have not encountered distorted picture in other cases where the projector INPUT (PCF "active area") does not match the container of the DCP played out from the server. E.g. when playing a 1998x1080 container with a SCOPE Input file (or vice versa) or when playing tripple flash 3D with a squeezed Input file on series-1 projector formatters lacking the required bandwidth.

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Carsten Kurz
Film God

Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009

posted 08-03-2017 05:12 PM

Everything that needs dedicated input files/macros to accommodate custom container types should not be used. The industry has been moving and is further moving towards a projecionist-less booth. DCPs thus need to follow very specific constraints.

- Carsten

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