Re-Wrapping a 4K DCP to 2K?

[Jim Cassedy]

Is it possible to re-wrap a 4k (unencrypted) DCP to 2K on DCP-O-MATIC™️?
(Please don't ask why- - I just need to know if it can be done) THANKS!
jc

[Marcel Birgelen]

It is.

Don't ask me how, but it can be done.


(I think there is even a re-encode option for it.)

[Ioannis Syrogiannis]

By "re-wrap", I imagine you are talking, not about re-encoding, but re-muxing. Having DCP-o-matic discard the extra JPEG2000 info and keep the rest intact, without any encoding load.
I would be surprised if there was such an option, but I would welcome it. Haven't seen any kind of checkbox for such a task, though.

[Jim Cassedy]

By "re-wrap", I imagine you are talking, not about re-encoding, but re-muxing. Having DCP-o-matic discard the extra JPEG2000 info and keep the rest intact, without any encoding load.
I would be surprised if there was such an option, but I would welcome it. Haven't seen any kind of checkbox for such a task, though.

Yes- Basically, I'm trying to turn a 4K DCP into a 2K DCP.

[Marco Giustini]

I know you said don't ask - I just wanted to make sure you are aware that any DCI server is perfectly capable of running 4K content, the decoder will only take the 2K part of it if 4K is not supported. The reason I am saying this is because I have seen main features being distributed in 2K and 4K sometimes and - AFAIK - for absolutely no reason.

[Ioannis Syrogiannis]

I just did a test, starting to encode a DCP with all settings same, except the resolution and (of course) the bandwidth, that I wouldn't know where to set.
The "Re-encode JPEG2000 data from input" box was not checked.
I was getting same frames per second I get when I encode from a 4K source ~4fps (I know, fast, right?).

So, unless there is something in the advanced settings I am not aware of, not-encoding is not an option.

Edit:
@Marco
The best reason I can thing about is cost of storage and logistics.

[Marco Giustini]

@Ioannis,

Logistics works better if you're shipping ONE version for both 2K and 4K systems!
Storage... I believe 4K is 25% larger than 2K (same bandwidth). So not a huge deal in 2023 but mastering, QC'ing and distributing twice as many version is - IMHO!

My guts feeling says: ignorance

[Carsten Kurz]

4k to 2k in DCP-o-matic will always perform a full re-encode incl. downscaling and recompression. Just the color-transform will be left out.

Don't see any reason for that operation - unless, it could be a 4k DCP at higher frame rates that are not supported by most servers/mediablocks or projector. In 2k, it would be more likely that they play at e.g. 50 or 60fps.

[Randy Stankey]

...I believe 4K is 25% larger than 2K...

For spacial resolution, 4K is FOUR TIMES larger than 2K. It's twice as wide and twice as tall.
2K is one QUARTER the size of 4K.

Temporal space varies by direct proportion. (e.g. 24 fps takes 0.8 times the space of 30 fps and 1.25 times the other way around.)

I forget how changing color space affects size.

[Marcel Birgelen]

We're talking about complex compressed formats here, so a 4-fold increase in resolution will not yield a strict 4-fold increase in file size. A simple frame in 4K can be less storage-intensive than a complex frame in 2K.

Color space affects every pixel, so in an uncompressed scenario, where all "color components" are mapped 1:1 to every pixel, it should be a linear increase. But that's not how we usually handle color in compressed formats, especially not moving pictures.

In an uncompressed, serial world, everything is linear and simple, in reality, everything is complicated. :P

[Marco Giustini]

As Marcel says, I was referring to the JPEG2000 compression. I am not 100% sure but I believe 4K was supposed to be 25% larger than 2K.

Uncompressed, sure, it's 4 times more pixels

[Randy Stankey]

Okay, teach me...

The way I understand, you calculate spatial differences then you calculate for compression. So, if you double or halve the frame size, you multiply or divide by four. Afterward, depending on what method you use to compress the individual frames, you decrease the file size, accordingly.

The same goes for temporal calculations. Going from one frame rate to another, you multiply or divide accordingly, then, depending on the way you apply I-frames, P-frames or B-frames (use your encoder settings) you shrink (or don't shrink) the file size even more.

Color settings, I don't understand as much. I rarely set color space manually. I usually use whatever color profile is appropriate for my intended display. I would decide on my color space before applying spatial and temporal sizes/compressions. Set it and forget it. Usually, color space is mandated for me, anyhow. (e.g. Cinema, Television, Computer, Print, etc. require you to use a particular setting for each.)

As I understand, all frames are I-frames in a DCP. Partial frames and Bi-directional frames aren't used. Essentially, time compression doesn't matter, much in DCP.

So, assuming color is set in stone, changing your frame size is the first factor in final file size then changing image compression comes next, followed by frame compression.

Is this right?

[Marco Giustini]

I'm not here to teach anything to anybody.

The figure I have in mind might be mistaken.
That said, the compression is chosen by the mastering facility. If you keep the bitrate identical, then the file is going to be identical regardless whether it's 2K or 4K. If you change the bitrate, I guess it'll be a subjective decision based on how the content look on a screen.

DCP-O-Matic has just a bitrate setting for encoding, as does Resolve. I did a test using H265 in Resolve, setting the quality to "best" and 4K is roughly twice as large as 2K.

As Marcel mentioned, when it comes to compressed files, things are not linear. As an extreme example let's take a completely black frame. That's going to be very easy to compress and the resulting file size is going to be very small regardless of the resolution.

[Ioannis Syrogiannis]

@Ioannis,

Logistics works better if you're shipping ONE version for both 2K and 4K systems!
Storage... I believe 4K is 25% larger than 2K (same bandwidth). So not a huge deal in 2023 but mastering, QC'ing and distributing twice as many version is - IMHO!

My guts feeling says: ignorance

I don't disagree, Marco.
Yet, I am under the impression you are thinking of a distribution company that is launching a title to be sent to a number of cinemas and may have more or less versions of a title.
I am not. I mostly think of an individual, preparing and managing (shipping or hosting) a single version.
Or, that manages a single version in-house (a festival, for instance), where intense traffic, overall storage and ingest time (times X) may make a difference.
Those two general cases would agree with the choice of DCP-o-matic, the intention not to harm (re-encode) the content and a feeling that it's not an endeavor to shout about.
Ignorance may equally well be.

That said, I repeat that I would welcome a feature of removing the extra layer of data the 4K adds to a DCP without re-encoding (so, way faster) and without deteriorating the 2K screening quality.
I do get into the position to need to transfer only once a DCP somewhere for a screening, and getting rid of even that 25% may save the day, when transfer speed (upload and/or download) is either restricted by ISPs, software, services of choice etc. (and it's -once again- a last minute notice .)

About color space and size, it doesn't matter between DCPs, but between video in general, it's mostly the color resolution.

[Jim Cassedy]

[edited] 4K is FOUR TIMES larger than 2K.

So, if I had $4k in one pocket, and $2k in my other pocket, I'd actually have $18,000 total?

Q > . . and if I did actually find $18,000 in my pockets, do you know what I'd really have?
A > - - - Somebody else's pants!