DCP-o-matic

Again - you spec a compression bit rate in DCP-o-matic. If it is set to 200MBit/s, your DCP will use THAT bitrate, no matter wether it's 2k or 4k. So both 2K and 4K DCP will have roughly the same size. Stay around 200MBit/s for both 2K and 4K DCPs, as that is the most compatible setting for ALL cinema projection systems currently installed.

If you have access to a DCP, it is easy to calculate the data rate of an existing DCP yourself. Take the size of the largest MXF file (if single reel), and divide by runtime in seconds. If it is a multi-reel DCP, you need to add up the individual video reel sizes first.

A 200GByte video MXF file with a runtime of 120min has a data rate of 27MByte/s, which equals around 220MBit/s (multiply MByte/s by 8)

200.000 Mbyte / 7200 s

Oh! I understand now. It's the same bitrate for 2K or 4K but that effectively means the 4K is 'less' resolution than than the 2K. It has to spread the 220Mbits/sec over a 4K resolution.

Again - if you specifiy a bitrate for the J2K compression, then that bitrate results directly into a specific file size, and that filesize is the same for 2k and 4k, if you set the birate the same for both encodings. 200Mbit per second is 200Mbit per second, no matter wether it's 2k or 4k. If for some reason you want a larger filesize for 4k, you have to set a higher bitrate. Bitrate is not a quality indicator, but a data rate.

Usually, these considerations are moot. J2K is a very efficient compression method. Just set your bitrate to something between 150 and 200 MBit/s and forget about it. The only reason to care about a higher bitrate would be higher frame rates, like 48 to 60fps.


It's not that the 4k version at the same bitrate has 'less resolution'. This is a complex transform coding which expresses the picture as a range of frequency components.