Why are UHD Blu-rays Often Sourced from 2K Files? Ask the Editors

uhd blu-rays

Q: I’ve have been reading about the image quality of various UHD Blu-rays. The majority of releases seem to be upscaled from a 2K file with only a slightly noticeable increase in quality compared with standard Blu-ray. Most of the 4K transfers seem to come from older titles that were shot on film. Although many newer movies are shot at higher resolutions, it’s disappointing that a 2K file is used for the 4K disc. It seems like a waste not to show off the format.

If a movie was shot at 4K or higher but shown at 2K in its theatrical release, how difficult/time consuming/costly is it to go back and create a 4K file for the UHD Blu-ray? Why is a 2K file is used to begin with?

– Mike Brennan


A: I don’t know the specifics of each studio’s process, but I can provide some general insight. Older titles that were shot on film are scanned—that is, converted to digital—at 4K resolution because that’s relatively easy with modern equipment. You scan the film, tweak the color, and you’re done. (Of course, that’s an oversimplification, but it gives you the basic idea.)

By contrast, many modern movies are heavily laden with CGI (computer-generated images), which is almost always done at 2K resolution. Why? Because doing it at 4K is too expensive in terms of rendering time and storage. So, even if the live action is shot at 4K, it must be downscaled to 2K in order to seamlessly integrate with the CGI. That’s one reason why many theatrical releases are delivered to the exhibitor in a 2K DCP (digital-cinema package). Another reason is that many commercial cinemas still have 2K projectors. When it’s time to create the UHD Blu-ray, the 2K file is upscaled to 4K/UHD.

Like you, I would much prefer the source files for UHD Blu-rays to be native 4K. I suspect that will happen as computer-processing power continues to increase and the cost of storage continues to decrease, which will make it cost-effective to do CGI at 4K. But for now, we’re stuck with many UHD Blu-rays sourced from 2K files.

There are several movie-industry professionals on AVS Forum, and I invite them to provide more insight into this in the comments.

If you’ve got an AV question, please send it to AskTheEditors@01900888.com.

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