Is 4K really the future?
By Chris Heinonen on
Judging from CES and CEDIA, companies seem to be convinced the 4K is the future for home video. There might not be any content yet, and we’ll need a new physical medium to support that much data most likely, but they are pushing towards it. Most films are mastered at 4K or higher resolutions, and so there will be a lot of content that could be ported to 4K very quickly and easily but is that really what we should be focusing on, or is this like the megapixel race in digital cameras?Just like with digital cameras, 4K is a number that consumers can easily understand and grasp onto. I know I was guilty about this a decade ago when friends were starting to buy some of the first affordable DSLR cameras on the market. I didn’t understand why they would buy a 6 megapixel camera that was so massive when I could buy a 12 megapixel camera that fit into my pocket. After all, more pixels are better, right? As anyone that has compared output from an SLR to a pocket camera can attest to, there is far more than a single number that makes up image quality.
Moving from 2K to 4K means having four times the resolution as before, which means to keep similar levels of detail we might need four times the storage space. That’s well past what Blu-ray can hold, and what we can stream in the USA or most countries, so we’d need to come up with a new medium to hold all of this extra data. What else could we do with this space instead of the extra resolution, and how would those improve picture quality?
If you’ve taken the ISF course, the most important element of a high quality image to them is dynamic range. Having a huge variation between white and black lets you get more shadow detail, more highlight detail, and have a far more realistic image than you can without it. Typically when you move up in megapixels but don’t increase the size of the chip or panel you are using, you lose dynamic range. On a DLP the mirrors will be smaller and reflect less light, and on an LCOS or LCD panel you can have signal bleed into adjacent pixels. If you were to put a 1080p plasma with poor dynamic range up against a 720p Pioneer Kuro with its much larger dynamic range, you’d almost certainly notice the increase in dynamic range before you noticed the loss of resolution.
Instead of using this extra storage space for more resolution, we could expand our bit-depth from 8-bits per pixel to 10 or 12-bits per pixel. This would allow us to have smoother gradients, more shadow and highlight detail, and allow for larger dynamic range than is currently possible with home video formats. Another improvement we could make would be with a larger colorspace than the current Rec 709 and HDTV standards. If you see a movie in the theater you can see colors that you don’t see at home since the DCI colorspace is much larger than the Rec 709 one.
I discussed using the Gretag Macbeth color checker chart to see how well a projector or display performed, but there are colors on that chart that you actually can’t see on your current home theater display. You can see these shades of blue and green in real life, but your display can’t produce them since it has a more limited color gamut than reality. Moving towards the DCI color gamut would let those colors be produced by your display and provide a more realistic image than is currently possible at home. Being able to produce these other colors and having a larger bit-depth to the image is one reason that a projected image at the theater has the potential to look better than anything you can do at home. Below you can see the DCI colorspace compared to the Rec 709 colorspace, which is the smaller triangle inside of the DCI space. Most of the difference is in reds and greens, but those are colors we can see in nature and can’t see on our displays.
Finally, we could also use less chroma subsampling than we currently do on Blu-ray to provide more color data for your display, as well as using more advanced compression formats than we currently do. Moving from 4:2:0 to 4:2:2 or 4:4:4 would provide 2-4 times as much color data as we have now, and also serve to eliminate some issues like chroma upsampling error on disc players. Since this loss of color data is less visible than a loss of black and white resolution, and it requires a lot of space to hold, this is unlikely to happen but using advanced compression formats would help with that possibility.
None of these are as sexy to people shopping in a store as 4K, and would take far more explaining by salespeople that often don’t understand it themselves. Many films that are shot digitally now are only shot at 2K resolution, but they are usually shot with a larger bit-depth and colorspace than our 2K displays can do, so we could still see our movies look better at home than the do already. The human eye is also only so sensitive to resolution that most people won’t see the full benefits of 4K unless they either buy a much larger display than they currently have, or sit much closer to the screen than feels comfortable to them.
4K isn’t a waste and does offer some benefits, but there are other things that could be done to offer larger benefits to home video, and if we are going to need a new physical media format for either one of them, I hope we can think outside the current box and take an approach that offers more benefits than just more pixels you might not see.