Saturday, April 07, 2007

Myth-busting : 4K vs 2K

I've be recently watching a few episodes of Mythbusters after my father said it was fun, and it is. So I figure that's what I been trying to do here, bust the myths like compression can't be used in post, or in this post measure the real difference between 4K and 2K aquisition; I just don't get to blow anything up.

This is not an blind image comparison test like I've done previously, after all we know 4K is better than 2K -- right? Instead I will investigate how much better 4K is than 2K under normal export scenarios. This is not so much a SI-2K vs Red One comparison, rather it is practical look at what happens when mixing camera sources or mixing shooting modes from one camera (all these bayer cameras tend to shot at a lower resolution when overcranked.) Also for anyone shooting with B4 or 16mm lens, you are only going to get 2K or maybe just 1080p, so it will be good to know what you are missing out on. For those who question my bias, yes we have licensed CineForm RAW compression (non-exclusively) to Silicon Imaging for their SI-2K camera, however CineForm is marketing our solution to all RAW shooters from Arri D20, Dalsa Origin and Red One users. We hope to benefit no matter what resolution you shot with.

Let's be real here, for today's delivery format will not have people finishing their productions at 4K (I doubt even 1% of 4K acquisitions will be finished at 4K 4:4:4), for those getting a filmout, they are typically done at 2K, even digital mastering for D-cinema is mostly 2K or 1080p for most E-Cinema and HD masters. So the 4K image is primarily going to buy you some flexibility for re-framing, the over-sampling will help clean up noisy sources, and ease pulling of keys, certainly a good thing. But assuming you have lit your project well and framed correctly in camera, how much difference is visible between shooting 4K vs 2k?

For this test I have a 4K sequence from a Dalsa Origin (4096x2048), I could do the same from any of the high resolution sources like Red One or Arri D20. To simulate the same sequence shot at 2K RAW took a little work. The original 4K RAW frame was compressed into a CineForm RAW AVI, this AVI was loaded into AfterEffects (the CineForm compression stage was needed as AfterEffect will not load the RAW DPX file from Dalsa, but as the compression is very light, this will not impact anything.) The AVI was placed into a 4K composite, in which I added a blur filter to simulate the effect of a optical low pass filter (OLPF,) which would be used on any decent bayer camera. Working out the amount of blur was tricky, more on that later. The 4K After Effects composite with the blur was then exported as a 2K DPX (RGB 4:4:4) sequence. As RAW bayer only has one chroma sample per pixel, not three like 4:4:4, this DPX sequence was then sub-sampled into a 2K RAW sequence (one third the size.) This new RAW sequence was encoded into a 2K CineForm RAW file, nearly a quarter the size of the original CineForm RAW 4K AVI (only 10MBytes/s.) The next step was to place both the original CineForm RAW 4K sequence and the derived 2K RAW sequence into a 2048x1024 project running Prospect 2K under Adobe Premiere. You will be able to see timeline playback and mixing of 4K and 2K in real-time at our NAB booth (SL7826) on mid-range PCs (any modern core dual Intel system.) I set the demosaicing quality to the best and then visually compared the look of 4K vs 2K. Oopss, something wasn't right.

The first thing I noticed was there was some color fringing in the 2K image, which you don't see on a real 2K shoot, yet the 4k look great scaled to 2k within Premiere. It turns out my guess for the blur level to simulate the OLPF was wrong -- too little and you get image aliasing, too much you get a blurry image. So a went back to composite and produced a range of sequences with different OLPF strengths, to get an image that wasn't overly blurred or aliased, just like a correctly configured camera.

The results of my test sequence is shown in these frames:


The CineForm RAW 4K exported as 2048x1024 (click on image to see a 1:1 scale)


The CineForm RAW 2K simultation exported as 2048x1024 (click on image to see a 1:1 scale)


There is a mild softening in the 2K RAW source, the difference is not massive, my guess is about a 10% reduction in something (frequency response maybe, but I wasn't directly measuring this.) However this 2K image is unsharpened, showing signs of the OLPF, so the next image adds a little sharpening, just like a regular camera (quick and dirty -- I didn't spend much time on this.)


The 2K RAW sharpened export as 2048x1024 is starting to look much more like to 4K downscale. (click on image to see a 1:1 scale)

As expected 4K still has an edge, but for practical workflows it is not going to greatly impact the results for a typical shoot. There will be no issues in mixing 4K and 2K sequences with your projects, your audience will not be able to tell. This test also shows how well a bayer "raw" image can maintain the look of a 4:4:4 image at the same resolution. 4K RAW downscaled to 2K gives a very good 4:4:4 source, and 2K RAW does a pretty good job of maintaining the 4:4:4 quality. For those who are still budgetting for 4K masters and 4K projection, make sure your audience sits in the front few rows. :)

5 comments:

Mike Hedge said...

great post see you at NAB

Rith said...

Thanks for sharing. See you at NAP

Anonymous said...

certainly a 4K image downconverted will only have the advantage of the original oversampling. The issues is more what happens when you convert the 2K to be projected at 4K?

David said...

Anon,

I'm confident it would still look great. I've seen HDCAM 1440x1080 3:1:1 through the Sony 4K project, and it looks awesome. If you are in the 0.1% of the market who will get a 4K distribution, 99% of your audience will not be able to tell the difference. Only a IMAX screening will benefit from a 4K master.

Kindari said...

Been working a lot in SD and HD, still learning alot...

It seems to me that the final 2KRAW to 2048 image is sharper in some areas than the 4KRAW to 2048. An example would be the sandy mud on the screen left actor's left elbow.

Am I not seeing this right?