So, director Rian Johnson has a critique of Red “hype” that has been attracting some attention in the Red community recently. The first version of it was rather rant-like, but there’s now a new version up, which attempts to be more of a straightforward technical critique. Unfortunately, it still contains quite a few technical inaccuracies. I’m going to go through at hit on several of those here, but it might be a good idea to read the full post for context.

I’m breaking this into two posts because the first one was getting rather long. Second post will probably be up tomorrow.

Johnson argues that the Red One is over-optimized for resolution at the expense of other criteria, like dynamic range. As part of this argument he makes the case that 2K is enough resolution for a feature. This is largely true, but there are several mistakes in the evidence Johnson presents for this. For instance, Johnson uses the fact that Kodak defined 2K as a standard resolution for DPX/Cineon files as evidence that Kodak considers 2K sufficient. He says:

Note that Kodak could have made any specs they felt were required here to fully achieve the quality of 35mm film: for example, they could have prescribed a file that has more resolution and less color information. Here’s one they could have used: 4K pixels, 3 channels-per-pixel, and 8-bits per channel — that would have actually been the exact same file size as the spec they did choose, and they even could have said the word, “4K.”

There are two problems with this. First, 8-bit 4K frames aren’t the same size as 10-bit 2K frames. If we assume a 16×9 frame, 10 bit-per-channel 2K is 30 bits per pixel * 2,359,296 pixels, or 8.44 MB/frame, while 8-bit 4K would be 24 bits per pixel * 9,437,184 pixels, or 27 MB/frame, over three times the size. Oops.

Second, Kodak didn’t define 2K as the standard resolution for digital film scans. They defined it a standard resolution, alongside, you guessed it, 4K. It’s pretty widely acknowledged that a 35mm original camera negative captures more than 2K worth of resolution. Certainly 2K acquisition and workflow can provide a perfectly acceptable level of quality. It’s more resolution than ends up making it into a theatrical release print. But Kodak didn’t declare that 2K is the end of the road, and nothing beyond it is worthwhile. It’s true that most features are currently finished at 2K rather than 4K, but in large part this is simply because it’s good enough and technically much less of a challenge… not because nobody should ever want anything better.

The resolution discussion continues, with an argument that because the Genesis and F23 capture 2K (or something close enough to it), they capture enough resolution that resolution doesn’t matter, and therefore Red’s higher resolution is not a meaningful advantage.

There a couple problems with this argument as well. The first is that it assumes (at least in the case of the F23) that three 1920×1080 sensors capture full 1920×1080 resolution. This just isn’t so. All general-purpose cameras have optical low-pass filters to avoid aliasing, which results in a loss of resolution. So The F23 necessarily resolves something less than 1920×1080. Johnson is right, of course, that the Red One doesn’t actually resolve 4096×2304, of course. Bayer-pattern sensors like the Red’s do resolve a fair bit less than their raw photosite counts might imply. But several tests have now shown the Red One does resolve about 3.2K, or about 1800 horizontal lines, which is over 60% more than the absolute maximum the F23 could possibly resolve.

This argument also seems to take it for granted that nobody would ever need more than 2K resolution even for an original camera negative (or digital equivalent). While it’s true that, as noted above, 2K is higher resolution than a release print, 4K projection is likely to be quite common in the years to come.

And consider: most still photo photographers shoot everything at the maximum resolution allowed by their cameras, even when their photos are going to end up being displayed at less than a megapixel on a web page. Few would be happy if you suddenly told them they couldn’t do this anymore. It gives them the flexibility to reframe later, to repurpose photos for higher resolution media later, to do things like masking on a large image before scaling down to the appropriate size for the deliverable. The only reason moving images aren’t regularly shot this way is because it’s technically difficult to build cameras that can do it, not because it wouldn’t be beneficial.

The discussion moves on to bit depth, noting that high-end HD cameras generally have 14-bit analog to digital converters (ADCs) and record 10-bit data (with a log curve applied, though Johnson doesn’t mention this). Moving on to the Red:

The Red [...] (still just talking about the image before it’s recorded), is so busy trying to outperform the other cameras in just the one area of superfluous resolution that it hasn’t got to the full cinema quality of the other requisite aspects. They’re using up all their photosites on resolution (so they can say 3.2K or 4K is a bigger number than 1.9K) and are truncating color information. The Red sensor only has one pixel per channel instead of three, which means it is 3-times color subsampled. The Red sensor samples at 12-bit (compared to 14-bit on the other cameras) for conversion to digital (in a seemingly unpublished bit-depth).

This seems to show a bit of confusion. Photosites are analog components. They don’t sample at a specific bit depth; bit depth is a function of digital encoding. A photosite simply generates an analog electrical signal that varies in strength according to how many photons strike that site. This analog electrical signal then goes into an ADC, and comes out as digital data with a specific bit depth. In the case of the Red One, the ADC outputs 12-bit linear data, which is recorded as… 12-bit linear data. Applying a log curve to image data puts more of your pixel luminance values “in the right place” (it distributes them across the tonal range of the image in a more useful way), so Red’s 12-bit linear doesn’t really provide an advantage over the 10-bit log of cameras like the F23. But the Red One isn’t really at a disadvantage here either. And keep in mind that the Red One is recording raw bayer data. My understanding (and I admit I’m not an expert on de-bayering algorithms) is that when generating an RGB image from a 12-bit bayer pattern, one can actually end up with a bit more than 12 bits of color data.

So, does that initial 14-bit ADC conversion benefit cameras like the F23 and the Genesis in any substantial way? Given that 12 bit encoding allows the Red One to record everything from the sensor’s noise floor (the camera doesn’t artificially clip to black below a certain point like traditional video cameras) up through its sensor-clipped highlights without any noticeable banding in images (even when you push them around in post, within reason), it seems unlikely that a 14-bit ADC would make any noticeable difference.

This entry was posted on Thursday, September 25th, 2008 at 02:10 PM and is filed under Red. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.