What’s up with Final Cut’s Color Wheels?

NOTE: The information presented here has been superseded by the release of FCPX 10.4.1 in April 2018. With that release the color wheels model has been changed. Please read the linked blog post for updated information.

Apple Final Cut Pro X 10.4 introduced new, advanced color correction tools to this editing application, including color wheels, curves, and hue vs. saturation curves. These are tools that users of other NLEs have enjoyed for some time – and, which were part of Final Cut Studio (FCP 7, Color). Like others, my first reaction was, “Super! They’ve added some nice advanced tools, which will improve the use of FCPX for higher-end users.” But, as I started to primarily use the Color Wheels with real correction work, I quickly realized that something wasn’t quite right in how they operated. Or at least, they didn’t work in a way that we’ve come to understand.

In trying to figure it out, I reached out to other industry pros and developers for their thoughts. Naturally this led to some spirited discussions at forums like those at Creative COW. However, other editors have noticed the same problems, so you can also find threads in the Facebook FCPX group and at FCP.co. It is certainly easy to characterize this as just another internet kerfuffle, surrounding Apple’s “think different” approaches to FCPX. But those arguments fall flat when you actually try to use the tools as intended.

The FCPX Color Wheels panel includes four wheels – Master, Shadows, Midtones, and Highlights. The puck in the center of each wheel is a hue offset control to push hues in the direction that you move the puck. The slider to the right of the wheel controls the brightness of that range. The left slider controls the saturation. One of the main issues is that when you adjust luminance using one of these controls, the affected range is too broad. Specifically, in the case of the Midtones control, as you adjust the luminance slider up or down, you are affecting most of the image and not just the midrange levels. This is not the way this type of control normally works in other tools, and in fact, it’s not how FCPX’s Color Board controls work either.

“What’s the big deal?” you might ask. Fair enough. I see two operational issues. The first is that to properly grade the image using the Color Wheels, you end up having to go back-and-forth a lot between wheels, to counteract the changes made by one control with another. The second is that using the Midtones slider tends to drive highlights above 100 IRE, where they will be clipped if any broadcast limiting is used. This doesn’t happen with other color tools, notably Apple’s own Color Board.

A lot of the discussion focuses on luma levels and specifically the Midtones slider, since it’s easy to see the issue there. However, other controls are also affected, but that’s too much to dissect in a single post. Throughout this post, be sure to click on the images to see the full view. I have presented various samples against each other and you will only get the full understanding if you open the thumbnail (which is small but also cropped) to the full image. I have compared the effect using five different tools – the Color Board, the Color Wheels, a color corrector plug-in that I built as a Motion template using Motion effects, Rubber Monkey Software FilmConvert (the wheels portion only), and finally, the Adobe Lumetri controls in Premiere Pro.

I am using three different test images – a black-to-white ramp, a test pattern, and a demo video image. The ramp without correction will appear as a diagonal line (0-100 IRE) on the scope, which makes it easy to analyze what’s happening. The video image has definite shadow and highlight areas, which lets us see how these controls work in the real world. For example, if you want to brighten the area of the shot where the man is in the shadows, but don’t want to make the highlights any brighter, this would normally be done using a Midtones control. Be aware that these various tools certainly aren’t calibrated the same way and some have a greater range of control than others. The weakest of these is FilmConvert’s wheels, since this plug-in has additional level controls in other parts of its interface.

Color science models

In the various forum threads, the argument is made that Apple is simply using a different color science method or a different weighing of some existing models. That’s certainly possible, since not all color correctors are built the same way. The most common approaches are Lift/Gamma/Gain and Shadows/Mids/Highlights. Be careful with naming. Just because something uses the terminology of Shadows, Midtones, and Highlights, does not mean that it also uses the SMH color science model. Many tools use the Lift/Gamma/Gain model, but in fact, call the controls shadows (Lift), mids (Gamma), and highlights (Gain). Another term you may run across is Set-up in some correction tools. This is typically used for control of shadows (equal to Lift), but can also function is an offset control that raises the level of the entire image. Avid Symphony employs this solution. Finally, both Symphony and Adobe SpeedGrade use what has been dubbed a 12-way color corrector. Each range is further subdivided into its own subset of shadows, mids, and highlights controls.

An LGG model provides broad control of shadows and highlights, with the midtones control working like a curve that covers the whole range, but with the largest effect in the middle. An SMH model normally divides the levels into three distinct, precisely overlapping ranges. This is much like a three-band audio equalizing filter. A number of the color correctors add a luma range control, which gives the user the ability to change how much of the image a specific range will affect. In other words, how broad is the control of the shadows, mids, or highlights control? This is like a Q control in an audio equalizer, where you change the shape of the envelope at a certain frequency.

Red Giant’s Magic Bullet Looks offers both color correction models with two different tools – the 4-way color corrector (SMH) and the Colorista color corrector (LGG). When you adjust the midrange control of their 4-way, the result is a graceful S-shaped curve to the levels on the waveform.

To study the effect of an LGG-based corrector, test the ramp. The shadows control (Lift) will raise or lower the dark areas of the image without changing the absolute highlights. The diagonal line of the ramp on the waveform essentially pivots, hinged at the 100 IRE point. Conversely, change the highlights control (Gain) pivots the line pinned to 0 IRE (at black). When you adjust the midtones control (Gamma), you create a curve to the line, which stays pinned at 0 and 100 IRE at either end. In this way you are effectively “expanding” or “compressing” the levels in the middle portion of your image without changing the position of your black or white points.

How the various color correction tools react

Looking at the luma control for the Midtones, two things are clear. First, all of these tools are using the LGG color science model. It’s not clear what the Color Wheels are using, but it isn’t SMH, as there is no bulge or S-curve visible in the scope. Second, the Color Wheels quickly drive the image levels into clipping, while the other tools generally keep black and while levels in place. In essence, the Midtones control affects the image more like a master or offset control would, than a typical mids or Gamma control. Yet, clearly Apple’s Color Board controls adhere to the standard LGG model. The concern, of course, is clipping. In the test image of the man walking on the village street, the sunlit building walls on the opposite side of the street will become overexposed and risk being clipped when the Color Wheels are used.

What about color? As a simple test, I next shifted the Midtones puck to the yellow. Bear in mind that the range of each of these controls is different, so you will see varying degrees of yellow intensity. Nevertheless, the way the control should work is that some pure black and white should be preserved at the top and bottom of the video levels. All of these tools maintain that, except for the Color Wheels. There, the entire image is yellow, effectively making the hue offset puck function more like a tint control.

One other issue to note, is that the Color Wheels offer an extraordinarily control range. The hue offset control RGB intensity values go from 0 (center of the wheel) to 1023. However, the puck icon can only go to the rim of the wheel, which it hits at about 200. With a mouse (or numerical entry), you can keep going well past the stop of the wheel icon – five times farther, in fact. The image not only becomes very yellow in this case, but you can easily lose the location of your control, since the GUI position in no longer relevant.

The working theory

The big question is why don’t the Color Wheels conform to established principles, when in fact, the Color Board controls do? Until there is some further clarification from Apple, one possible explanation is with HDR. FCPX 10.4 introduced High Dynamic Range (HDR) features. One of the various HDR standards is Rec. 2020 PQ. In that color space, the 0-100 IRE limitations of Rec. 709 are expanded to 0-10,000 nits. 0-100 nits is roughly the same brightness as we are used to with Rec. 709.

Looking at this image of the man walking along the street – where I’ve attempted to get a pleasing look with all of the tools – you’ll see that the Color Wheels in Rec. 709 don’t react correctly and will drive the highlights into a range to be clipped. However, in the bottom pane, which is the same image in Rec. 2020 PQ color space, the grade looks pretty normal. And, in practice, the Color Wheels controls work more or less the way I would have expected them to work. Yes, the same controls work differently in the different color spaces – properly in 2020 PQ and not in 709.

But why is that the case? I have no answer, but I do have a wild guess. Maybe, just maybe, the Color Wheels were designed for – or intended to only be used for – HDR work. Or maybe there’s conversion or recalibration of the controls that hasn’t taken place yet in this version. If the tool is only calibrated for HDR, then its range and weighing will be completely wrong for Rec. 709 video. If you increase the Midtones luma of the ramp in both Rec. 709 and Rec. 2020 PQ, you’ll see a similar curve. In fact, if you overlay a screen shot of each waveform, placing the full Rec. 709 scope image over the bottom portion of the Rec. 2020 PQ scale, you’ll notice that these sort of align up to about 100 IRE and nits. It’s as if one is simply a slice out of the other.

Regardless of why, this is something where I would hope Apple will provide a white paper or other demonstration of what the best practices will be for using this tool effectively. If it isn’t intentional, and actually is a mistake, then I presume a fix will be forthcoming. In either case, put in your feedback comments to Apple.

A word about HDR

Over the course of testing this tool and this theory, I’ve done a bit of testing with the HDR color spaces in FCPX. If you want to know more about HDR, I would encourage you to check out these contrary blog posts by Stu Maschwitz and Alexis Van Hurkman. I tend to side with Stu’s point-of-view and am not a big fan of HDR.

The way Apple has implemented these features in Final Cut Pro X 10.4 is to allow the user to set and override color spaces. If you set up your project to be Rec. 2020 PQ (and set preferences to “show HDR as raw values”), then the viewer and a/v output (direct from the Mac, not through a hardware i/o device) are effectively dimmed through the Mac’s color profile system. When you grade the image based on the 0-10,000 nits scale, you’ll end up seeing an image that looks pleasing and essentially the same as if you were working in Rec. 709. However – and I cannot over-emphasize this – you are not going to be able to produce an image that’s truly compatible with Dolby Vision and actually look correct as HDR, unless you have the correct AJA i/o hardware and a proper display. And by display, I mean a top-end Dolby, Canon, or Sony unit, costing tens of thousands of dollars.

As I understand the PQ specs, the bulk of the higher range is for the highlights that are normally constrained or clipped in our current video systems. However, that 10,000 nits scale is weighed, so that about 50% of the image value is in the first 100 nits, making it of comparable brightness to the current 100 IRE. The rest of that range is for brighter information, like specular highlights. You don’t necessarily get more brightness in the shadow detail. Therefore, if you are grading a shot in FCPX in a 2020 PQ color space and you only have the computer display to go by, you’ll grade by eye as much as by scope. This means that to get a pleasing image, you will end up making the average appearance of the image brighter than it really should be. When this is viewed on a real HDR monitor, it will be painfully bright. Having a higher-nits computer display, like on the iMac Pro (up to 500 nits), won’t make much difference, unless maybe, you crank the display brightness to its maximum (ouch!).  “Mine goes the 11!”

Right now, HDR is the wild, wild west. If you are smart, you’ll realize that you don’t know what you don’t know. While it’s nice to have these new features in FCPX, they can be very dangerous in the wrong hands.

But that’s another matter. Right now, I just hope Apple (or one of the usual suspects, like Ripple Training, LumaForge, or Larry Jordan) will come out with more elaboration on the Color Wheels.

©2018 Oliver Peters

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