I have a confession to make. I walked around for years as a working colorist without a full understanding what people meant when they used the term “rolloff.” Generally, I knew that it was something that happened in shadows and highlights, and sometimes it was a bad thing. But when the conversation would come up, I would just nod my head and hope no one asked me any direct questions.
Maybe I’m on my own, but I have a hunch that a lot of us don’t understand exactly what rolloff is. I want to remedy that today. I’m going to give you a simple and concise definition of what rolloff is and why it matters to colorists.
Visualizing rolloff
We’re not going to be doing much hands-on color grading today. Instead, I’m only going to use DaVinci Resolve as a visual aid to give us a clear understanding of what rolloff is and the role it plays in color grading.
To start, I’m going to pull up my scopes so I can visualize what we’re looking at. The exposure chart DCTL that I’m using is a free download that you can grab by clicking here.
I’m using this DCTL to render a very simple virtual scene that has a dynamic range of thirteen stops. Middle gray is in the middle, and there are six stops below middle gray, six stops above. But there’s already a problem, isn’t there? The problem is there is no visual difference in our upper white patches. They’re all identical in value. Why is that happening? What’s going on?
This is a very common problem in color grading and image mastering in general. You see, there’s a difference between the dynamic range of an image source, or a camera, and the dynamic range of a display. These two don’t always agree. In the example above, our display has less dynamic range than our source camera.
That’s why our scopes have a nice, gentle, slowly-climbing curve that just crashes right through the ceiling. After that, it becomes a flat white line. This is because our Waveform representation cannot discern any difference between the white patches on our exposure chart.
The challenge
Our challenge with rolloff is that we want to preserve the gentle curve in the middle of our tone scale. That’s the spot where things like skin tone live. So, we generally want to leave that part of the tone scale alone to preserve the exposure relationship at the center of the image.
If we want to pull our highlight values back, we need to do something other than only grabbing the Gain wheel. Look at what happens when I pull the Gain wheel back.
We can now see the distinct patches of gray up in our highlight register, but that move has cost us a lot. Look at how far back we’ve had to move every single one of our patches to get our very brightest patch to reproduce on the screen. That’s a pretty gnarly compromise and one that we don’t want to make.
This is where rolloff comes in. The idea behind rolloff is that at a certain point in the tone scale, we need to slowly reduce the distance between our highlight patches so that we are reducing contrast in that area. But at the same time, we need to maintain the relationship between the tones in the middle of our curve to preserve things like skin tone.
Here’s what rolloff looks like in Resolve. I’ve used a Color Space Transform to do some luminance mapping. Now I can see the individual stops in our highlights that were obscured just a minute ago. If you look at the Scopes, you can see how I have rolled off those highlights.
Why is it called that?
The “S” shape you see above is where the term “rolloff” comes from. It’s when we “roll off” our highlights and shadows, or close the gaps between our exposure patches. If you look closely, you can see that the distance between our exposure patches gets smaller and smaller as we approach the top of our highlights. Likewise, we are losing contrast in the upper register of our tone scale.
But what we are gaining is the ability to discern those distinct patches of light gray. And that loss of contrast is only happening in the upper part of our tone scale. Our mid-tone range is almost completely unaffected by the changes we made in Luminance Mapping. That’s what rolloff means. Rolloff is a way of taking a large dynamic range and compressing it into a smaller dynamic range in such a way that we can see details without darkening or flattening out our entire image.
Creative rolloff
Okay, that covers the technical definition of rolloff. All we’re really trying to do with rolloff in its most basic mode is fit the camera or the source into what the display can do. It’s not that creative in nature. But rolloff can be used creatively. Let’s take a look at how that would work.
If you look at the image above, you can see that I’m already using the same Luminance Mapping scheme that I was using on our exposure chart. And by turning Luminance Mapping on, I’m doing a much better job of holding onto those crazy highlights in the door than by having no tone mapping at all.
Now, this is just a technical rolloff. But what if I wanted to shape the contrast in the highlights even further? There are lots of different ways to do it, but one great example would be to use my Kodak 2383 LUT. This is a free download that you can grab here. It’s a great look to work with especially if you are working color-managed like I do.
This is my 2383 LUT. This look will introduce a form of creative rolloff to our image. To get a better sense of how it works, let’s look at how this LUT affects our grayscale ramp.
You can see on the Scopes that our highlights are being rolled off. We’re flattening out the contrast and decreasing the distance between the patches in the upper section of our tone scale. If we go back to our main image, you can see that, although we are increasing the overall brightness in the area around the door, we’re getting a much flatter reproduction of the highlights. We can see that better if we bring the exposure down using Offset. See how highlights are flatter than what we started with?
Creating rolloff manually
We can even create a rolloff effect by hand if we wanted to. To do this, go to Custom Curves and add two control points, one to roll things off on the top end of the curve and another to keep everything in the middle nice and pinned down.
This is an extreme example of preferential rolloff, but let’s pretend that we want a creamy, flat matte reproduction in our highlights. If that’s what we want, this method is a great option.
We can do the same thing at the bottom of the curve. Again, set two control points and pull up the bottom of the curve. The key is not to touch the middle section of the tone scale. We want to keep the middle part of our curve the same as it was in our original image. But at the bottom of the curve, we want to flatten out the shadows and bring them closer together.
This is a dramatic change, but hopefully it illustrates a kind of creative rolloff you can do in Resolve. More importantly, I hope you have a better understanding of what exactly people mean when they say “rolloff.”
Rolloff and film
As a closing note, I want to point out that maintaining strong mid-scale contrast while flattening out the bottom and top of the image is something that film does really, really well. That’s one of the main visual characteristics people want to see when they say they want a filmic look. It’s that eye-catching mid-scale contrast paired with a retention of highlights and shadows as we approach the bottom or the top of our dynamic range.
That’s all for now! I hope this arms you with a clearer idea of what rolloff is and the different forms it can come in.
