Advanced Curves Techniques Every Video Editor Should Know
Tweaking colors with Curves is really intuitive once you “get” how they work.
Take a look at the Beginners guide to curves if you need an introduction to Curves. Combined with the scope I use the most, the RGB Parade, it’s a super tool for quick and precise color adjustments. Granted, if you have a control surface with trackballs, you’ll work fast with the Color Wheels too–but if you’re limited to working with a mouse or a pen and tablet, the Curves will be a really fast alternative.
The most common set of Curves is the RGB Curves. We use it for all the most common tasks, like white balance, black level tweaking, and shot matching. Let’s have a look at some easy tasks first.
(For purposes of this tutorial, I’ll be using Adobe Premiere CC, but these concepts apply to all NLEs that can use Curves. Also, this is an “advanced” curves tutorial, so I’m assuming you have a keen understanding of color correction tools like Curves, Gamma, wheels, etc.)
Easy Black and White Level Balancing
Curves also give you better results than other widely used color grading tools. Take a look at this image with a blue cast.
A common way to fix the cast would be to drag the white balance slider. But adjusting white balance often affects the black levels too much or too little, causing tinted blacks. This makes the blacks and the shadows look a bit dirty or muddy.
By adjusting the color cast with the RGB Curves, we can protect or even clean up, the blacks. In this example, I just dragged the curve for each color until the whites and blacks were close to 0 and 100 on the scope, and matched. This was easy because the RGB Parade and the RGB Curves combination is so intuitive. Drag the blue curve to adjust the blue waveform, etc.
I spent less time tweaking the Curves in the second example than I spent dragging White Balance, Exposure, Shadows and other sliders in the first one. The sliders compete with each other, so it ends up being a push-and-pull exercise, requiring lots of tweaking. The result I got with curves is also a lot better, especially in the shadows. So in this case, I got a better result in less time. I also believe Coca-Cola would be much more satisfied with the red color on the tuk-tuk in Figure 03.
Notice that the R, G, and B curves are still straight lines; this was a very simple adjustment. I only adjusted the overall gamma with the white curve. A color cast from a camera is often just a gain imbalance for the R, G, and B color channels and doesn’t introduce gamma errors in each color channel. So, to fix this kind of color cast, it’s best to stick to straight lines – something that’s very hard to achieve when you work with color wheels and sliders, but easy when working with Curves.
Prepare Textures for use as Mattes
I use textures a lot, especially as mattes to roughen up text, logos and other graphics. Curves are perfect for adjusting the black level, white levels and midtone contrast in the mattes. Other tools, like Levels, give you less control over midtone contrast, where the partial transparency lives. With Curves you get granular control over midtone contrast. You can use soft curves, steep curves and S-curves, so the results are much better.
Let’s do a quick change in the timeline. I’ve copied the concrete wall image to video track 3 and adjusted it with curves. I also put a Track Matte Key on the text layer, and changed the blending mode to Overlay.
I used the Hue/Saturation curve to desaturate the concrete wall image, and the white curve in RGB Curves to increase contrast a lot. The desaturation isn’t really necessary, but I find it much easier to visually evaluate a matte when the colors are gone.
Next, I changed some settings in the Track Matte Key effect. I set it to take the matte from video track 3, and to use the Matte Luma. I also activated the “Reverse” checkbox, to invert the matte. The result is that the text is only visible where the concrete wall image is dark. Now it looks like it’s been painted on the wall a long time ago.
By changing the curve while looking at the final result, I can make the text as aged and worn as I want.
As you can see, the curves make it super-easy to adjust the look and feel of this effect.
Since I like to keep my mattes black and white, even though the colors don’t really matter for the effect, I’ve made a Lumetri preset in Premiere that removes the colors and increases contrast. It’s just a preset, so everything can be tweaked from there. But using a preset takes me faster to my goal.
Fast and Easy Shot Matching
Shot matching is probably the task we do most often when color grading. Again, using the combination of RGB Curves and RGB Parade it’s fast and easy to spot differences and match the shots.
With RGB Parade, RGB Curves and a good workspace, it gets really easy to compare images and waveforms to quickly match shots to a hero shot. In most NLEs, we can only have one scopes panel, but if it follows the active monitor like it does in Premiere, it’s easy to compare waveforms by toggling between monitors.
Let’s decide on the first shot as the hero shot. We will make all the other ones look like this one. Using the Crop trick explained in the article “How to Use and Read the Four Primary Video Scopes” I can isolate the drawing, and see that the peaks of red trace goes up to around 65, the green to 60 and the blue one to about 45. See figure 14.
With those numbers in mind, let’s move the playhead in the timeline to the second clip, and compare.
After adjusting the white curve, the RGB parades seem to match the first shot pretty well, so we don’t need any individual adjustments on the color curves. That’s not a big surprise since it was shot in the same room.
Now let’s move to the third clip, and see how it matches. This was shot in another room, and the lighting is a bit different. We’ll have to decide what part of this image we want to match the drawings. I’ll go for the lower part of the stack of drawings, and crop away everything else.
Playing through the sequence, I could see that the third shot still stands out a little, as you can see above. We probably did a bad choice of area to compare in the stack of drawings. I tweaked that shot a bit further by eye, lowering the top of the blue curve, and raising midtones in the white curve. See the final result below.
Just so you can see what we have achieved with this, here’s a before and after, side by side. The two lower images have now been tweaked to look like they’re a part of the scene.
If you find out after you’ve matched all the shots that you want another look, don’t adjust the first shot and start the matching all over again, tweaking individual shots. Instead, do the same changes to all the clips using an adjustment layer, a node, nesting or whatever your NLE has to offer. Now that the shots are matched to each other, it’s easy to adjust them as a group.
Fix that Dull Sky
With the Hue/Saturation curves we can do saturation adjustments to the whole image, or isolate the adjustment to only affect some colors. Let’s see how we can get some deeper blues in this slightly over-exposed sky.
Of course, dragging the midtones down in the RGB Curves will help a bit, but not enough. So I selected the blues in the Hue/Saturation curve and increased the saturation only for these colors.
The result is much better than it would be if I used secondary color correction for this. The image has quite a lot of sharpening from the camera, and this creates a halo along the clouds that would really stand out if I tried to isolate the sky with the qualifiers. Just a few clicks with the mouse was enough to improve this shot with curves.
Some Useful Curves Presets
You don’t want to start from scratch every time you bring up a new shot. Instead, you want to have presets that you can use as a better starting point, because they’re already close to what you need. So you’ll get to the final image with fewer clicks.
In addition to the high-contrast matte preset mentioned above, I have several other Curves presets that I use on a daily basis. “Warmer” and “colder” presets, etc. You can easily create such presets yourself. But let’s look at some other useful presets for quick grading and for special effects. (Check the “After” shots to see the curves adjustments used to create the respective presets).
Sometimes you’ll get dull footage, because they’re poorly exposed, because they’re shot in Log, or because of haze. An S-curve can make such shots look great with very little effort. Here’s a collection of shots with different S-curves on them. The split screens show the original on the left and the result on the right.
Remember, I didn’t tweak these curves, I just dropped a preset on them. If you have 10 shots from the same camera, you can add the S-curve to all of them in less than a second! To create a preset in Premiere, just tweak the curve as you want, then click the panel menu in the Lumetri Color panel and choose Save Preset.
Next time you see dull footage, don’t find a LUT, just throw your favorite S-curve on it to get its levels in the ballpark. Then tweak further with another set of curves, or by modifying your preset curves.
Some video formats can record levels above legal video levels without clipping them. With curves, you can get your seemingly clipped highlight details back while maintaining your overall look.
Of course, this trick won’t work if the highlight information has been clipped.
Crushing the blacks
When you deliberately shoot flat, the blacks get a bit milky. If the rest of the image looks OK, but the blacks are grey, throw this preset on, and it’s done. This preset curve only affects the blacks, leaving the rest of the image untouched.
Old grayscale images look older when you add a little bit of sepia to them, and fresh video recordings can look old. With this preset, the extreme blacks and whites are still clean, but the midtones get a nice sepia color cast. Add desaturation before the curves to make the preset usable on any footage.
Simulating Color Negative
In the Beginner’s Guide to Curves for Color Correction article, Jason Boone created a cool “negative” look by inverting the Luma channel.
This is a great start, but the skin color is still skin color even though the blacks got white and the whites got black. But we can take it one step further to fully emulate a color negative if we invert each color channel.
Since the color negative doesn’t do an identical inversion of every channel, but rather looks more orange, we can tweak the curves further to get the look we want.
There are more curves!
The RGB curves and the Hue vs Saturation curves that we’ve used in this article are the two most common ones, but there are other curves you may want to explore.
DaVinci Resolve and FCP X (ver 10.4) have Luma vs Saturation Curves. These are great for cleaning up the blacks and whites after other adjustments or LUTs have been applied. With this curve, you can tweak the saturation differently in the shadows, midtones and highlights.
Since Premiere doesn’t have this kind of curve, I made a preset using secondary color correction qualifiers that does the same thing. But it would be much better if Premiere had this curve, since it’s easier to tweak, which can be a timesaver.
Resolve and FCP X (ver 10.4) also have a Saturation vs Saturation curve, which lets you do things like increase the saturation of less saturated pixels without affecting the pixels with more saturation—again without the need for secondary color correction qualifiers, which can get messy and complicated. The Hue vs Hue curve lets you adjust the hue of pixels within a narrow hue range.
As you’ve seen in this article, Curves give you very precise control over exposure, contrast and colors—often a lot more control than you get with other tools. When you’ve mastered the RGB curves and the Hue vs Saturation curve I urge you to explore other curves. What is your favorite technique when color grading with curves? Let us know in the comments!