Color Correction Tools (Part One): Saturation, Levels and Curves

We know that color correction is nothing but changing the color values of a pixel. In the article I’ve linked to, we also looked at some popular software and hardware devices. What differentiates these, and how could a newcomer be expected to know the difference or importance of each feature?

In this article I’ll explain the six key tools that any color correction or grading application should include. In fact, these six tools can cover 99% of color scenarios in both amateur and professional productions.

The major color correction tools are as follows:

  • Brightness
  • Contrast
  • Saturation
  • Levels
  • Curves
  • Masks
  • Mixers
  • Tracking

 
I won’t be covering brightness and contrast since they are ubiquitous tools, and are self-explanatory. In this part we’ll cover saturation, levels and curves in detail.

Saturation

A saturation tool is usually a simple slider:

Saturation Tool

Saturation is the simplest tool you can find, and works similarly to brightness. It is a linear slider that moves from 0 (black) to 255 (8-bit fully saturated image). Because we have defined our image as an 8-bit image, with a fixed color space, it is impossible to go beyond 255. We call this value ‘full saturation‘.

A pixel that reaches this value is said to be fully saturated. Reducing the saturation (removing the color) is called desaturation.

In an RGB image/pixel, each of the three channels red, green and blue can reach 255 or full saturation. When all three channels are fully saturated we get pure white.

The saturation slider usually only allows all three channels to be moved up or down simultaneously. There are other tools for more control, but sometimes you just want to adjust the overall color in an image. The saturation tool gets this done in one click.

One thing to remember is that saturation never goes all the way to white when you push it to the max. The reason for this is that all the colors are not represented linearly in all pixels – after all, an image is a complex bunch of pixels. Pushing the slider moves all of them at the same time, and just because the slider reaches the end doesn’t mean all pixels have reached full saturation in all channels.

E.g., if I have two pixels, one black (first box in the above image) and the other dark red (second box in the above image), and I push the slider all the way to the limit, the pixel values in the second box will reach the end before the first.

Levels

Levels is a combination of two tools, called the Input and the Output tool. They are both sliders, but each affect the image from a different perspective:

Levels Tool

When you select the Levels tool you get the histogram of the image or frame. Both the input and output tool shows up at the same time. The easiest way to start thinking of these tools is as follows:

  • Input – Contrast Tool
  • Output – Brightness Tool

 
Remember, it’s not only a contrast+brightness tool. The levels tool offers far more control. The ways it does this are:

  • You can adjust the Gamma.
  • You can adjust the levels of each channel separately or as a whole.
  • When you adjust each input slider, the gamma shifts – this is why it behaves like a contrast tool.
  • When you adjust each output slider, the gamma stays unchanged – this is why it behaves like a brightness tool.

 
The input tool has three controls:

  • Black – The lowest value is 0. Increasing the black input level adds black (a process called crushing the blacks) to the mid-tones. Think of it as black water creeping into the mid-tones like a flooding river. Changing this also shifts the gamma, but notice the white stays fixed.
  • White – The maximum value is 255. Lowering it adds white to the mid-tones. Notice how the black stays fixed.
  • Gamma – the levels tool allows you to change the gamma precisely.

 
All three controls also allow for numeric values, so you can precisely enter the values you prefer.

The output slider has two controls:

  • Black – Lowest value is 0. Increasing this value brightens the image.
  • White – Highest value is 255. Reducing this value darkens the image.

 
I cannot begin to tell you the power that the levels tool holds. You can grade an entire movie with this one tool, if you know what you’re doing. It is the tool I use the most, and is easily my favorite color correction or grading tool.

Curves

The curves tool does exactly what the levels tool does, but it is more intuitive.

Curves Tool

It’s just two different ways of working. Neither is better. Some prefer the ‘precision’ of levels (like me), while others prefer the intuitive nature of the curves tool. Why not learn both? There’s good reason to.

The Curves tool has both Input and Output components. The key difference between it and levels is that levels allow you to manipulate gamma directly as a value, while the curve allows you to manipulate gamma as a curve.

When you open the curves tool, the curve is always straight – this is represented by the thin blue line. You can click on any point on this line, and it becomes a dot (control point). You can then move this dot around, and the curve changes shape. You can add as many points as the tool will allow, and make different shapes. The above image shows two points, and the popular ‘S’ shape.

See how this works? When you move the points, you are varying gamma, while the white and black points stay put. But, you have the added control of manipulating the overall look of the image by just moving a few control points. To do the same thing with the levels tool means playing musical chairs with all the sliders – see where ‘intuition’ comes into play?

In addition to this cool feature, you can also vary the black and white points of both the Input and Output tool. This works exactly as the levels tool.

How do you know when to use the levels tool over the curves tool?

When your gamma shift is linear and easily defined (one control point), use the levels tool. It’s faster, and you’ll get to your result faster.

When your gamma curve is complex (more than one control point), use the curves tool. The rest is the same, no need to get confused.

Learn both!

In Part Two we’ll cover the more complex tools – mainly masking, mixers and tracking.