Contrast/Saturation Splitting

Contrast and saturation go hand by hand. When using global contrast adjustments like Curves, for example, we can observe our colour saturation shifting. Depending on the tool and software we use, the opposite could also be valid - while adjusting saturation our contrast is also changing. More contrast means more saturation, less means less and this is usually a natural way to work with images.

So, why would we want to separate these close friends - the Contrast and the Saturation? Let's put it simply - because we value colour. Our clients also do, by the way.

An example:

We are doing a composite on a kitchen. We have nailed the product colour. We happily save the image but on the following day, we notice that it actually lacks some contrast. We go in and make the desired adjustment, however, with our contrast added now the colour of our product becomes saturated. We can easily fix this - we add a mask for our doors, de-saturate the colour, compare to what we had before until we nail it again.

But what if we could actually add that contrast without affecting the saturation in our scene?

Often times we do actually pay attention to every individual colour in an image and therefore global adjustments often tend to ruin our work. That's why this additional control can be beneficial in everyones workflow.

We use Fusion and Photoshop in our pipeline so we are going to look at them for this example.

Fusion generally works way better than Photoshop in this manner as the only tool which affects Saturation while adjusting Contrast is the Curves Tool. Other tools like Colour Correct have the ability to adjust contrast and saturation separately.

Photoshops' tools, on the other hand, do affect both contrast and saturation at the same time.

Fusion Curves Workflow:

METHOD 1

This is our base image which lacks some contrast. Let's put some Curves on.

Now we've got the contrast but the gold, the coffee foam, the plant, the clementines have all become a bit too saturated for my liking.

At this point, we can simply make the Curves affect only the Luminosity of the image without affecting Hue or Colour.

To do this we have to change the Colour Space in our Curves to HLS (Hue/Lightness/Saturation). We then need to reset the curves for the Hue and Saturation (assuming that we didn't do a curve for the alpha, which we shouldn't :)). To do that just untick the Lightness channel so we don't affect it, right-click on the graph and choose Reset. Of course, you can do your curves in HLS mode to start with, so you don't have to go through this additional step.

We can notice the difference between Curves in RGB (top) and Curves in HLS affecting only the Lightness (bottom).

METHOD 2

With this method, we are essentially splitting the colour and the luminosity changes made by the Curves and then combining them back for maximum control.

We do this with a couple of Merge nodes. We set the blending mode of the first one to Luminosity and the second to Colour. We now input our Curves into the Foreground of both and then connect our Composition thus far into the Background.

If we now preview our Curves and compare to what we have after our second Merge both images should look identical.

We can now control how much of the saturation from the curves do we want in our image simply by adjusting the Blend amount of our second Merge node. In the end, if we don't want any of it, we can just remove the second Merge entirely.

Drawbacks of these methods:

An important thing to note is that both methods don't produce identical results in terms of contrast. Therefore you can check these yourself, see the difference and choose the most appropriate for your situation.

A drawback to this type of splitting, in the end, is the splitting itself. When we start to change the position of our White and Black points we can get some artefacts. The reason for this is that we shift our contrast values but don't affect our colour values (and as mentioned in the very beginning contrast and colour go hand by hand).

With Method 1 these can appear like broken pixels in the highlights and somewhat significant colour shifts in the shadows.

With Method 2 there's just a somewhat unnatural colour shift mostly around the highlights. This is why, for general use, Method 2 is the better one.

Photoshop Contrast/Saturation Workflow:

For the Photoshop users, the workflow is quite simple: If we use Curves on a separate layer (which we should) we put this curves layer in Luminosity mode.

For Saturation/Vibrance (adjustments that work with colour) we can use the Colour mode. In other words, this method is identical to our second method in Fusion and therefore has the same issues that you should be mindful of.