Curvy Curves

 German version

As promised in the previous Blender-Tip I want to provide some unusual things to do with the Curves in the compositor. I must say that I will use the words Value, Luminance and Brightness in a loose way; In fact they do not mean the same but are special terms according to different color spaces, but let's not care about this for out purposes today ;)

What does an RGB Curves node do in the normal case? It is a function that reflects a relation between the input values on the x-axis and the corresponding output values on the y-axis. It is displayed a graph so that we may interact with it.  If we lower the values of the R-curve on the left half and raise them on the right, all pixels that do already have a low red value become even less reddish and vice versa for example.
Not only male artists like curves much because they provide a graphical way to do value-dependent color manipulation.

But who say that we can only control the RGB Values?
We could also come up with the idea of playing with the HSV channels!
To do that, all we have to do is to reassign the HSV channels as the RGB channels, apply the curves and then revert the channel assignment. That'd look like this:

Here we are able to manipulate the Saturation using the G-Curve whilst the B-Curve provides control over the Value channel. The latter is useful in many situations because we may change the luma gradiation, but leave the chrominance as it is. We control the Hue with the R-Curve, but the Hue Correct node does a better job for that.

However, do we always need to control a channel in relation to its own input value? We could also try to alter the saturation as a function of the brightness using the G-Curve!
To realise that we do also assign the value to the G-Channel, compare its value after the curve with the unaltered one and then apply the change to the saturation channel.
Looks like this:

Now we can control the Saturation as a function of luminance.

So the Principle is to manipulate the Channel that we use to control any Value with the Curves and then extract the changes the Cuves have made by substraction so that we can add them to the Value we want to alter. Again this technique represents a useful tool because it gives us the power to alter everything we can imagine as a function of any other value!

I'd like to mention another useful application of that principle:
We can control the RGB Values in relation to the Brightness. That gives us the option to control the color balance in the image very precisely.

That's it for today.
The next time we will look into frequency extraction and I will explain a technique to soften skin tones in a natural looking way!

1 Kommentar:

  1. Nice, thanks.
    It would be nice to have an before and after image to compare