The Channel Mixer 100% Concept

There is persistent and pervasive advice that you should make your channel mixer output slider values sum to 100% in order to preserve overall image brightness, exposure, luminosity, etc. The concept certainly has its merits, but being dogmatic about it can lock us out of some very interesting Channel Mixer effects for different kinds of saturation and color grading. And the 100% rule/guideline has a fundamentally more important feature:

100% slider sums preserves neutrality!

Neutrality means keeping gray things gray. Gray means that the R, G, and B values are identical, or R=G=B.

Let’s see how that plays out. Channel Mixer (CM) slider values are shown as percentage values, so for example, if I set the red output sliders to 120/0/0, I will boost reds in regions where the base image has high R values. And my sliders definitely do not add to 100 in this case.

For a middle (50% brightness) gray pixel or color, my RGB values would be 128/128/128. 120% means 1.2 in mathematical terms, so my new pixel value for that CM setting will be:

So clearly, neutrality is broken, and we will impart a red color to our grays.

On the other hand, if we set our red output channel to 120/-10/-10, our sliders sum to 100%, so what does that do to a middle gray pixel?

And that remains true for any set of slider values that sum to 100%, or any gray value where R=G=B.

And what happens to our brightness with that 120/-10/-10 setting? We can look at two measures of brightness. The first is HSB brightness, which is simply the highest of the R, G, and B pixel values normalized to 1 by dividing by 255, the maximum possible R, G, or B value in an 8 bit color scale.

The other measure of brightness is luminosity, a measure of perceived brightness. Luminosity is calculated as a weighted sum of R, G, and B values based on the human perception that green is brighter than red, and much brighter than blue. Photoshop calculates luminosity on a normalized basis using rec. 601 weights, close to:

By either of these measures, any gray pixel will not change its brightness or luminosity if our sliders sum to 100%.

But that situation changes when we look at colored pixels. For example, let’s take a red pixel with RGB values of 148/100/100 (any RGB combination where R>G=B is pure red). As the R value of 148 is the highest, our HSB brightness for that pixel is simply 148/255=0.58 (58% in Photoshop terms) and our luminosity is:

Now let’s apply our neutral 120/-10/-10 Channel Mixer Red channel sliders.

So with this neutral transform (since our sliders summed to 100%), which is widely reputed to preserve brightness, our HSB brightness has actually changed from 0.58 to 157.6/255=0.62, a 4% increase. Meanwhile, our luminosity is:

So by both measures of brightness our 100% summed Channel Mixer slider values are imparting changes to brightness. That’s just one example pixel, but the changes are more apparent looking at an RGB hue strip that I’ve created in Desmos. For that, let’s use a neutral-preserving global saturation Channel Mixer setup where the sliders sum to 100% for each output channel:

So where did this 100% to preserve brightness concept come from? Well, it arises because since we’re preserving neutrality when we sum to 100, that also means a completely white pixel (RGB 255/255/255) will also be unperturbed when we satisfy this condition. So preserving neutrality means no change to the white point. But preserving neutrality, or grayness, is an also a fundamentally important consideration, a principle that’s gotten lost in the discourse about channel mixer values.

But is neutral preservation that critical? In some cases, yes, you do want to preserve neutrality, keeping gray things gray throughout your image. Particularly if you’ve already set your white balance and are happy with it. But keep in mind, white balance is itself not neutral preserving. In fact that’s its whole point, to shift the color of grays/neutral until they are in fact neutral. Alternatively, many experienced users modify white balance for creative effect, breaking neutrality on purpose!

The benefits of breaking neutrality often outweigh the merits of keeping it. In fact, I would argue that you’re missing out on a whole host of useful Channel Mixer color grading effects if you insist on summing your sliders to 100%. We’ll discuss a very interesting use case when we explore subtractive saturation using the Channel Mixer, something I’m pretty sure one cannot cleanly do with an HSL adjustment.