Curves Adjustments - Follow-up Part II

Okay, let's talk about curves adjustments and color. This is where quite a few people flew off the boat in my explanation on the wonder of the Aperture 3 curves tool. As can be seen in the title this is part II of follow-ups to the original post. In case you missed the last two here's part I.

The photo at the top displays yet another shot from RAW material made for my Mirror-Mirror project. It has much the same set of stuff discussed in a quite radical re-vamp for illustration discussed in that first post. There are two big differences comparing this set of adjustments to that first set. First – overall color balance via the white balance control is much different than the as shot WB provided by the X100S auto mode. It's far warmer. The second big difference is in the color toning I added to the highlights via curves. The focus of our discussion today.

The toning on this image is all in the upper mid-tones and especially the highlights. Here's a full resolution screenshot showing the curves in RGB that makes the toning happen (download or open in a new window to see full size). To see the underlying red, green, and blue adjustments go to the channel drop down at the top of the control and make sure Show RGB Overlays is checked.

We'll discuss these adjustments in depth but first lets have a look at their impact on the image. Here's a screen shot with that color toning effect turned off.

There are a few things that should be clear from those illustrations. The first thing is that I manipulated both the red color channel and the blue channel to varying degrees away from that center gray line. The other thing that should be clear is that the effect is the strongest in the highlights, far less in the mid-tones, and nonexistent in anything below the mid-tones. That strongest strength corresponds to where the red and blue curves are farthest away from that strait gray line you see.

Let's dive in and look at those red and blue curves adjustments a bit more. First the red one that you can see to the left here. There are three points on the curve. Let's call the right most point the targeted tonal range I wanted to manipulate. The other two I added to neutralize the effect going towards the mid-tones and also get rid of the opposite effect (less red) that occurred in the shadows. The reason I needed both is because the amount I pulled up the red was pretty extreme on that first point in the highlights. I used the other two to pull that red line back towards the strait gray reference. Play with it yourself – you'll see what happens and get a better feel than my words will give you.

Now let's take a look at the blue channel as can be seen on the right. It's a bit less extreme and targeting the same tonal range so I only needed one additional point to neutralize it's effect on mids and shadows. The red channel adds red to highlights and the blue channel takes away blue from the same range. Nows a good time to do some remedial color math in the RGB color mode. Pretty much stuff you should remember from 4th grade art class if you were paying attention and not eating paste.

Remedial Color Math

  • Adding red does exactly that but it also reduces cyan.
  • Blue, same principle, subtracting blue adds yellow.
  • Green… ah green. Subtracting green adds magenta.

So in the RGB space you have three colors to manipulate. They happen to be the primary colors adding or subtracting one of those primary colors has the exact opposite effect on the primary color's opposing secondary color. So the blue channel adds/subtracts blue which is the same as subtracting/adding yellow. Red – same thing, it's opposing secondary is cyan. Green as mentioned, has an opposing secondary of magenta which you should know from the ubiquitous white balance controls. So if you want to manipulate something that is yellow you are also primarily talking about messing with blue, etc, etc.

Advanced Color Math

Well not really – just pointing out something that may not be intuitively obvious. From a color point of view subtracting blue can be looked at as adding yellow. It can also be looked at as adding green and red. The two channels that are not blue. Why is this? Well back in that 4th grade art class you should know mixing red and green will give you yellow. You can also look at it this way, if it makes more sense… Taking away blue leaves behind green and red – the other two channels in RGB hence it leaves behind yellow. We talked about what opposing secondary is associated with red, green, and blue. Now let's just list what happens when we mix two of each of those. It should be obvious but let's beat this horse dead…

  • Red plus green make yellow.
  • Green plus blue makes cyan.
  • Blue plus red makes magenta.

See how that works out to taking away the color channel not mentioned in any of the combos. The horse is not completely dead yet. For completeness of execution of the color horse how about an enumeration of mixing those secondary colors…

  • Yellow plus cyan give us… green
  • Cyan plus magenta gives us blue.
  • Magenta plus yellow gives us red.

Ta….Daaaa see how they are all connected. If you look at any of the 360 degree diagrams that represent a color spectrum you will see the mixing we are talking about leaves the result in the middle. Take a look at the handy dandy one here if you wish. Obviously these are continuous and the proportion of the mix will skew one way or the other in between colors. Example: Orange is a reddish yellow.

Practical Matters

Okay if you survived the above you should now see that adding red and subtracting blue (leaving yellow behind) in the above curves is like mixing red and yellow giving us some degree of orange in the highlights. Typically you will want to add one thing and take away another rather than just adding something. While perfectly valid, doing so will also affect the luminance or brightness of the tonal ranges affected. This is not absolutely bad but could be… use your eyes. What if we just wanted the added red you ask rather than orange… well you could bump up the red and get rid of both green and blue. If you did that you would need less of each movement to have the same amount of effect because both add red and take away everything else.

So my primary goal here was to add an orange-red to the highlights. Why? Well the area behind Katya felt cold to me and ramping up WB was not going to help for two reasons. First off is that the shadow side and darks are already plenty red and warm. Second – the relationship wouldn't change which means that that area would always be cooler relative to the shadows. Sometimes this is a bigger deal in perception of color than the actual warmth/coolness overall. As an exercise let's take some red out of the lower mids and shadows here. There are a lot of options as you now know to do that using curves. I'll use the last one I suggested but in reverse.

Here I took red out of the shadows and added blue and green. You can see it in RGB mode with Show RGB Overlays. I hope that helps a few of you that had color questions related to curves. If not feel free to question or comment.


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