HSL

Learning goal

You will be able to separate each color in a photo not by its brightness, as the tone curve does, but by the color itself, and change them one by one. You will explain the HSL model, which splits color into three separate properties, that is, hue, saturation and luminance, and read hue as a hue angle on a color wheel, saturation as distance from gray, and luminance as lightness. You will show that the color mixer's channels work not with sharp boundaries but with wide bands that blend into each other, so changing one channel spills onto neighboring colors too, and notice how traces like banding and noise appear while darkening a blue sky. You will check color spill across the frame, a color changing everywhere in the photo at once, and check what happens on different skin tones while touching the orange and red channels.

The question

The drummer opens a landscape they shot from their balcony: a deep blue sky, a lush green hill in front, orange-tiled roofs at the foot of the hill, and among the roofs, in a garden, their little sister in a blue T-shirt. They want the sky a little darker and deeper. When they try it with the tone curve's blue channel, the sky darkens but the tiles slip toward gray and the hill toward a murky olive. The curve knew tone, not color.

In the color mixer you lower only the blue channel's luminance. The sky darkens and the clouds stand out; the tiles and the hill don't change at all. But two things catch your attention: the little sister's blue T-shirt has darkened too, and in the sky's light area near the horizon, thin, stepped rings have appeared. How do you change only a blue sky? And why does "only blue" sometimes change more than the sky, and sometimes only part of the sky?

Context

In the basic editing module you got to know the color mixer, splitting colors into channels and changing each one's hue, saturation and luminance (Lesson 17.5). There you learned this tool by its results; in this lesson we take up the logic behind it, that is, the HSL model, which thinks of color as three separate properties, and the limits of this model.

The tone curve sliced the photo by lightness; HSL slices it by color. Neither takes the other's place; they complement each other. When you want to adjust a photo's sky, leaves, skin or a piece of clothing one by one, you first need to know which tool catches that area most cleanly. This lesson tells what HSL catches well, what it can't catch, and when you need to move on to the next tool, masks.

Simple explanation

How is color split into three properties?

The HSL model is a color model that defines a color by three independent properties: hue, saturation and luminance. Its name comes from the initials of these three properties in English. Hue is the color itself: is it red, yellow, blue. Saturation is how intense the color is; zero saturation corresponds to gray, high saturation to a vivid color. Luminance is how light or dark the color is.

A hue angle is the place of a color's hue on a color wheel. Colors are thought of as lined up around a circle: starting from red, going to orange, yellow, green, cyan, blue, purple and magenta, and from there back to red. A hue shift is turning a color on this circle toward its neighbor. Saturation can be thought of as how far the color is from the gray at the center of the circle. This picture makes it easier to understand which way you are going when changing a color: pulling a green leaning toward yellow toward green is a small turn on the circle; deepening a faded blue is a step outward from the center.

HSL's strength is in being able to change these three properties separately. To darken the sky you lower blue's luminance; its color doesn't change, its intensity doesn't change, it only darkens. This gives a much more natural result than trying to do the same job by raising saturation. Changing a color's luminance is also the basis of deciding which color turns into which gray when converting a photo to black and white; you will meet this use again in the module's later lessons.

Where does a channel begin and end?

A channel boundary is the soft transition zone where one color channel's effect passes into the neighboring channel. The channels in the color mixer aren't separated by sharp boundaries; each channel affects a wide band on the color wheel, and this band overlaps with neighboring channels. When you change the blue channel, pure blues are affected the most; blues leaning toward cyan or purple are affected less, but still affected. This soft transition prevents breaks forming between colors.

Its effect is seen most in the sky, because the sky isn't a single color: at the top it is a deep blue, toward the horizon a light blue leaning toward cyan. When you change only the blue channel, the sky at the top changes strongly and the sky near the horizon more weakly; this can create an unwanted transition in the sky. A small adjustment to the cyan channel too closes this difference. In the same way, leaves leaning toward yellow and green are split across two channels; to change all the leaves the same way you need to touch both channels together.

Banding is a smooth color or tonal transition appearing split into stepped rings instead of a continuous flow. Pushing a wide, smooth transition like the sky hard with HSL, especially in a compressed file carrying little tonal information, makes the tonal steps visible. The same forcing also brings out noise in the sky; noise catches the eye more easily in a dark sky. Working with a RAW file, making adjustments in small steps and not going to the extremes in wide transitions prevent banding and noise.

What can't HSL catch?

Color spill across the frame is a change made to a color channel changing that color everywhere in the photo at once. HSL selects a color not by its position but by its color; it doesn't tell the blue of the sky apart from the blue of a T-shirt, a sea or an eye. The darkening of the drummer's little sister's T-shirt came from this. This limit is HSL's most important property: if a color is found in only one place, HSL is a perfect tool; if the same color is found in several places, to change only one of them you need to move on to the masks you will see in the next lesson.

You can also manage the relationship between colors with HSL. Darkening the blue sky makes the orange tiles look brighter and more prominent, because these two colors are the complementary colors that bring each other out the most. Pulling the leaves' yellow a little toward green gives a summer air, pulling their green a little toward yellow an autumn air. These decisions are aesthetic choices; they can't be measured as right or wrong, but it shouldn't be forgotten that each one also affects the colors in the rest of the photo.

Because most skin tones sit in the orange and red channels, touching orange to change the tiles or autumn leaves in a landscape also changes every face in the frame. On dark skin, lowering the orange channel's luminance can pull the skin toward an ashy, lifeless brown; reducing its saturation dims its warmth. On light skin, raising the red channel's saturation flushes the cheeks and nose, and shifting orange's hue toward yellow yellows the skin. Even if there is only a small figure in a landscape, after every change made in the orange and red channels you need to enlarge the face and keep the skin close to the person's real color; if they can't be separated, it is more correct to apply the change only to the tiles or the leaves with a mask.

Visual evidence

In the drummer's landscape you lower the blue channel's luminance by a medium step. The sky at the top drops to a deep blue, and the clouds stay white and full; but the light area near the horizon, leaning toward cyan, has hardly changed, and a strange transition has formed in between. When you lower the cyan channel's luminance a little too, the transition disappears. The stepped rings that appeared near the horizon, on the other hand, are erased only when you pull the blue adjustment back a little and start again from the RAW file.

The tiles now look warmer and more prominent in front of the blue sky. You turn the hill's green, which leans toward yellow, a little toward green in the yellow channel; the hill comes together. The little sister's blue T-shirt in the garden, on the other hand, has darkened along with the sky; her dark skin hasn't changed at all in the orange channel, but HSL isn't enough for the T-shirt. You note that you need a mask to separate the T-shirt from the sky.

Common mistakes

Summary

The HSL model splits a color into three independent properties as hue, saturation and luminance; hue is the hue angle on the color wheel, saturation the distance from gray, luminance the lightness, and each can be changed separately. The color mixer's channels work not with sharp boundaries but with overlapping wide bands; colors split across several channels, like the sky, are adjusted together, and pushing wide transitions hard leaves banding and noise. Because HSL selects color not by position but by color, color spill across the frame happens; if the same color is in several places you move on to a mask, and when the orange and red channels are touched, it is checked by enlarging the face that dark and light skin keep their own color.

Mini assignment

In this assignment you will edit a landscape and a frame with a small human figure in it only with HSL.

1. In a photo with a blue sky, darken the sky first by raising saturation, then by lowering blue's luminance, and compare the two results.
2. Prepare two frames, adjusting only the blue channel and adjusting blue and cyan together; look at the transition in the sky near the horizon.
3. Take blue's luminance to the extreme, enlarge the sky and see whether there is banding or noise; then make the step smaller.
4. In a landscape with a person in it, change the orange channel and check how the face is affected by enlarging it; list in which other places the same color is found in the photo.

Check yourself

• When changing a color, can you decide whether you need to change the hue, the saturation or the luminance?
• Can you explain which channels the sky is split across and how neighboring channels are adjusted together?
• When you touched the orange and red channels, did you check whether the skin of the faces in the frame changed?

Glossary

Mini quiz

1. In the HSL model, how does lowering luminance change a color?

• A) It shifts its color to a neighboring color
• B) It brings the color closer to gray
• C) It darkens the color while keeping its hue and intensity
• D) It makes the color more vivid

2. When you lower only the blue channel's luminance, what is the reason the sky's area near the horizon hardly changes?

• A) Because the sky near the horizon leans toward cyan, it partly falls into another channel
• B) The sky near the horizon is more saturated
• C) The blue channel affects only the clouds
• D) There is no light in the sky near the horizon

3. What is the reason the drummer's little sister's blue T-shirt darkened along with the sky?

• A) The T-shirt's noise
• B) Banding
• C) The channel boundary being sharp
• D) HSL selecting color not by position but by color

4. In a landscape, when you lower the orange channel's luminance to strengthen the tiles, the face of a dark-skinned person in the frame slips toward an ashy brown. Which is the most fitting solution?

• A) Lower orange's saturation too
• B) Apply the change only to the tiles with a mask, or take back the orange adjustment
• C) Raise the red channel's luminance
• D) Crop out the part of the photo where the face is

Answers

1. C — Luminance changes only the lightness; hue and saturation stay in place.
2. A — The channels work with overlapping bands; blue leaning toward cyan sits largely in another channel.
3. D — HSL changes the same color everywhere in the photo together; it doesn't tell the T-shirt's blue apart from the sky.
4. B — Because skin and tile sit in the same channel, the way to separate them is selecting by position.

Summary card

• HSL: hue is the hue angle, saturation the distance from gray, luminance the lightness.
• To darken the sky, lower luminance, not saturation.
• The channels overlap; the sky and leaves split across two channels, adjust them together.
• Don't push wide transitions hard; banding and noise appear.
• HSL changes a color everywhere; skin is in orange and red, check by enlarging the face.

Bridge to the next lesson

In the drummer's landscape the sky was now as deep as they wanted, but their little sister's blue T-shirt had darkened along with it. The same evening they also showed you a portrait of their family: their mother's face was bright in the window's light, while their father's face had stayed in her shadow, a little in the dark. Changing them with a slider changed both at once. How do I brighten only a face? The next lesson tells how to apply an edit to only one area of the photo with brush, gradient, sky and subject masks.