Using LAB Color Adjustments
Intro | Multi-RAW Processing | Creating HDR Images by Hand [Part I] | Creating HDR Images by Hand [Part II] | Sharpening in LAB Color | Converting to Black and White | Using LAB Color Adjustments | Inverting Backgrounds with LAB | Intro to Compositing | HDR in Adobe Photoshop CS5 | Using Image Apply Image | Aging Photos Roundup | Making Colors Pop in Photoshop
Intro Image: The color effects in this image of a Poppy core were achieved using LAB color adjustments.
In a previous column, Sharpening in LAB Color, I showed you how to use the LAB Color space for compositional sharpening. Using LAB sharpening has been used by knowledgeable digital photographers for quite a while—and comparable sharpening techniques using only luminance information can be found in other color spaces—but the creative possibilities inherent in the LAB model of color are less well known and unique.
Sure, you can use LAB color swaps to alter the color of objects if you want to turn a green car red, or change the color of a model’s dress. But this kind of pseudo-realistic adjustment is only the beginning. The whole topic of LAB color adjustments comes alive when you start to address the fantastic color “painting” possibilities that LAB opens up.
It’s well known that you can specify “impossible” colors using LAB. These are colors that do not appear in nature and cannot be reproduced using printing inks or the phosphors in a monitor. The gamut of LAB colors is larger than is possible in RGB (monitors), which itself has a larger gamut than CMYK (printing).
Outside the gamut means outside the gamut—these LAB colors cannot be reproduced, and you probably won’t ever see them, either.
I’m not suggesting that you go after these impossible colors, but their existence in LAB is a good hint that LAB is the most exciting color space to use when hunting for exotic color effects.
In the earlier Sharpening in LAB Color article I explained the basic concepts of LAB. You might want to refer back to that article for an overview of LAB basics; however, this article will revisit the conceptual underpinnings of LAB in Photoshop from the viewpoint of what you need to know to succeed with color manipulations. I’ll also go over converting to LAB color.
Next, I’ll show you how to do simple color swaps. Getting creative, I’ll explain two of the key LAB color adjustments, inversions and equalizations. Finally, I’ll show you how to use LAB color adjustments paired with channel blending modes for an infinite palette of spectacular color variations.
Fasten your seatbelts! It’s going to be a colorful and exciting ride!
Additional images that will be covered in this tutorial:
LAB Color: The Largest Color Gamut
A color model—sometimes called a color space—is the mechanism used to display the colors we see in the world. This mechanism involves disassembly and assembly. A color model has to be able to accurately pick apart—or specify—the apparent color seen.
You are probably already familiar with two color models, RGB—Red, Green and Blue—and CMYK—Cyan, Magenta, Yellow and Black. In these models, from the specifications of the colors associated with an object, devices get a way to reproduce the object’s color: the RGB primaries combine on a monitor or television, and combined CMYK inks reproduce via “process” printing.
In contrast to RGB and CMYK, LAB is largely a theoretical color model. Scanners and digital cameras do not capture in LAB. You can’t input to your computer in LAB color. Printers don’t print in LAB. You can display LAB on your computer monitor if you are working in Photoshop—in fact, Photoshop does many of its internal calculations using LAB—but you can’t save a LAB JPEG, and outside Photoshop for the most part you won’t be able to display images on a monitor with colors encoded using LAB.
If this all sounds hugely impractical, wait! Theory has its place: there are far more colors in LAB than can be specified in RGB as shown in Figure 2—that’s what being a larger gamut means. As an aside, you may be interested to know that RGB has a larger gamut than CMYK. Within RGB, color spaces like ProPhoto RGB and Adobe RGB have more gamut than sRGB, which is the lowest common denominator color space specified by default for monitors and televisions. By the way, that’s why your working RGB space should not be set to sRGB when you work on your images in Photoshop—Adobe RGB (1998) or ProPhoto RGB are better choices, with (as noted) more color range.
Figure 2: LAB Color has a bigger gamut (more colors) than the other color spaces.
As a practical matter, LAB is structured a little differently than the other color spaces. There are three channels:
- The L channel contains luminance, or black and white, information. In the Photoshop Channels palette, the L channel is called the Lightness channel.
- The A channel provides information about greens and magentas.
- The B channel provides information about yellows and blues.
Both the A and B channel are organized in a scheme that is sometimes called color opponent. This means that complete information about the two opposite colors is contained in each channel.
In Photoshop, each LAB channel value ranges from 0 to 100. At a given point, the color information is completely specified using the two channels and the range shown in the accompanying table.
Table 1: Each of the color channels in LAB contains complete information about an opponent pair of colors.
Don’t worry if you don’t understand the theory behind LAB very well at this point! If so, you are not alone. The good news is that using LAB color to achieve stunning impact is actually pretty intuitive—and as you get practice using the techniques I’ll demonstrate in this column you will most likely absorb the theory by osmosis.
The one point you really do need to get out of this discussion is the concept of two opponent colors that are defined by a single channel. The practical implication is that controlling the curve for this color-opponent channel can be very potent. As you’ll see, small moves on a LAB color curve produce big shifts in color in your images—and if you want big shifts, like swapping blues for yellows and vice versa—then LAB is the color space for you.
Converting to LAB Color
The first time you convert to LAB color in Photoshop, you should select Edit > Convert to Profile and choose LAB from the Profile drop-down list in the Destination Space area as shown in Figure 3.
Figure 3: Use the Convert to Profile dialog in Photoshop to convert an RGB or CMYK photo into the LAB Color space.
Make sure to set the Conversion Options as shown in Figure 3. The Profile should be set to LAB Color, the Engine to Adobe (ACE) and Intent should be Relative Colorimetric. Check the Use Black Point Compensation box.
On subsequent soirees into the LAB color space you can convert to LAB simply by choosing Image > Mode > LAB Color. The options you chose in the Convert to Profile dialog will be applied with this quicker method of conversion.
Since you can’t output a LAB image, at some place in you workflow you will need to convert back to RGB (or CMYK). To convert back to RGB from LAB once you are through working in LAB, simply reverse the process by selected Edit > Convert to Profile or Image > Mode > RGB.
In workshops I am sometimes asked whether the roundtrip to LAB causes image degradation in a significant way. It’s a good question, but hard to come up with a definitive answer, although in my experience I am not aware of major problems related to LAB conversions.
I’d always suggest looking closely at any image at a high magnification where critical reproduction is an issue.
It’s certainly fairly easy to introduce problems in LAB by specifying colors that are not easily reproducible in smaller-gamut spaces such as CMYK. In addition, the channel equalization move that I demonstrate later in this column can add noise, so this is something to watch carefully. Then again, there are many ways to add problematic elements to digital imagery—so if there are problems related to LAB this is certainly not unique.
With the preliminary issues out of the way, it is time to have some fun with LAB!
Take a look at the studio shot of radishes, taken on a black velvet background, shown in Figure 4.
Figure 4: Studio shot of radishes on a black background.
Suppose you need to convert the radishes from red (more accurately, magenta) to green. By the way, I’m not really sure why you’d want to do this. But trust me, in my years at the frontier and intersection of commercial photography, fine art photography, and digital manipulation I’ve had many weird requests that made even less sense than green radishes.
In LAB color, this is an easy move that doesn’t involve any selecting. In “real life” you may need to make a selection before adjusting colors to make sure you don’t change colors that shouldn’t be altered.
Open the channels palette. Make sure the A channel is active (as indicated by the blue selection shown in Figure 5). With all the channels visible (this is indicated by the eyeball icons in the left-most column shown in Figure 5), choose Image > Adjustments > Invert.
Figure 5: The LAB A channel is selected in the Channels palette.
That’s all there is to it! With a single LAB move, your radishes are now mostly green with a hint of cyan.
Figure 6: Following the A-channel inversion, the red radishes are green.
The functional equivalent of the Image > Adjustments > Invert command applied to the A channel is to reverse the curve applied to the A channel using the Curves window. This is often a preferred technique because there’s a great deal of flexibility. Small adjustments to the curve lead to huge variations of the color in the image.
To reverse the colors in the A channel using curves, open the Curves window by selecting Image > Adjustments > Curves. Next, choose the A channel in the drop-down list in the Curves dialog (see Figure 7). The normal curve for the A channel will go from green at the lower left to red at the upper right (of course, it is technically a “curve” even though the line is apparently straight).
Figure 7: The “normal” curve for the A channel is shown, going from green to magenta.
To reverse the curve, drag the end-points of the A-channel curve from bottom to top (at the green end of the graph) and from top to bottom (at the magenta end of the graph). Viola! Once again you have successfully created an abomination of nature, green radishes.
Figure 8: “Reversing” the curve reverses the colors in the channel.
In my creative work, I tend to find swaps—another word for inversion—that involve luminance information more interesting than simply reversing the curve of the A or B color channel. For example, the high-key photo of flowers on a white background shown in Figure 9 is pretty cool as it stands. Let’s see what happens when you perform simple LAB adjustments on the overall image and on the LAB channel.
Figure 9: I created this high-key image of poppies by photographing straight down, using a light box for the background.
To invert the overall image, make sure that all three LAB channels are selected in the Channels palette as shown in Figure 10.
Figure 10: Make sure all the LAB channels are selected.
With all the LAB channels selected, choose Image > Adjustments > Invert. All color values in the image will be swapped with their LAB color antagonist counterpart. As you can see in Figure 11, this turns the white background black and the delicate red and white flowers blue.
Figure 11: With a complete LAB inversion, the white background becomes black and the flowers turn blue.
It’s probably a more useful starting place for creative work in the Photoshop darkroom to see what happens when you invert the L channel (as opposed to all the channels). To do this, make sure the Lightness channel is selected in the Channels palette with all three channels visible as shown in Figure 12.
Figure 12: If you select only the L channel, you can swap whites and blacks.
Once again choose Image > Adjustments > Invert. This time the lightness values in the image are swapped, with white becoming black and black becoming white (see Figure 13).
Figure 13: Inverting the L channel puts somewhat more realistic flowers on a black background.
A good step to take with this kind of luminance inversion is to use a curve to lighten the image and bring out details in the flowers as shown in Figure 14.
Figure 14: Adjusting the L Channel curve provides more detail in the flowers.
Inverting the L channel of a high-key photo is a great way to create images that are apparently on black. However, typically it takes a bit more work to come up with a final image that I’m happy with. This involves using a number of LAB adjustments as individual layers and blending them together. I’ll explain a bit more about the technique later in the article, but for now keep in mind that the finished version of the poppies on black, processed to emphasize transparency of the flower petals and shown in Figure 15 combines eight LAB variations as layers.
Figure 15: The finished image of poppies on black started with an L-channel inversion.
Note: Future Photo.net articles will cover background inversion techniques in detail and explain how to best use creative blending modes.
Equalization is an adjustment that spreads out the values in a color channel. When you consider the nature of the LAB color-opponent scheme, you can see that equalization of an LAB channel might lead to some pretty interesting effects. Essentially, equalizing a LAB channel exaggerates the color values that are already there.
To apply an equalization adjustment, select a LAB channel (or channels). Next, choose Image > Adjustments > Equalize.
Starting with the Poppy image on white shown back in Figure 9, the results of equalizing each of the L, A, and B channels are shown in Figure 16 – Figure 18.
Figure 16: Equalize adjustment applied to the L Channel.
Figure 17: Equalize adjustment applied to the A Channel.
Figure 18: Equalize adjustment applied to the B Channel.
I’m sure you can see the fantastic color opportunities that LAB equalization adjustments can provide, given appropriate imagery.
Blending Layers for More Creative Goodness
Although many LAB color moves can be gorgeous, and useful, most of my work in the Photoshop darkroom isn’t finished with one or even two LAB moves. I use a variety of blending modes in the Layers palette combined with masking to combine my creative adjustments in striking ways.
For example, creating the apparently transparent poppy petals shown in the final version of the poppies on black in Figure 15 took the five different LAB adjustments blended as layers shown in Figure 19.
Figure 19: LAB adjustments can be combined as layers using different blending modes.
The combination of LAB adjustments and effective use of layer blending modes is a one-two knockout punch. You can use these techniques to create a variety of interesting effects ranging from the subtle rendition of the Anemone shown in Figure 20 to the surreal view of an ancient Bristlecone Pine shown in Figure 21.
Figure 20: This version of an image of an Anemone was created starting with an L channel inversion with other LAB variations layered on top.
Figure 21: To create this effect, I used several different LAB adjustments, combined using the Overlay, Difference, and Exclusion blending modes.
You can learn more about the in-depth specifics of working with LAB color in my book The Photoshop Darkroom: Creative Digital Post-Processing (Focal Press). I particularly suggest you have look at the spread on blending modes on pages 70-71, the extended case study explaining how to work with LAB creatively starting on page 154, the chart showing the impact of different LAB adjustments on pages 168-169, and LAB landscape enhancement on pages 193-194.
- Explained that LAB has a wider gamut of colors than any of the other available color spaces
- Showed you the structure of the color-opponent LAB channels
- Explained how to convert to LAB color with the correct options set
- Showed you how to “swap” opponent colors, such as red and green
- Introduced the curves adjustment, applied to individual LAB channels
- Showed you how to invert an entire image in LAB
- Explained working the L channel inversions
- Showed you how to work with LAB channel equalizations
- Gave examples of some of the creative possibilities available in Photoshop when you use blending modes to combine several LAB adjustments
Other Photoshop Tutorials
Creativity in the Photoshop Darkroom by Harold Davis: Multi-RAW Processing | Creating HDR Images by Hand [Part I] | Creating HDR Images by Hand [Part II] | Sharpening in LAB Color | Converting to Black and White
Harold is the author of The Photoshop Darkroom: Creative Digital Post-Processing (Focal), Creative Composition: Digital Photography Tips & Techniques (Wiley), Creative Night: Digital Photography Tips & Techniques (Wiley), Creative Close-Ups: Digital Photography Tips & Techniques (Wiley), Practical Artistry: Light & Exposure for Digital Photographers (O’Reilly Digital Media) and other books. Harold gives frequent digital photography workshops, many under the auspices of the Point Reyes National Seashore Association.
Text and photos ©2010 Harold Davis.
Text and photos ©2010 Harold Davis.