Lab Color Pyrotechnics

I’m sure you’ve heard the term “Lab” thrown about more than once. The September 2006 issue of Digital Graphics magazine featured my article about the advantages of sharpening and enhancing Lab images in Photoshop. This article focuses on a couple of techniques that take Lab color a few steps further. If you missed that article, here is a brief explanation of how this versatile color mode works.

LIGHTNESS AND COLOR

Lab is more easily understood by comparing it to other color models. Like RGB, Lab images contain three color channels. Unlike RGB the information is divided into an L channel and two color; a and b. The pixels in the L channel are assigned any of 100 potential levels of brightness with dark values represented by lower numbers and light values by higher numbers. Absolute black equals 0 and the pure white equals100.

The a channel contains a spectrum of 256 shades of magenta and green. Negative values between -128 and -1 indicate green while positive values between 0 and 127 indicate magenta. The b channel represents yellow and blue. Values between -128 and -1 indicate blue and positive values between 0 and 127 indicate yellow. (See Figure 1)

Figure 1: A diagram of the Lab color model shows that Lab color has three channels—an L representing Lightness an a channel representing magenta and green, and a b channel representing blue and yellow.

Unlike RGB color that configures information into color that devices like scanners, monitors and digital cameras can produce, or CMYK which is a break-down of the density colored ink mixtures, Lab describes all of the colors visible to the human eye and is device independent. It was developed by the International Commission on Illumination (Commission Internationale d'Eclairage) in the 1930s and is sometimes referred to as CIELAB.

Why is using Lab mode advantageous? Because having the access to the lightness information separated from the color information enables you to control each independently.

CREATING IMPRESSIONIST EFFECTS WITH LAB COLOR

Because the colors in Lab mode can vary significantly from the more natural looking RGB, some very nice impressionist effects can be created with Lab color. The technique described here in Photoshop creates a soft grainy image with pastel colors. This effect is achieved by converting a copy of the Lightness channel into what looks like an ink drawing, adding noise to it and then using it as a mask. You can control the effect to a degree by varying the amount of noise. The effect changes the color relationships to produce a grainy pastel like image that has a lot of graphic appeal.

1) Open an RGB image. Choose Image > Mode > Lab Color to convert the image to Lab mode.

2) Press Cmnd/Cntrl-A to select the image and then Cmnd/Cntrl-C to copy it to the clipboard.

3) In the Channels palette, click on the Lightness channel and drag it to the New Channel icon to make a copy of it, as in Figure 2.

4) With the copy of the Lightness channel active choose Image > Adjustments > Brightness and Contrast. Click and drag the contrast slider to the left to -60 to decrease the contrast.

5) Click on the Lightness channel. Choose Filter > Stylize > Find Edges to make the edges of the channel distinct as in Figure 3.

6) Drag the Lightness Copy channel to the load selection icon.

7) Choose Filter > Noise > Add Noise. Enter an amount. The higher the resolution the more noise will be needed. For distribution click the Uniform radio button. (See Figure 4)

8) Deselect. Click on the Lightness channel, copy and choose Image > Adjustments > Invert.

9) In the Channels palette click the Lab channel. Drag the Lightness Copy to the Load Channel as Selection icon.

10) Drag the inverted Lightness Channel copy to the Load Selection icon again.

11) Choose Filter > Noise > Add Noise.

12) Choose Edit > Paste Into to partially restore the original image.

13) In the Layers Palette adjust the Opacity and choose Blend mode for the new layer that was created further enhance the picture as in Figure 5.

Figure 2: An altered copy of the Lightness channel is used as a mask for applying filters.	Figure 3: The Lightness channel is modified with the Find Edges filter to look like a pencil sketch.
Figure 4: Noise is added through the mask to soften the look of the image.	Figure 5: The opacity and blend mode of the layer enhance the final result.

BLACK AND WHITE TO COLOR IN LAB

This technique is to say the least, an unusual approach to coloring black and white images. It requires the channels palette and three tools from the same flyout: the Dodge, Burn and Saturate tools. It is a good exercise in how Lab color can vary on each channel. Painting with the Dodge and Burn tools directly on all the a channel which contains magenta and green and the b channel which contains cyan and yellow can produce all the hues. Working the L channel with the Dodge and Burn tools will control the darkness and lightness of colors. Applying the Sponge tool to the Composite channel will saturate or desaturate the colors. Modifying the exposure of the Dodge and Burn tools or the Flow of the Sponge tool precisely controls the intensity of the effect.

The table below lists the affect these tools have on the various channels:

Effect of Dodge Burn and Sponge Tool on Lab Channels

Tool	Channel	Result
Dodge	a	Changes colors towards magenta
Dodge	b	Changes colors towards yellow
Dodge	L	Lightens values
Dodge	Lab	Lightens values
Burn	a	Changes colors towards green
Burn	b	Changes colors towards blue
Burn	L	Darkens values
Burn	Lab	Darkens values
Sponge	a	No effect
Sponge	a	No effect
Sponge	a	No effect
Sponge	b	No effect
Sponge	L	No effect
Sponge	Lab	Saturates/Desaturates

The advantage of this technique is its directness. You work directly on the image with manual tools to produce colorful if not slightly unnatural results. This example shows how the technique works on a portrait. I recommend that you set the exposure in the Dodge and Burn Options bar or the Flow of the Sponge tool to relatively low values and make multiple passes for more control.

1) Start by opening a grayscale image. (See Figure 6) Convert the image to Lab mode. The a and b channels will be consistently 50% gray or have Lab values of 0. Click on the a channel in the Channels palette to target it.

2) Click on the visibility icon of the Composite channel to display the image in the image window as in Figure 6. Choose the Dodge tool from the Tools palette. In the Options bar set the opacity to 10% or less and the range to midtones. Note: When using the Dodge, Burn or Sponge tools, tools set the range to Highlights if you are working onlight areas, Midtones for medium areas, or Shadows for dark areas.

3) Choose a soft brush from the Brushes palette. Click and drag over the face with the brush. You can see the gray shift towards magenta. (See Figure 7)

4) Click on the b channel paint over the face again. As you add yellow to the face the flesh tones start to emerge.

5) Click on areas you want to paint blue. Choose the Burn tool from the Tools palette. Set the Opacity to 10% Paint on the b channel with the Burn tool. The areas become blue. Make multiple passes until you achieve the hue you want. If the color becomes too bright, dilute it by painting on the area with the Dodge tool.

6) If there is an area you want to paint green, choose the Burn tool and paint on the a channel.

7) Target the Lab channel. Choose the Sponge tool from the Tools palette. In the Options bar, choose Saturate and set the flow to 15%. Drag over the areas you would like to intensify. Choose Desaturate to tone down areas.

8) Target the L channel. With the Dodge tool drag over the areas you would like to lighten or the with the Burn tool, drag over the areas that you would like to darken.

When you’re finished dodging, burning and desaturating, target each of the color channels and gently apply the Gaussian Blur filter to soften the colors a little. Figure 8 shows the results.

Figure 6: A grayscale image is opened and converted to Lab mode.

Figure 7: Dodging or burning the a or b channels produce color.

Figure 8: The final image is a vivid example of how Lab can be used to colorize an image.

Converting your images to Lab color mode certainly has it’s advantages as far as the aesthetic application of color goes. The examples presented in this article illustrate techniques that extend the power of Photoshop. There are other advantages to working in Lab color that are less exotic but extremely useful like the ability to sharpen or reduce noise or enhance color. It’s advisable to convert your image back to RGB before you print which won’t affect the quality of the image but will accelerate your printer’s processing time.

Sign & Digital Graphics

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