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Adjustment Layers

Every one of the commands I’ve discussed in this chapter is applicable to a single layer at a time. If you want to correct the colors in multiple layers, you have to create a special kind of layer called an adjustment layer. Adjustment layers are layers that contain mathematical color correction information. The layer applies its corrections to all layers beneath it, without affecting any layers above.

You can create an adjustment layer in one of two ways:

Choose Layer New Adjustment Layer. This displays a submenu of color adjustment commands, ranging from Levels and Curves to Invert, Threshold, and Posterize.

Click the half-black, half-white circle at the bottom of the Layers palette, as shown in Chapter 14. Choose any one of the remaining 12 options to make a new adjustment layer.

Figure 17-40: Click the black-and-white circle to display a pop-up menu of dynamic fill and adjustment layers.

If you choose a command from the Layer New Adjustment Layer submenu, Photoshop displays the New Layer dialog box, which permits you to name the layer, assign a color, and set the blend mode. You can also group the adjustment layer with the currently selected layer, which means that the adjustment layer will affect only that layer. If you choose an option from the black-and-white circle in the Layers palette (labeled in Figure 17-40), Photoshop bypasses the New Layer dialog box and heads straight to the selected correction. Choosing Curves, for example, displays the Curves dialog box. (Invert is the only option that produces no dialog box whatsoever.) Change the settings as desired and press Enter or Return as you normally would. If you hold down Alt or Option while clicking the adjustment layer icon in the Layers palette, you’ll get the New Layer dialog box just as if you’d chosen from the menu commands.

Regardless of the color adjustment you select, it appears as a new layer in the Layers palette. In Figure 17-41, for example, I’ve added a total of three adjustment layers. Photoshop marks adjustment layers with special icons that look like miniature versions of their respective dialog boxes. This way, you can readily tell them apart from image layers. As you can also see in Figure 17-41, adjustment layers automatically come with accompanying layer masks. You can take advantage of this fact (as in the top adjustment layer), ignore the mask (as in the bottom adjustment layer), or delete the mask by dragging it to the trash can icon (as I’ve done with the middle adjustment layer). More on those layer masks in a minute.

Figure 17-41: Here I’ve created three layers of color correction in front of a single background image.

The advantages of layer-based corrections

At this point, you might think, “Big whoop. You can apply corrections to multiple layers. That doesn’t seem like such a great feature.” But lest you judge too hastily, here are a few reasons adjustment layers are so great:

Forever editable: As long as the adjustment layer remains intact, stored in one of the three formats that support layers (native Photoshop PSD, TIFF, or PDF), you can edit the color correction over and over again without damaging the underlying pixels. Unlike standard color corrections, which alter selected pixels directly, adjustment layers have no permanent effect on the pixels. On the slightest whim, double-click the adjustment layer icon to bring up the color correction dialog box, complete with the settings currently in force. Tweak the settings as the mood hits you and press Enter or Return to make changes on-the-fly. Toggle the visibility of the adjustment layer by clicking the eyeball icon for the layer. You can’t get any more flexible than this.

Versatile layer masking: You can likewise adjust the affected area to your heart’s content. Unless it’s grouped with another layer, an adjustment layer covers the entire image like an adjustable wall-to-wall carpet. You modify the affected area by painting inside the layer. An adjustment layer doesn’t contain pixels, so painting inside the layer changes its layer mask. Paint with black to remove the correction from an area; use white to paint the correction back in.

Reorder your corrections: As with any layers, you can shuffle adjustment layers up and down in the stacking order. For example, if you decide you don’t want the correction to affect a specific layer, just drag the adjustment layer to a level in the Layers palette below the layer you want to exclude. If you’re juggling multiple adjustment layers, as in Figure 17-42, you can shuffle the adjustment layers to change the order in which they’re applied. This includes the standard reordering keyboard shortcuts, Ctrl+[ and Ctrl+] (z -[ and z -] on the Mac).

Fade corrections: You can fade a standard color correction right after you apply it using Edit Fade. But you can fade a correction applied with an adjustment layer any old time. Just change the blend mode and Opacity settings in the Layers palette.

Correct using blend modes without ballooning file sizes: Some folks prefer to correct overly light or dark images using blend modes. Take an image, copy it to a new layer, and apply the Multiply mode to darken the layer or Screen to lighten it. The problem with this trick is that it increases the size of the image in memory. Duplicating the image to a new layer requires Photoshop to double the size of the image in RAM.

Change one adjustment to another: After applying one kind of adjustment layer, you can convert it to another kind of adjustment layer. For example, you could swap an existing Levels adjustment for a Curves adjustment. To do so, choose the desired color adjustment from the Layer Change Layer Content submenu. Photoshop doesn’t try to preserve the prior color adjustment when making the conversion — in other words, it can’t convert the Levels information into Curves data — but it does preserve the layer mask, the blend mode, and other layer attributes.

Correcting a flat image using layers

Although many artists use adjustment layers to edit multilayer compositions, adjustment layers are equally applicable to flat photos. Originally printed in the February, 1996 issue of Macworld magazine, Figure 17-42 shows how I corrected an image shot with a Polaroid PDC-2000 using a total of three color corrections, layered one on top of the other. (Much of the following text also comes from that same article.) At first glance, the original photo on the left side of the figure is a textbook example of what happens if you ignore backlighting. But as we saw in “The Shadow/ Highlight command” section earlier in this chapter, an image that appears black may actually contain several thousand colors just itching to get out. Adjustment layers make it easier than ever to free these colors and make them fully visible to the world.

Because my image was in such rotten shape, I decided to start with the Curves command. I clicked the black-and-white circle button and selected Curves from the pop-up menu. Then I used the pencil tool in the Curves dialog box to draw a radical upswing on the left side of the graph, dramatically lightening the blacks right out of the gate. I clicked the Smooth button a few times to even out the color transitions, as demonstrated in the second example in Figure 17-42.

All this lightening resulted in some very washed-out colors (a typical side effect), so I created a second adjustment layer using the Hue/Saturation command. By raising the Saturation value, I quickly breathed a little enthusiasm into those tired oldhues — a sufficient amount, in fact, to make it clear how soft the focus was. So I went back to the original image layer and applied the Unsharp Mask filter. Had it not been for the advent of adjustment layers, I would have had to sharpen the image either before color correcting it, making it impossible to accurately gauge the results, or after correcting it, which might bring out compression artifacts and other undesirable anomalies. With adjustment layers, however, I can sharpen and correct at the same time, giving no operation precedent over the other.

The hedge monster remained a little dark, so I selected it with the Color Range command and then created a third adjustment layer for the Levels command. Using Levels, I quickly enhanced the brightness and contrast of the green beast, bringing him out into the full light of day. As I mentioned earlier, Photoshop automatically generated a layer mask for my selection, which appears as a tiny white silhouette in the Layers palette.