Last, but certainly not least, the Blend If options allow you to composite layers based on the numerical values of their pixels. For some images, this means that you can create very convincing composites without having to resort to making intricate selections or layer masks. As with the earlier color channel-blending options, however, these controls can cause much furrowing of the brows when you first encounter them. Let's try this feature using the car and clouds layers:
Set the clouds layer blend mode back to Normal. Double-click the car layer to bring up our old friend, the Layer Style dialog box.
In the lower portion of the advanced blending controls, you'll find the Blend If section. Start with the underlying layer. Click the slider on the right and move it in to about level 200 (see Figure 18). In the image, you'll see that the highlight areas of the clouds are beginning to blend through into the car layer.
Figure 18 With the car layer active, adjusting the Blend If values for the underlying layer (the clouds) causes the bright highlights of the clouds to show through in the car layer.
Like most aspects of digital imaging, this effect is firmly based on numbers. As a result of the adjustment, the underlying layers are showing through wherever the pixels on those layers are brighter than level 200. So, in the white vertical band on the right, those pixels take over almost immediately. Next to blend through are the bright highlight areas of the clouds.
Although this creates an interesting effect, the edges of the blended layers are still somewhat rough and jagged. Fortunately, there's an easy way to create a softer, more natural blend. Notice that the slider you moved is divided into two sections. Hold down the Alt key (PC) or the Option key (Mac), and click the right side of the highlight slider; then drag to the right to separate it from the left half (see Figure 19). This creates a gradual blend between the pixels that do blend through and those that do not. I eventually settled on a spread of 194 to 225. This shows all pixels on the clouds layer that are brighter than level 225 and then gradually shows less until it reaches level 194, when it stops blending pixels from the underlying layers.
Figure 19 Alt-click (PC) or Option-click (Mac) the slider to separate it. This creates a softer, more natural-looking blend of the tones between the two layers.
Set the Underlying Layer slider back to its default starting point and switch to the This Layer slider. Move the highlight slider on the right in and to the left, to about level 155, and you'll see the sky and clouds layer begin to appear in the front end of the old car. With this setting, you are telling Photoshop to not show any pixels on the car layer that are brighter than 155. Because this includes most of the pixels that describe the car, the sky from underneath shows through here.
Try splitting the two halves of the slider to soften the effect. You might also try setting this slider back to its default and working with the left shadow slider. Figure 20 shows an example of the effect I got by selectively hiding the darker pixels on the car layer.
Figure 20 More advanced layer blending using the Blend If sliders in the Layer Style dialog box.
Figure 21 shows another example of this feature, using two images found in the stock art folder on the Photoshop 6 application CD.
Figure 21 Using the Blend If sliders to quickly add a new sky to an image.
The Blend If settings are really one of Photoshop's hidden gems. Like the rest of the advanced blending controls, I suspect that many people ignore them simply because they look and sound somewhat intimidating. Once you take the time to unlock their secrets, however, the rewards can be great. Now that you know a little more about how these settings affect your images, you can use them with more confidence.