When you blend layers or groups using the Blend Mode pop-up in the Layers palette, all the data from the default channels is blended. You can specify the level of opacity for the top layer, but that's about the only control you can exercise.
Suppose that you need not only to specify which layers and channels to blend but also to limit the blending to shadows, midtones, or highlights. The Blend If feature is made for just such a supposition. It works on an "If true, then do" basis. You specify the criteria that have to be met; the feature applies the blends if your criteria are met. For example, if you want to blend only the blue pixels between two layers and restrict the blending between levels 50-150, you can do so. You can also take the harsh edge off the transitional areas by partially blending pixels with specific values. In fact, Blend If is a great feature for blending pixels without having to create masks in order to restrict the blending.
Now, take a deeper look at the feature, starting with a simple blend that knocks out a block of color. Admittedly, you can accomplish more or less the same with the Magic Wand or the Background Eraser, but Blend If gives you the option both of bringing back the deleted color because it was only masked and also of creating a smooth, transitional edge between the masked color and the visible content.
Before calling the Blend If dialog box, select the layer to be blended. Next, choose Layer ^ Layer Style ^ Blending Options, or click the Add a Layer Style button in the Layers palette and then choose Blending Options. When the dialog box opens, you will see two gradient ramps in the bottom part of the dialog box, the Blend If section—the top one named This Layer, which refers to the active layer and the bottom one named Underlying Layer, which refers to not only the layer immediately below the active layer but also any visible layers below it.
Just below the gradient ramps are sliders that control the brightness range of the blended pixels on a scale from 0 (black) to 255 (white). For the white background in Figure 6-13 to be masked, the white slider for This Layer was moved inward to level 240. This action masked all the levels from 240-255 but also created a sharp cutoff at level 240. For a smooth transition, or blend, to be created, the slider was split by Alt-clicking (Windows), Opt-clicking (Mac OS) and the innermost half dragged to level 230. This action effectively created a 10-level transparency ramp from level 240 to 230 that allowed the pixels to blend smoothly.
One upside of using Blend If is that it behaves a little bit like Layer Styles in as much as you can modify the settings or undo them altogether at any time. To do so, simply call up the dialog box again and make any changes. The mask used to make the blend will be altered or deleted and the layer returned to normality.
When you apply Blend If to a layer, you will not see a visible indication to set the layer apart. This lack can lead to confusion. To save frustration at a later date, either color it or add Blend If to the layer name as a reminder.
When you blend data from the layers below the active layer, the feature behaves slightly differently and can lead to confusion or simply a quick exit! It's worth taking a little time to learn how the This Layer and Underlying Layer sliders act on the active layer. Here's how they differ in their approaches.
When you use the sliders for This Layer, data is clipped in the active layer to reveal data below (Figure 6-14). When you use the sliders for the Underlying Layer, the data punches through the active layer. For example, to blend all detail from level 0-50 and to create a smooth zone between levels 40-50, move the black slider inward and stop when you see the figure above the gradient ramp read level 50. Next, Alt-click (Windows), Opt-click (Mac OS) the slider to split it and move the left half to the left until the figure above the gradient ramp reads 40/50.
NOTE The sliders for the underlying Layer affect all underlying layers and not just the one immediately below the active layer.
nOTE| To blend all channels, leave the pop-up menu above the sliders set to Gray (the default) or click it and select a channel (see also "Excluding Channels when Blending," which follows this Note). Furthermore, you can choose a blend mode and opacity in the General Blending section or select it in the Layers palette.
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