The most direct method for juggling multiple images is "sandwiching." By this I mean placing a heavily filtered version of an image between two originals. This technique is based on the principal that most blend modes — all but Multiply, Screen, Difference, and Exclusion — change depending on which of two images is on top.
For example, Figure 13-13 shows two layers, A and B, and what happens when I blend them with the Overlay mode. When the leaf is on top, as in the third example, the Overlay mode favors the woman; but when the woman appears on the top layer, the Overlay mode favors the leaf.
As I mentioned earlier, the Overlay mode always favors the lower layer. Its opposite, Hard Light, favors the active layer. Therefore, I could have achieved the exact effect shown in the third example of Figure 13-13 by placing the leaf underneath and setting the woman layer to Hard Light. Flip-flop the layers and apply Hard Light to the leaf to get the last example.
Other blend modes have opposites as well. Take the Normal mode, for example. When you apply Normal, whichever image is on top is the one that you see. However, if you change the Opacity, you reveal the underlying image. At 50 percent Opacity, it doesn't matter which image is on top. The color of every pair of pixels in both images is merely averaged. So an inverse relationship exists: If the filtered image is on top, an Opacity setting of 25 percent produces the same effect as if you reversed the order of the images and changed the Opacity to 75 percent.
The other obvious opposites are Color and Luminosity. If I were to position the green leaf in front of the woman and apply Color, the woman would turn green. The same thing would happen if I placed the woman in front and applied Luminosity.
The moral of this minutia is that the order in which you stack your layers is as important as the blend modes you apply. Even filters that have no stacking oppo-sites — Soft Light, Color Dodge, Hue, and others — produce different effects depending on which layer is on top. Just for your general edification, Figure 13-14 and the possibly more enlightening Color Plate 13-2 show a few examples.
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