The Clouds filter creates an abstract and random haze between the foreground and background colors. Difference Clouds works exactly like layering the image, applying the Clouds filter, and selecting the Difference blend mode in the Layers palette.
Why on earth should Difference Clouds make special provisions for a single blend mode? Because you can create cumulative effects. Try this: Select blue as the foreground color and then choose Filter ^ Render ^ Clouds. Ah, just like a real sky, huh? Now choose Filter ^ Render ^ Difference Clouds. It's like some kind of weird Halloween motif, all blacks and oranges. Press Ctrl+F to repeat the filter. Back to the blue sky. Keep pressing Ctrl+F over and over and notice the results. A pink cancer starts invading the blue sky; a green cancer invades the orange one. Multiple applications of the Difference Clouds filter generate organic oil-on-water effects.
Tip To strengthen the colors created by the Clouds filter, press Shift when choosing the command. This same technique works when using the Difference Clouds filter as well. In fact, I don't know of any reason not to press Shift while choosing one of these commands, unless you have some specific need for washed-out effects.
Color Plate 11-10 shows some entertaining applications of the Clouds filters. With the foreground and background colors set to blue and orange, respectively, I applied the Clouds filter to a layered copy of the rose image. For maximum effect, I pressed Shift and chose the filter to create the top-left image in the color plate. I then pressed Shift and chose the Difference Clouds filter to create the purple montage in the figure, and pressed Ctrl+F ten times to achieve the top-right image. Looks to me like I definitely have something growing in my petri dish.
Yeah, so really groovy stuff, right? Shades of "Purple Haze" and all that. But now that I've created this murky mess, what the heck do I do with it? Composite it, of course. The bottom row of Color Plate 11-10 shows examples of mixing each of the images from the top row with the original rose. In the example on the left, I chose the Overlay option from the Layers palette. In the example in the middle, I chose the Screen mode. And in the last example, I chose Hue. This last one is particularly exciting, completely transforming the colors in the rose while leaving the gray (and therefore unsaturated) background untouched. Without a mask, without anything but a rectangular marquee, I've managed to precisely color the interior of the rose.
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