Combining masks
As described for the Apply Image command, the Channel popup menus may offer Selection, Transparency, and Layer Mask as options. But here they have more purpose. You can composite layer masks to form selection outlines, selection outlines to form masks, and all sorts of other pragmatic combinations.
Figure 1328 shows how the Calculations command sees selected areas. Whether you're working with masks, selection outlines, transparency masks, or layer masks, the Calculations command sees the area as a grayscale image. So in Figure 1328, the white areas are selected or opaque, and the black areas are deselected or transparent.
Source 1 Source 2
Figure 1328: Two selections expressed as grayscale images (a.k.a. masks). The left image is the first source, and the right image is the second.
Source 1 Source 2
Figure 1328: Two selections expressed as grayscale images (a.k.a. masks). The left image is the first source, and the right image is the second.
Assuming that I've chosen Image ^ Calculations and selected the images using the Source 1 and Source 2 options, the only remaining step is to select the proper blend mode from the Blending popup menu. Screen, Multiply, and Difference are the best solutions. The top row in Figure 1329 shows the common methods for combining selection outlines. In the first example, I added the two together using the Screen mode, just as in the preceding steps. In fact, screening masks and adding selection outlines are equivalents. To subtract the Source 1 selection from Source 2, I inverted the former (by selecting the Invert check box in the Source 1 area) and applied the Multiply blend mode. To find the intersection of the two masks, I simply applied Multiply without inverting.
The Calculations command doesn't stop at the standard three — add, subtract, and intersect. The bottom row of Figure 1329 shows three methods of combining selection outlines that are not possible using keyboard shortcuts. For example, if I invert the Source 1 mask and combine it with the Screen mode, I add the inverse of the elliptical selection and add it to the polygonal one. The Difference mode adds the portion of the elliptical selection that doesn't intersect the polygonal one and subtracts the intersection. And inverting Source 1 and then applying Difference retains the intersection, subtracts the portion of the polygonal selection that is not intersected, and inverts the elliptical selection where it does not intersect. These may not be options you use every day, but they are extremely powerful if you can manage to wrap your brain around them.
Depending on how well you've been keeping up with this discussion, you may be asking yourself, "Why not apply Lighten or Add in place of Screen, or Darken or Subtract in place of Multiply?" The reason becomes evident when you combine two soft selections. Suppose that I blurred the Source 2 mask to give it a feathered edge. Figure 1330 shows the results of combining the newly blurred polygonal mask with the elliptical mask using a series of blend modes. In the top row, I added the two selection outlines together using the Lighten, Add, and Screen modes. Lighten results in harsh corner transitions, and Add cuts off the interior edges. Only Screen does it just right. The bottom row of the figure shows the results of subtracting the elliptical mask from the polygonal one by applying Darken, Subtract, and Multiply, and occasionally inverting the elliptical mask. Again, Darken results in sharp corners. The Subtract mode eliminates the need to invert the elliptical marquee, but it brings the black area too far into the blurred edges, resulting in an overly abrupt interior cusp. Multiply ensures that all transitions remain smooth as silk.
Invert + Darken Subtract Invert + Multiply
Figure 1330: When adding softened selections (top row) and subtracting them (bottom row), the Screen and Multiply modes provide the most even and continuous transitions.
Invert + Darken Subtract Invert + Multiply
Figure 1330: When adding softened selections (top row) and subtracting them (bottom row), the Screen and Multiply modes provide the most even and continuous transitions.
The reason for the success of the Screen and Multiply modes is that they mix colors together. Lighten and Darken simply settle on the color of one source image or the other — no mixing occurs — hence the harsh transitions. Add and Subtract rely on overly simplistic arithmetic equations — as I explained earlier, they really just add and subtract brightness values — which result in steep falloff and buildup rates; in other words, there are cliffs of color transition where there ought to be rolling hills. Both Screen and Multiply soften the transitions using variations on color averaging that makes colors incrementally lighter or darker.
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