Another very good method of making selections that are based on tone is the Calculations tool. This tool works on the channels instead of the layers. Calculations works by blending two channels into a new channel (or the same channel blended into itself).
Figure 1 shows an image that is dominated by two colors. However, it is the difference in tonality of these two colors that is important and that will allow the selection to be made.
In this image, it is desired to select the yellow leaves for editing. As in the case of the Channel Mixer, the channels must first be studied in order to understand the contrast in each of the channels. For this, the Channels palette is selected. Figures 2 -- 4 show the three channels.
An examination of the channels quickly shows that the Blue channel has little contrast between the yellow leaves and the pine forest background. The green channel has some contrast. However, it is the Red channel that has the most contrast. Therefore, the red channel will be used with Calculations.
Calculations creates a new channel named Alpha 1 and places it in the Channels palette (see Figure 7). The new Alpha 1 channel is shown in Figure 8. While this image shows a better separation of the areas to be selected from the rest of the image, the contrast needs to be increased further. This is very easy to do. The Calculations procedure is simply repeated. The only difference this time around is that the Alpha 1 channel is used in Calculations, and the Vivid Light Mode is used since it creates the best contrast enhancement. Figure 9 shows the Alpha 2 channel that is created from the second iteration of Calculations.
The Alpha 2 channel has increased contrast, but even more contrast is needed. One more pass with Calculations is performed for this image (see Figures 10). This pass uses the Alpha 2 channel from the prior pass. Figure 11 shows the Channels palette after the Calculations procedure is complete.
To turn the channel into a selection, the Alpha 3 channel is selected, and the Load Channel as Selection icon at the bottom of the palette is clicked.
Back in the Layers palette, the selection can be seen (see Figure 12). After some clean-up work, the selection is slightly blurred and turned into a mask.
Figure 13 shows the final mask, and Figure 14 shows the final Layers Pallet after the mask was applied to a Curves layer.
The Gradient Tool is a bit different than any of the other tools that have been presented thus far. All of the other tools create a selection. The selection is then turned into a mask. Used in the manner shown in this article, the gradient tool does not create a selection. Rather, it used to directly create a mask.
Figures 15 and 16 show a scene that contained a dynamic range greater than the DSLR could handle. So, two images were taken. The exposure for the first image was set for the foreground, but this exposure blew out some of the detail in the clouds. The exposure for the second image was set for the sky. This exposure produced good sky detail, but it lost detail in the darkest parts of the image. Combining these two images in Photoshop will produce a final image with both foreground and sky detail (keep in mind that the rock in this image is supposed to be very dark since it is in silhouette; however, the water in the tidepools should have detail).
In order to produce and use the mask that will combine the two layers, a layer mask is added to the Sky layer. This is done by selecting the Sky layer and choosing Layer/Layer Mask/Reveal All. The Layers palette now looks as shown in Figure 19.
In order to create the mask, the Gradient tool is chosen from the Tools palette (see Figure 20).
The last controls that need to be set are the left Color Stop (the small rectangular object just below the Gradient Bar at the left end) and the right Color Stop (the small rectangular object just below the Gradient Bar at the right end). These settings are critical. They determine how fast the mask will progress from black to white. The farther apart the two stops are, the more gradual the transition will be. Since the Gradient Tool will be used to create the mask, this will determine how fast the mask will progress from black to white. Subsequently, this will determine how the two images will blend.
To determine the proper Color Stop settings for an image, the image needs to be analyzed to determine how the mask (and thus the gradient) should appear. For the current image, the mask will start a little bit below the horizon. The mask will start off black. Since the mask will be applied to the layer mask of the Sky layer, this will ensure that the detail in the image, where the mask starts, will come from the Background layer. As the mask moves across the ocean and toward the sky, it will become lighter. Thus, the detail in the image will transition from being sourced from the Background layer to being sourced from the Sky layer. By the time the mask reaches the horizon, most of the transition from black to white should be complete (the transition will be less noticeable if it occurs over the dark ocean than if it occurs over the much lighter sky).
To achieve such a mask through the use of a gradient, the left Color Stop is moved all the way to the left (this guarantees that the gradient will start its transition to white right away; if the left Color Stop were moved to the right, the gradient would stay solid black for some distance). Since this image requires that the transition be relatively short (so that the transition is complete by the time the mask reaches the horizon) the right Color Stop must be moved quite a ways toward the left. Figure 24 shows the Gradient Editor after the Color Stops have been set.