One of the ways that Photoshop can help blend multiple images together is through using the “Blend If” sliders in the Blending Options of the Layer Styles dialog. In this example I want to blend the clouds from the first image into the sky of the rock image.
With the Cloud image selected on the Layers panel, I choose layer > Layer Style > Blending Options (or, you can use the fx icon at the bottom of the Layers panel and choose Blending Options…).
In the Layer Styles dialog, I moved the black slider for the Underlying Layer to the right to hide the dark foreground values of the cloud image. In order to create a smooth transition, Option + (Mac) | Alt + (Win) click and drag the black triangle to split it into two. The values to the left of the split triangle will be completely transparent the values between the split triangle will transition from transparent to opaque, and the values to the right of the second split triangle will be fully opaque.
It’s OK if the rock in the foreground is semi transparent at this point – you want to focus on the “transitional areas” – where the new sky (the clouds) will meet the ocean and the top of the rocks.
To bring back the solid rocks in the foreground, I made a copy of the rock layer and moved it above the new sky (the clouds) layer in the Layers panel. Then, I added a layer mask and painted with black to hide the drab sky and reveal the clouds below, while keeping the rocks solid.
Because this example has a fairly straightforward horizon to mask, you might feel that I’m making this process or technique overly complicated. However, the Blend if sliders can be tremendously useful when masking detailed objects such as a tree against a sky. Notice that you can even change the Blend If options to blend individual color channels.
When working with Layer Masks, the Properties panel can be used to non-destructively soften the edge of the mask enabling it to be re-edited at a later time. This can be very useful if the size of the original file or even layer needs to be changed in the editing process.
To add a non-destructive feather, select the desired layer mask on the Layers panel. Then, on the Properties panel, adjust the Feather slider as needed. Although this feature was in Photoshop CS5 (via the Masks panel), in Photoshop CS6 the Feather values support decimal places.
Note: in Photoshop CS6, the Marquee, and Lasso tool’s feather values (in the Options bar), support decimal places as well.
When adding a mask to a layer in Photoshop, instead of guessing at the size of a feather that should be applied (because we know that the feather amount will vary depending on the resolution of the image and the desired softness of the edge that is being feathered), use the non-destructive Feather option on the Properties panel. To access the mask properties, click on the mask icon on the Properties Panel. Now, you can add a feather to soften an edge yet, if the image is resized or other adjustments need to be made at a later time, the feather can be appropriately adjusted.
And while you’re in the Properties panel, and the focus is on the mask, be sure to check out the non-destructive Density slider to reduce the opacity (density) of the mask.
When working on intricately composited, multi layered documents, I often find it useful to check each of the layer masks before finalizing the image. To do this, Option (Mac) / Alt (Win) -click on the mask thumbnail in the Layers panel to display it. With the mask visible, check to see if there are any unwanted awkward transitional areas that might not have been visible in the complex composite (a sharp edge from a selection or gradient for example).
• “Q” will toggle between Quick Mask and Normal Mode.
• Double click the Quick Mask icon on the tool panel to view Quick Mask Options.
• Command (Mac) / Control (Win) + 2 to view the Quick Mask, yet edit the composite image. Note: You can view the Channels panel to see what’s happening under the hood – notice how Command (Mac) / Control (Win) + 2 targets the RGB image instead of Quick Mask and that Command (Mac) / Control (Win) + 6 will target the Quick Mask).
Option-Command (Mac) / Alt-Control (Win) + D is the shortcut to add a feather to a selection. But remember, in Photoshop CS4, if you’re adding a soft edge to a mask, I find that using the nondestructive Feather slider in the Masks panel much more flexible.
After adding a layer mask to hide portions of a layer, it can sometimes be difficult to determine if there are any small bits of the layer that have been accidently left behind. In this case, it might be helpful to temporarily add a layer effect such as a bright red stroke ( Layer > Layer Style > Stroke and click the color swatch to choose a vibrant color) . The stroke will now appear around any small areas of the mask that you may need to clean up. When finished, simply remove the layer effect by dragging the “fx” icon on the Layers panel to the Trash icon).
•Command (Mac) / Control (Win) -click on a layer or layer mask thumbnail in the Layers panel to load it as a selection.
• Command (Mac) / Control (Win) + Shift to add additional layer and or layer mask to the selection.
• Option-Command (Mac) / Alt-Control (Win) to subtract another layer and or layer mask from the selection.
• Option-Command (Mac) / Alt-Control (Win) + Shift to create the intersection of two layers and or layer masks.