Flattening and Merging Layers in Photoshop

Being the true layers evangelist that I am, I have spent the last two chapters touting the glories of layers. And they are wonderful. But they do have a dark side. They can make your file go from slim and trim to bulky and bloated. A consequence of a larger file size is that it slows your computer system performance. And you are limited in the types of file formats that allow you to save layers. You can preserve layers only in Photoshop’s native format (.psd), TIFF (.tif), and PDF (.pdf). If you save your file in any other format, Photoshop smashes your layers down into a background. This file limitation often forces users to save two versions of every layered file — one as a native Photoshop file and one as something else, such as EPS or JPEG, to import into another program.

Merging layers combines visible, linked, or adjacent layers into a single layer (not a background). The intersection of all transparent areas is retained. You can merge layers or layer sets. You can also merge adjustment or fill layers but they cannot act as the target layer for the merge. Merging layers can help to decrease your file size and make your document more manageable. Flattening an image combines all visible layers into a background. Photoshop deletes hidden layers and fills any transparent areas with white.
Flattening is usually reserved for when you’re completely finished editing your image. When you convert an image from one color mode to another, it may cause the file to flatten. Look out for the warning dialog box that prompts you of this result and go back and save a copy of your file as a native Photoshop file, thereby preserving your layers.

Merging layers
You can merge your layers several ways. Here’s the first option:
1. Ensure that all the layers (and layer sets) you want to merge are visible.
Note that Photoshop prompts you with a dialog box informing you that all hidden layers will be discarded. Click OK.
2. Choose Merge Visible from the Layers palette pop-up menu or the Layer menu.
All visible layers are smushed into a single layer. The newly merged layer takes on the name of the topmost layer. To merge visible layers without discarding your hidden layers, press Alt (Option on the Mac) when choosing Layer-->Merge Visible. You may also want to check out the “Alternative methods for merging layers” sidebar.

Alternative methods for merging layers
Because merging layers is such an important aspect of your Photoshop experience, I thought you’d want to know that (as with other Photoshop options and tools) there’s more than one way to merge layers. In fact, here are two sets of steps to help you get the job done. I know, I know, I’m such a giver. Try this method to merge layers:
1. Link all the layers and layer sets you want to merge.
2. Choose Merge Linked from the Layers palette pop-up menu or the Layer menu.
Photoshop combines all linked layers into a single layer. The newly merged layer takes on the name of the topmost layer. Or you can try this method:
1. Position the layer or layer sets you want to merge adjacent to each other in the Layers palette.
2. Select the top layer of those you want merged.
3. Choose Merge Down from the Layers palette pop-up menu or the Layer menu. If the top layer is a layer set, the command is called Merge Layer Set.
Note that Merge Down merges your active layer with all layers below it, so make sure that every layer underneath your active layer should be merged.

Stamping layers
Photoshop also allows you to stamp your layers. Stamping merges layers onto a target (or active) layer (you can leave some layers untouched). Stamping layers is similar to using the Merge Visible command while holding down the Alt (Option on the Mac) key.
To stamp layers, follow these steps:
1. Move the layer you want to stamp to above the layer you want to stamp from. Both layers must be visible.
2. Select the top layer and press Ctrl+Alt+E (Ô+Option+E on the Mac).
You can also stamp linked layers by selecting one of the linked layers and choosing the same command.
If you want to stamp all visible layers, select the layers (or layer sets) and press Shift+Ctrl+Alt+E (Shift+Ô+Option+E on the Mac). You may need to borrow a friend’s fingers for that keyboard combo.

Flattening layers
To flatten an image, follow these steps:
1. Ensure that all the layers you want to retain are visible.
Remember that Photoshop will discard all hidden layers.
2. Choose Layer-->Flatten Image, or choose Flatten Image from the Layers palette pop-up menu.
Note that Photoshop doesn’t prompt you with a warning like, “Are you sure you really want to do this?” But if you mistakenly flatten your image, you can undo the command by immediately choosing Edit-->Undo. If you’ve gone ahead and performed another action, then undo your mistake by using the History palette.