Not all transparency is alike. I've already talked about transparency at the document level and the layer level, but there are also many different ways to create transparency, and for certain types of output some techniques are more appropriate than others. For example, you might need GIF transparency for a Web page, but for a national magazine you might need a transparent PSD file or an EPS file with a clipping path. It's always best to start from your output goal to determine which technique to use. Table 13.1, next page, condenses the most common uses of transparency and what to do to get there. In the rest of this chapter I cover the ideas in Table 13.1 more specifically.
Table 13.1. Ways to Apply Transparency
Make a painting tool partially transparent
Set the Opacity value in the options bar for the tool
Make an entire layer transparent
Set the Opacity value for that layer in the Layers palette
Make part of a layer transparent
Paint black in a layer mask, or draw a vector mask
Create transparency based on color combinations between layers
Apply a blending mode
Make a transparent document background for Adobe InDesign[*]
Remove opaque background layer, save as PSD
Make a clipping path
Draw a clipping path, save document as EPS or TIFF
Make a transparent document background for Web pages
Remove opaque background layer, save as PNG or GIF
Permanently delete areas of a layer
Paint a layer with the eraser or background eraser tool
[*] or other programs that can read transparency from a Photoshop-format document
Permanent and Reversible Transparency
In the most general sense, there are two ways to make part of a layer transparent. You can apply a mask or blending mode, or you can simply erase parts of the layer. Whenever possible, an eraser tool (such as the background eraser) should be the last resort. It's better to use a mask or other method that doesn't involve erasing, because erasing is permanent. Once you erase layer content, you can't get it back unless a previous version of the layer exists in the History palette. In contrast, when you apply a mask, a blending mode, or another similar method, it's reversible. With masks, you can restore hidden areas by painting white into a mask instead of black. When you use an eraser tool, you can't change your mind later. If you want to preserve maximum flexibility in your workflow, use masks and avoid the eraser tools.
Opacity isn't just for creating transparency. For example, when you adjust the opacity of an adjustment layer, you reduce its effect on the underlying layers. Sometimes that's easier and faster than adjusting the controls in the adjustment layer's dialog box.