Hands on: Using layers in Photoshop

Layer masks
Layer masks are greyscale channels that hide or display pixels on the layer to which they apply, depending on the greyscale value of the mask pixels. Black pixels hide detail, white pixels display it at full opacity. Grey pixels in the mask display layer pixels at varying degrees of opacity, depending on how light or dark they are.

Using a layer mask rather than erasing unwanted detail from a layer gives you more options. If you later decide you want the detail back, you can edit the mask. What’s more you can group and ungroup layer and mask, which means you can move the layer around within the mask or slide the mask around over the stationary layer ­ a useful trick for putting together montages.

Vector layers
Most image-editing applications provide vector drawing tools. Vector images are usually confined to vector layers, which are resolution-independent but are off-limits to many tools and effects designed to work on pixels.

Photoshop’s shape tool offers a variety of geometric shapes as well as a pre-set shapes library, all of which produce a solid filled layer with a vector mask. Text layers are likewise vector based.

Layer opacity
Drag the opacity slider in the layers palette to range from fully transparent to fully opaque. For more control over layer opacity, right-click the layer in the Layers palette and select Blending options. This dialogue box allows you to control transparency in different parts of the tonal range. You can make the highlights transparent so the underlying layer shows through.

Layer styles
Photoshop’s layer styles include drop and inner shadows, inner and outer glows, bevel and emboss, as well as stroke. Layer styles can help you create anything from aqua-style buttons to metallic type. You can even apply Layer styles to type without rasterising it.

Align and distribute layers
One advantage of keeping elements on separate layers is that you can align and distribute them. This is useful for text and for arranging images in a grid.

Layers outside Photoshop
To retain all the layer information in an image, save it in Photoshop’s .psd format. You can also save layered files as compressed TIFs. Layered files can grow large, so while it’s a good idea to keep a layered original for editing; for other uses save a flattened file.

Many applications now support layered Photoshop files, so if you plan to import your work into another image editor, a compositing application or a 3D modelling program, save it as a layered PSD. The importing application may recognise the layer structure but lack support for features such as layer styles and adjustment layers. Depending on how these are handled it’s often better to merge anything problematic prior to saving the file.

Adobe Indesign has good support for layered Photoshop files and allows you to select layer visibility for placed PSDs. This can be useful if you produce a map in Photoshop with multiple overlays displaying different features ­ you only need to save one file then select the relevant overlay when you place the file on the Indesign page.

Layer tips and tricks

Blend mode tip 1
The Multiply blend mode can be used to boost contrast and reduce haze in landscape photos. Open an image and drag the background layer onto the new layer button in the Layers palette to duplicate it. Set the blend mode of the top-most layer to multiply and reduce the layer opacity to soften the effect.

Blend mode tip 2
Use the Difference blend mode to identify differences in processed versions of the same image, for example where noise reduction has been employed or to determine whether the image has been sharpened. Superimpose the two images and set the top-most layer blend mode to Difference. Where overlying pixel values are the same the image is black, otherwise the greater the difference in value, the lighter the pixels.

Adjustment layer tip
You have to be careful when using the Clone Stamp tool on layers that have had layer adjustments applied, as cloned pixels will be sampled in their adjusted state. If you are cloning to a layer above the adjustment layer and you want the cloned pixels to match the source, this is fine. But if you’re cloning to the same layer or another layer below the adjustment layer, the cloned pixels will be subjected to the adjustment layer twice and will not match the source.

The simplest way around this is to do your retouching before applying adjustment layers. Alternatively, turn the adjustment layer off before cloning. Photoshop CS3 has an ignore adjustment layer option precisely to deal with this problem.

Layer mask tip
Use a layer mask with a gradient to apply a graduated effect to a layer. Duplicate the background layer then press Ctrl & Shift & U to desaturate it. Click the Add Layer Mask button on the Layers palette, then select the Gradient tool. Make sure the default background to foreground gradient is selected in the Tool Options palette and the default foreground and background colours are selected (press D), then drag with the gradient tool from roughly a quarter of the way in from the top of the image to a quarter of the way in from the bottom.

Vector layer tip
To apply filter effects to vector and text layers, first duplicate the layer before rasterising it. To rasterise a vector shape in Photoshop select Layer >Rasterize>Layer. Should you need to subsequently edit the type or vector shape you’ll still have the original to work on.