10.3 Image Layers

As will be explored in much greater detail later in this book, an extremely powerful method of improving the visual impact of bird photos is to process the bird separately from its background, to make the subject stand out more prominently.  More generally, it’s often highly desirable to process different parts of the image differently—for example, to darken one part of an image while lightening another, or to sharpen the subject while blurring the background, etc.  In many cases (though certainly not all), this is best accomplished via the use of image layers.
    Layers in photoshop are a lot like layers in a layer cake.  If you imagine a layer cake in which the layers are different shapes and colors, and if you imagine looking down on the cake from above, you can envision that the top layer will eclipse a certain portion of the layers beneath it, but that some parts of those lower layers may be visible nonetheless, if they
poke out around the edges of the top layer (and any other layers above them). 
    The figure below illustrates this more concretely in the case of bird photography.

Fig. 10.3.1: Layers in Photoshop.  Separating elements of the image
into different layers is convenient because it affords much greater
efficiency when applying effects intended for only one part of the
image.  Merging the layers then produces a complete image.

In this case, we’ve partitioned the original image into two layers: layer 2 contains all of the pixels making up the bird, while layer 1 contains everything else.  If you were to merge these two layers together, you’d get back the original image.  The reason it’s useful to separate image elements into separate layers is that it makes it very convenient for when you want to apply any sort of digital manipulation (e.g., adjusting the sharpness, exposure, saturation, contrast, etc.) to just the bird, or just the background, or to some other important element in the photo which naturally stands by itself or perhaps simply requires the most extensive processing.
    Making layers is easy in Photoshop.  Once you’ve got some important part of the image selected (selection is discussed section 10.6), you can make a new layer containing just that part of the image by pressing Cmd-J on a Macintosh computer (Ctrl-J on a Windows PC).  A new layer will be created and can be selected by clicking on it in the layers panel (typically located near the lower-right part of the screen).  The figure below shows a typical layers panel.  In this example we’ve created four layers: a text layer (for signing the photo), a cloud layer (for adding some artifical clouds in the background), a background layer, and the full image which includes the subject (named here
Background because that’s the default name assigned by Photoshop to the original layer comprising the entire image).

Fig. 10.3.2: The Layers panel in Photoshop.  Layers can be
renamed, moved higher or lower in the stack, toggled off
and on, given layer masks that make parts of a layer
transparent, blended with other layers in various ways,
and made semi-transparent to any integral degree.

     The layers panel has a number of powerful features.  First, notice the eyeball icons to the left of each of the four layers in the figure above.  Clicking on any of these will cause that layer to toggle between the visible and invisible states.  This is especially useful for special-effect layers, because you can repeatedly toggle the layer on and off until you decide whether the image looks better with or without that effect.  In the upper right corner of the layers panel you’ll see that there’s an opacity setting, which can be set differently for each layer.  When the opacity is less than 100%, the layer becomes semi-transparent; this is useful for special-effect layers that might look overly gaudy or artificial at 100% opacity, in which case a lower opacity might produce a more tasteful degree of subtlety.  The blending mode (upper left corner of the panel) is a powerful tool that I almost never use, but that can come in handy in rare cases. 
add a mask button along the bottom of the panel creates what we call a layer mask, which shows up as an additional icon in the layer bar.  This is a very powerful tool which takes some experience to master but can save considerable time once you’re comfortable using it.  The mask is a pattern that determines which parts of the layer are visible, and which are (fully) transparent.  You can select the mask and paint on it using the brush tool (section 10.1).  Any region of the mask that you paint in black causes the corresponding part of the layer to become transparent.  The nice thing about masks is that if you make a mistake, you can easily fix it by switching your brush color between black and white and re-painting that part of the mask.  You can also use shades of gray to indicate partial transparency.  Masks facilitate one form of nondestructive editing, in which edits that you made previously can be easily changed later by simply re-painting parts of the mask.  They’re especially useful for blending only part of a layer with other visible layers, by painting the mask in various shades of gray (such as via the gradient tool—section 10.1).
    The figure below shows just one of the many useful things you can do with layers.  In this case, we’ve created a special layer with artificial clouds rendered by Photoshop’s built-in cloud effect.  The top image shows the result of enabling the cloud layer, while the bottom image shows the result of disabling the cloud layer.  By repeatedly toggling the layer on and off, the pros and cons of utilizing the layer in the final image can be assessed visually without having to rely too much on imagination.

Fig. 10.3.3: Assessing a special effect via layer toggling.  Top: an
image in which an artificial cloud layer is toggled on.  Bottom: the
same image with the cloud layer toggled off.  The ability to toggle
layers like this on and off significantly eases the task of assessing
the aesthetic value of various processing options.

    Note that images that contain layers should ideally be saved in Photoshop’s proprietary PSD format, to retain the maximal amount of information.  In order to export the image to JPEG you’ll need to flatten the image (either permanently or temporarily), which simply means that you need to merge the layers into a single composite image.  I recommend keeping both the original RAW file from the camera and the PSD file containing any layers you’ve created, in additional to any flattened JPEG’s you’ve extracted from the PSD file.  The JPEG’s are useful for posting images on the internet or for making paper or canvas prints.  The PSD’s are useful if you need to touch-up some part of the image (e.g., if after printing the image you find that the bird doesn’t stand out from the background enough), or if you need to extract additional JPEG’s at different resolutions.  The RAW files should, in my opinion, never be deleted, since you never know when you might find some flaw in a processed image that can only be fixed by going back to the RAW image and re-processing it from scratch.
    One disadvantage of the use of layers is that each additional layer requires more memory (and hard drive space) to store, and can slow down your computer if you use too many of them.  This problem can be partially alleviated by installing more RAM (memory) into your computer, though disk space remains an issue.
    As we’ll see in section 10.6, a simpler, quicker, and somewhat less powerful alternative to layers is the use of saved selections.  The ideal balance between the use of these various techniques (i.e., layers versus saved or de novo selections) is one which each photographer needs to find based on his or her own postprocessing style and hardware.