In the old days of photographic imaging—in the time before computers—a mask was a piece of semi-transparent orange film called rubylith that was designed to conceal a portion of an image from the process camera’s eye. Another type of mask was the sheet of frisket film used to shield a photograph from the overspray of a touch-up airbrush. These masks were cut buy an expert who hand-wielded an X-Acto knife to ensure that the exposed areas were precise. All things considered, they certainly performed well in the realm of analog images. But compared to the digital masks of today, manually cut masks seem primitive and limited in their capabilities. 

 

Digital masks are actually a paradox. They simultaneously perform two opposite functions. Some types of masks protect areas from being affected while at the same time contain other areas to be edited.

 

Other types conceal regions from view while at the same time display the other areas. Among Photoshop’s mask-making features are a group of selection tools, channel-based masks to store selections, layer-based masks that conceal and reveal layer content, vector masks that are composed of paths or shapes, type masks that create character based selection outlines, and filter masks that reveal and conceal the effects of filters.

 

Masks are essential to Photoshop’s workflow. Almost every image that is opened and edited is masked. Choosing which type of mask to use will depend on the particular situation. It’s advisable to know what types of masks are available in Photoshop to be able to make an informed decision as to which works best for your particular task. This article describes all of Photoshop’s masks and explains how they are used to accurately isolate regions of an image for precision editing.

 

MASK BASICS

All of Photoshop’s masks share similar characteristics. Depending on the type of mask, and the current viewing mode, most are displayed either as a black and white or gray thumbnail in one of the panels, or as a colored overlay on the image. Masks can be modified with specific tools or commands.

 

Each image opened in Photoshop can present a different set of selection and editing problems. Consequently, the masking techniques built into Photoshop are designed to cover any imaginable situation. Masking can be quite complex and labor-intensive, and sometimes situations arise where one masking technique is not quite sufficient and must be combined with other masking techniques. 

 

MASK TYPES

Eight different types of masks are available in Photoshop:

 

Figure 1: A selection outline is usually made with one of the four selection tools. It surrounds the isolated area with an animated marquee.

Selection Outline: (see Figure 1) The most basic masks are temporary selections that define an area for instant editing. These selection outlines are sometimes called marching ants because the animated marquee looks like a column of tiny insects encircling the selected region. 

 

A selection outline is made with one of the selection tools or with the Color Range command. These outlines enable the region within their boundaries to be edited by a tool or command. The area outside the outline remains protected from editing.

 

Figure 2: An Alpha Channel is a selection that has been saved to the Channels panel.

Alpha Channel: (see Figure 2) This is a fancy name for a mask created by saving a selection. Alpha channels reside in the Channels panel, immediately under the color channels. Alpha Channels can be altered with the painting and editing tools, the Stroke and Fill commands, some of the Filters, transformation commands, and with many of the adjustment features. When an Alpha Channel is loaded, it is converted into a selection outline. An Alpha Channel can contain up to 256 shades of gray. White areas on an Alpha Channel’s thumbnail represent regions on the image that will be selected when the channel is loaded. Black areas represent parts that are protected, and gray areas represent parts that are partially selected. 

 

Figure 3: A Quick Mask is a technique used to edit selections using the painting and editing tools.

Quick Mask: (see Figure 3) The usual method for making a Quick Mask is to make a selection first and then click on the Quick Mask mode icon in the Tools panel (or press the Q key). A red overlay is displayed that defines the protected region of the image. The hole in the overlay represents the selected area when the Quick Mask is converted into a selection outline. To convert the Quick Mask to a selection outline click the Edit in Standard Mode icon in the Tools panel, or press the Q key again. 

 

Quick Masks are very similar to Alpha Channels in that they are grayscales that support up to 256 levels of gray and can be edited with the same tools and commands. The only real difference is that they are temporary. 

 

Figure 4: A Layer Mask conceals regions of the image by painting on the mask with black.

Layer Mask: (see Figure 4) Layer Masks are used to reveal or conceal pixels on their content layer. On an adjustment layer they reveal or conceal tonal and color alterations A Layer Mask is created by choosing Layer > Layer Mask > Reveal/Hide All. If an area on the image is selected prior to making the Layer Mask, choose Reveal > Hide Selection to base the mask on the selected area. A Layer Mask thumbnail resides in the Layers panel to the right of the layer thumbnail, as in Figure 4. 

 

Like Alpha Channels, they support 256 levels of gray. Black areas on the thumbnail represent parts that are hidden, and white areas represent parts that are visible. Gray represents areas that are partially visible. Layer Masks can be edited with the same tools, commands and filters that are used to edit Alpha Channels. Layer Masks make it possible to erase areas from the image without eliminating any content. Areas that are concealed by painting black on the mask can later be revealed by painting those same areas with white.

 

Figure 5: A Clipping Mask combines the content of two consecutive layers and is defined by the shape of the pixels on the bottom most layer.

Figure 6: Cool overlay effects can be achieved by combining Clipping Groups with Layer Masks and combining the two layers with blend modes and opacity adjustments.

Clipping Mask (Group): (see Figure 5) A Clipping Mask is created by placing pixels that are surrounded by transparency below one or more layers in the stacking order. The layers are “clipped” together to form a Clipping Group by pressing Opt/Alt and clicking on the line in the Layers panel that separates them in the stack. The top most pixels become visible, conforming to the shape of the bottom pixels that are defined by the transparency that surrounds them. A Clipping Mask can be edited by adding or removing content from the bottom layer. Cool overlay effects can be achieved by combining Clipping Groups with Layer Masks, as in Figure 6.

 

Figure 7: Vector Masks are created with the pen or shape tools and conceal all of the content on the outside of the path.

Vector Mask: (see Figure 7) These masks are composed of paths made with the pen or shape tools. They reside in the Layers panel and are represented by a gray and white thumbnail. The white areas of the thumbnail reveal content and the gray areas conceal. Vector Masks are editable with the vector tools such as the Path Selection and the Direct Selection tools and the Add, Subtract, and Convert Anchor Point tools.

 

Figure 8: Type Masks create selection outlines in the shape of text characters.

Type Masks: (see Figure 8) Type Masks are generated by choosing the Type Mask tool and clicking on the image, then typing. The mask appears as a translucent red field with transparent text characters. When the mask is committed by pressing the enter/return key or clicking on the Commit button, the characters convert to selection outlines in the shape of characters. Before conversion to selection outlines, the text characters can be edited with the type tools or transformed (scaled, rotated, moved, etc.) by pressing the Cmnd/Cntrl key and dragging the points on the bounding box that is displayed. After being committed, the text is no longer editable as type but can be altered with the selection tools. Type Masks do not create tile layers. When a Type Mask is created, a content layer must first be targeted in the Layers panel.

 

Figure 9: Smart Filter Masks control the location of a filter effect on a layer.

Smart Filter Mask: (see Figure 9) These masks conceal the effects of filters. Smart Filters are attached to Smart Objects. A Smart Object is made by targeting a layer in the Layers panel and choosing Convert to Smart Object from the Layers panel pull-down menu. A filter is then applied to the image. In the Layers panel, a mask thumbnail is created that can be modified with the Brush tool. Black painted on the mask conceals the affects of the filter and white reveals the affect. Gray partially reveals. If an area is selected on the smart object layer, the mask that is created will be based on the selection.

 

Figure 10: The Masks panel controls the edges of either vector or Layer Masks.

THE MASKS PANEL

The Masks panel: (see Figure 10) is a feature that further controls either vector or layer masks. After a mask has been created, the Masks panel can refine its density (opacity) and its feather radius. By choosing Refine Mask Edge from the panel, a dialog box similar to the Refine Edge feature used for selections is displayed. The sliders enable precise control over the mask’s edges so that the mask performs to its optimal capabilities. 

 

Although they can be edited with Photoshop’s tools and commands, selections, Alpha Channels, Quick Masks and Type Masks are not part of the image, per se; in other words, they are not printable. They are independent tools that are created and used to alter the image. Similarly, Layer Masks, Vector Masks and Smart Filter Masks do not contain layer content. They are tools that are made to reveal or conceal regions of the image. Clipping Masks are unique in that they actually contain content and constitute a relationship between two or more consecutive layers.