Photoshop
Adobe Photoshop is a powerful 2D imaging program that was originally developed for enhancing photographs. However, it has always offered tools for digital painting as well, particularly since the implementation of a radically improved brush engine in version 7. It has become a popular painting tool for professionals and hobbyists alike. This article is not intended to be a complete reference for the program; only Photoshop techniques that are pertinent to digital painting will be addressed here.
Note: Most of the techniques described here rely on the use of a graphics tablet, such as those manufactured by Wacom and numerous other companies.
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Basic Painting Techniques
Image Size
It is generally a good idea to start with a smaller "canvas", particularly for artists with slower computers. (It's possible to cover more of a small image at once than with a large one; large brushes tend to cause computers to lag.) When starting a painting, many artists will start with an image that doesn't exceed 1000 pixels on either side. Once major forms are blocked in, the image can be scaled up using Image > Image Size. Then details can be filled in using smaller brushes.
Note that resolution is only important if the painting will be printed at some point. Higher resolutions mean higher-quality printed output, but they also necessitate larger pixel dimensions to produce the same physical dimensions. (All else being equal, resolution does not affect the display of a painting within Photoshop.)
Brushes
The Brush tool (hotkey B) is the only tool that is absolutely necessary to Photoshop's form of digital painting. It is represented on Photoshop's tool box by the 
icon.
Photoshop's brush engine has never been as complex as those of programs that specialize in painting, such as Corel Painter. It operates on a simple principle: a grayscale image ("brush tip") represents how much color should be applied at every point, and this tip is repeated at regular intervals along the path of the mouse or tablet stylus. As a result, it is difficult to paint with multiple colors simultaneously, produce impasto effects, or perform a number of other actions that are easily accomplished with Painter or traditional paints.
However, the simplicity of the brush engine can also make it a very effective tool for conveying artistic ideas. Indeed, it is often wise to paint with one of the simplest brushes available. All options can be found on the Brushes palette. Under Brush Tip Shape, pick a hard-edged circle. Set "Spacing" to roughly 5%. Deactivate Shape Dynamics. Under Other Dynamics, set both "Control" pull-down menus to Pen Pressure. Make sure that Smoothing is activated, as well. The resulting brush will lay down more color when the stylus is pushed into the the tablet with more force. (Some artists activate Shape Dynamics and/or deactivate Other Dynamics, using the "Opacity" field near the top of the screen to control opacity.) In general, avoid using soft-edged brushes. These tend to limit control and produce sloppy, blurry results.
Colors
Choosing colors can be difficult for beginners. It is a common mistake to pick unrealistic colors. (This article assumes the reader has at least a basic understanding of color theory; its purpose is to illustrate how to use Photoshop to pick appropriate colors.)
The Color Picker is accessed by clicking the large, colored "Set Foreground Color" square near the bottom of the tool box. This dialog box contains versatile controls for picking nearly any color imaginable. By default, saturation and brightness can be controlled by clicking within the large box; hue is controlled using the slider to the right of that.
The easiest way to learn more about which colors look good is to sample colors from photos or other paintings using the Eyedropper tool. Over time, you can build up a mental library of where to find useful colors in the Color Picker. Avoid sampling colors and painting immediately with them, though. At least look at the color's location in the picker, reset the colors (hotkey D), and manually pick the color over again.
Many objects trace a "curve" of colors on the Color Picker. Sampling a number of colors from the same object in a photo can reveal the shape and location of this curve. For many objects, this curve is shaped like a backwards C, with the highest saturation in the middle values.
Unfortunately, the Color Picker cannot be assigned a keyboard shortcut unless you download a third-party program such as AutoHotkey (see Suggested Wacom Tablet Setup).
Blending
The simplest way to blend colors is to simply change the pressure of the stylus against the tablet. However, this often produces unsatisfactory results when used alone; further blending is usually necessary. There are several ways to do this.
Most artists recommend not using the Smudge or Blur tools (hotkey R). While these do have some uses, beginning artists should generally avoid them. Instead, try to blend using only the Brush tool. The following method is popular for its simplicity, its subtle texture, and the impressive level of control it offers.
When adjacent areas of color require blending, it's possible to sample colors directly from the painting. Press and hold alt while clicking one of the color masses; this temporarily activates the Eyedropper tool and grabs whatever color is clicked. Then paint with light pressure over the boundary between colors. Now sample the new colors and paint over the larger, more diffuse boundary. Keep repeating this process until a sufficiently smooth transition is achieved.
Layers
The ability to use layers while painting can be a bit of a double-edged sword. Using too many can make it impossible to work in an organized way; using too few can limit creative options. In general, speedpaints and sketches require few layers, while finished pieces may necessitate a greater number of layers.
By default, a new Photoshop document contains just one layer titled Background. Artists may paint as much or as little as they wish on this layer. Foreground elements are often relegated to new layers (created by clicking the 
icon at the bottom of the Layers palette). This allows a unique advantage: if the artist wishes to paint freely on a foreground object without worrying about brush strokes spilling over the carefully-drawn edges, he or she can press the 
icon at the top of the Layers palette. This makes it impossible to paint in areas of a layer that are already transparent. (Clicking the icon again unlocks the transparency.)
Layers can also be used for atmospheric effects such as haze or lighting. The pull-down menu at the top of the Layers palette can be used to change a layer's blending mode. This is set to "Normal" by default, but a plethora of other options exist. Each causes the layer to affect underlying layers in a different way. Most of these are beyond the scope of this article, but a few are relevant to digital painting.
- Screen/Multiply: These two modes are opposites. Screen has a brightening effect on lower layers; Multiply has a darkening effect. Pure black and pure white are completely invisible in each of these modes, respectively.
- Color: This mode causes the layer to only affect the image's hue and saturation. Brightness values are preserved from lower layers. This is useful for colorizing grayscale artwork (as is Multiply).
- Overlay/Soft Light/Hard Light: These three are similar, but subtly different. They can be useful for deepening colors in select areas.
- Color Dodge/Linear Dodge: These similar modes are most often used for adding in bright highlights. Use sparingly.
Advanced Techniques
Custom Brushes
Developing a thorough understanding of the world's optical properties is much more important than picking fancy tools. However, customizing Photoshop's brushes can yield more interesting results if basic artistic principles are understood.
The key to custom brush design is experimentation. However, here are a few tips.
- The biggest way to add variety to brushes is using new brush tips. Selecting an area of an image and hitting Edit > Define Brush Preset will add it to the Brushes palette. Then all of the normal options will be available. Experiment with unusual patterns, either hand-painted or photographed. Even areas of previous paintings can be useful.
- Dual Brush and Texture are good ways to add grit to a brush.
- Pushing "Angle Jitter" (under Shape Dynamics) to 100% can introduce an interesting sense of chaos to a brush. The Scattering options are similarly useful.
Advanced Blending
As mentioned above, the Smudge tool (hotkey R) is not normally a good tool for blending. When used with default settings, it does little more than smear digital paint around in an unpleasant manner. However, the principles of custom brush design can be carried over to the Smudge tool, and it can become a delightful way to blend colors—when used in moderation. Here are some general principles for using the Smudge tool as a blending tool.
- In general, use Scattering. The Smudge tool is rarely useful without a moderate degree of Scattering.
- Spiky or crinkled brush tips with hard edges are generally the most effective. These often produce watercolor-like blends.
- Activating or deactivating "Both Axes" (under Scattering) can result in different blends. For beautiful running-wash effects, try turning this option off and blending a variegated area with horizontal strokes. The digital paint appears to run down the canvas.
- If a custom brush seems to be particularly effective as a blender, try it with the Brush tool.
- For the smoothest blend possible, use one of Photoshop's default 0%-hardness circle tips and a medium-small scatter value. The resulting blend is easy to control yet incredibly smooth.
- Bear in mind that the Smudge tool offers six blending modes in addition to the default Normal: Darken, Lighten, Hue, Saturation, Color, and Luminosity. All of these, the first two in particular, can be useful while painting.
Color Correction
Photoshop's tools for enhancing photos can be used similarly for paintings. Most of these are found under Image > Adjustments. Hue and Saturation (ctrl-U) and Levels (ctrl-L) are the two most commonly used color correction tools. The first allows the colors of an image to be subtly or radically altered; the second allows the brightness to be altered. While artists should strive to pick "correct" colors from the start, these tools are helpful when small tweaks are desired.
Filters
Filters are a dangerous temptation for many newcomers to Photoshop. They should mostly be avoided by beginning painters, as they can quickly become crutches. Nevertheless, they can occasionally be the right tool to get a job done.
Filter > Render > Lens Flare is possibly the most infamous offender. This easily-recognizable light source is frequently seen in ads, television commercials, and photomanipulation projects. It should be avoided simply because it is so recognizable. However, a lens flare on a black layer set to "Linear Dodge" can be used subtly if the effect is blurred using the Gaussian Blur or Radial Blur filters and recolored, perhaps using Hue and Saturation or Color Balance.
Miscellaneous Tips and Tricks
Important Hotkeys
The following hotkeys are extremely helpful when painting. Generally, the artist should keep his or her weaker hand either on the keyboard or on the keys of the graphics tablet. (Note that these hotkeys and the dozens of others in Photoshop can be customized via Edit > Keyboard Shortcuts.)
- B: Switches to Brush tool.
- Z: Switches to the Zoom tool, which is used for zooming in.
- ALT: 1. Temporarily switches to the Eyedropper tool (when Brush tool is active). 2. Temporarily causes the Zoom tool to zoom out rather than in.
- SPACE: Temporarily switches to the Hand tool, which is used for moving the view of the image.
- R: Switches to the smudge tool.
- ALT-CTRL-Z: Step Backward (like Undo, but with multiple possible Undos).
- F: Toggles between three different view modes.
- TAB: Toggles the visibility of palettes. Pressing F two times and then TAB gives the most unobstructed view of an image possible within Photoshop.
- [, ]: Decreases and increases the brush size, respectively. Very useful for changing on the fly.
- ALT-CTRL-0: Actual Pixels; zooms in or out to 100% size.
- E: Switches to the Eraser tool. Useful when the boundaries of a layer need to be cleaned up.
- CTRL-R: Toggles rulers on and off.
- CTRL: Temporarily switches to the Move tool.
- V: Switches to the Move tool (required if an existing guide must be moved).
Mirroring the Image
Image > Rotate Canvas > Flip Canvas Horizontal is an often-overlooked way for artists to discover and correct mistakes in their work. The human eye tends to overlook mistakes after seeing them enough times, and certain idiosyncracies can become ingrained in the artist's head and hands. Flipping the canvas throws into sharp relief any problems with perspective and composition. (Flipping it vertically is somewhat less helpful, but it has its uses as well.)
Tool Presets
The Tool Presets palette allows the artist to compile frequently-used brushes and tools in one place. Try leaving "Current Tool Only" unchecked; this will allow easy access to brushes and Smudge blenders at the same time. Remove any tools from the palette that don't seem absolutely essential.
Perspective Layout
There are numerous ways to set up perspective guidelines in Photoshop. One common method involves using guides—bluish horizontal or vertical lines that can be created in PSD images, but appear neither in printed material nor in other file formats. A guide can be created by clicking and dragging from the rulers at the top or left side of the image. (They can be repositioned if the Move tool is activated by hotkey V; it is also possible to delete a guide by dragging it back into the ruler.)
Create a horizontal guide that represents the horizon line. Now create one or two vertical guides that intersect with the horizon line at the vanishing point(s). (Three-point perspective can be achieved by adding a second horizontal guide and a third vertical guide.) Once vanishing points are established, create a new layer. Switch to the Line tool (hotkey U) and click the 
icon near the top of the window. Set the Weight value to 1px or 2px. Now repeatedly click-and-drag outward from the vanishing points in random directions, creating guidelines that radiate outwards. Turn the Opacity of the layer down to about 20%, allowing the guidelines to be slightly visible over the painting. Allow this layer to sit on top of all the painting layers and turn it on and off as desired.
Underpainting and Overpainting
Creating a finished painting from a blank white canvas can be a daunting prospect. Underpainting and/or overpainting can greatly reduce this angst and allow for a much more controlled workflow. Using a small, hard brush, quickly outline the basic composition of the painting. This can be done either on the Background layer or on a new layer. In the case of the former, the artist then creates a new layer and begins painting above that. In the case of the latter (a more common technique), the artist reduces the sketch layer's opacity and paints on one or more layers beneath it. Typically, the original sketch layer is made invisible for the final painting.
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