There are two types of computer graphics: raster images and vector drawings.
Raster images, the primary domain of Photoshop, are dependent on a grid of square pixels (in broadcast media, the pixels are non-square, but the principle is the same). Each raster image is defined by its resolution, or the number of pixels occurring within every inch of the grid. Thus raster images are resolution dependent, meaning they look best when displayed on a monitor or other device that fits the same number of pixels within an inch.
Because a pixel is the smallest unit of measure in a raster image, it cannot be divided. All image data must partition evenly into the pixel grid. Each pixel must also be solideither filled with a single color, empty to create transparency, or semitransparent so that, when laid atop another object, the pixel appears to be filled with a single solid color. One pixel equals one color; it cannot be half one color and half another. The fewer pixels per inch (ppi), the less detail and color variations possible.
Scaling a raster image up to a larger size increases the ppi, forcing the existing filled image pixels to expand across more pixels. For example, a 100- by 100-pixel image blown up 200% horizontally and vertically spreads the image data from each individual pixel across the adjacent 31 pixel becomes 2 horizontally, 2 vertically, and 2 diagonally (see Figure 16.1). At the same time, new pixels mix the colors from adjacent pixels above, below, and to both sides because every pixel is expanding to fill four slots. This is why blown up images appear fuzzy.
Figure 16.1. Original image magnified 8x (top). The same image blown up 400% of the original size and magnified 4x (center). The same image reduced to 50% of the original size and magnified 8x (bottom). Note the pixel grid visible in the original and the quality loss resulting from enlargement and reduction in the lower two.
Shrinking or scaling down images is even more destructive (though it typically looks better) because, to fit X number of pixels into Y number of slots is impossible. The pixels that don't fit must be thrown away. Scaling the same 100- by 100-pixel image down to a 50- by 50-pixel image, for example, throws away exactly half of the original pixel data.
Because they are resolution dependent, raster images are limited in their utility. Sizing or viewing a raster image to or at any percentage other than 100% results in quality loss. By contrast, vector drawings are resolution independentthey are not based on a pixel grid, and quality cannot be lost by resizing. A vector does not have a size.
Fonts are vectors. Each character or glyph in a font is a vector path, which is what enables type to display and print sharply at any resolution.
Vector is geometry. The anchor and end points of a path are plotted by their coordinates in space relative to the axesx for horizontal positioning, y for vertical location, and z for depth positioning. Recall that path segments are mathematically defined curves or straight lines that connect points. Neither points nor lines have size or resolution, merely location.
Zooming in on a raster image quickly shows its grid of solid pixels. Angled edges that appear smooth at 100% become jagged or stair-stepped (see Figure 16.2). Vectors may be zoomed in to the one-billionth percentage without any sign of jaggedness in smooth angled edges. Moreover, even zoomed, more paths may be added without any difference between the new and old paths.
Figure 16.2. A raster image of a 72-ppi angled shape (left), magnified 6x. Compare with the same shape still in vector (right) magnified by the same amount.
An original 1- by 1-inch vector drawing may be scaled up to 100 by 100 feet while remaining perfectly sharpthis is an example; it may actually be scaled ad infinitum in either direction without quality loss. The versatility of vector artwork to be put to use in almost any situation, at any imaginable size, is its greatest strength. Indeed, that is why many professional designers and illustrators choose to create in Illustrator rather than immediately reaching for Photoshop.
Points and Paths
Paths are the actual drawings to which fills, strokes, and effects may be applied. Other than imported or placed raster images, everything created in Illustrator is built from paths.
Points are the basic building blocks of paths. Between points are path segments, lines that are either straight or curved. End points define the beginning or end of a path, with a single path segment running into, or out from, the end point. Anchor points have path segments on two sides, and occur everywhere the path changes direction or curvature. Paths must include a minimum of two points to begin and end them.
On the artboard, points and path segments are drawn as small boxes and lines in the color assigned to the layer on which the path resides.
Paths may be either open or closed (see Figure 16.3). Open paths have beginnings and ends, with two end points defining such. Whereas closed paths have no discernible beginning and end. In closed paths, points connect to other points infinitely via path segments, and end pointswhere the path drawing actually began and finishedare indistinguishable from anchor points.
Figure 16.3. Open (left), closed (center), and compound (right) paths.
Compound paths are objects comprised of two or more paths. For example, the letter O drawn in vector has two pathsthe outside path (shape) and the hole. Though they form a single object, the paths themselves never meet; they are two disconnected loops. Together they form one object, compounding each otherthe inner path creates a hole in the fill of the outer path.
Vectors Across the Creative Suite
Borrowed from Illustrator by both Photoshop and InDesign are the Pen tool for precision drawing, several of the Shape and Line tools to create common geometric path shapes, and the Direct Selection tool for modifying paths (see Figure 16.4). Although the Pen tool is, but for a few keyboard shortcuts, identical across the Creative Suite applications, the Shape and Line tools tend to vary. Only Illustrator, for example, has a vector-based Flare tool or the Spiral tool. Still, Photoshop and InDesign have common shape tools such as Rectangle, Ellipse, and Rounded Rectangle. They both also have the Line tool, which creates perfectly straight path segments between two points.
Figure 16.4. The Tools palettes from Illustrator (left), Photoshop (center), and InDesign (right) showing the common vector tools.
Illustrator is the vector drawing powerhouse of the Creative Suite, with the tools and commands to create almost anything imaginable. The reason InDesign and Photoshop include the most basic of Illustrator's drawing and shape tools is not to usurp the role of Illustrator in the creative workflow; when only a simple object like a rectangle is required, it's more efficient to remain in InDesign or Photoshop and draw it directly than to open Illustrator. Each of the Creative Suite applications has its own distinct role, but the roles often overlap for convenience.
Because the functionality of the common vector tools is identical or nearly identical between the applications, and because the underlying technology is superposable, paths created in any one application can be manipulated in the others. An ellipse drawn in InDesign, for example, may be copied from the InDesign document and pasted into Illustrator to take advantage of Illustrator's more advanced, purpose-built vector features. After the Illustrator work is completed, that path may be dragged from Illustrator and dropped back into the InDesign document or even into Photoshop for an effects stopover before returning to InDesign.
For more on drawing and working with paths in Photoshop, see Chapter 7, "Using Selections, Paths, and Transforms in Photoshop"; for information about using paths in InDesign, see Chapter 30, "Creating and Modifying Paths in InDesign."