The Digital Eye

Figure 1: Photoshop’s vector tools.

There are two fundamentally different types of graphic images. Vector graphics are composed of mathematically defined points, curves and shapes called objects. An object can be filled with a solid color, a gradient or a pattern and stroked with a colored outline of a specific weight. Raster images use a grid of colored squares called pixels that render variations of color. Though Photoshop is primarily used for pixel manipulation, there is one section in the Tools palette that contains a few vector tools (see Figure 1). Conversely, most of the tools and processes in Illustrator generate and edit vector objects with a few filters and effects that modify pixels.

VECTOR GRAPHICS
Vector objects are composed of paths and points that define their shapes. Both Illustrator and Photoshop have Pen and Shape tools that generate these paths. In Illustrator the path generating tools are used for drawing objects. In Photoshop the vector tools can be used to select an area of a raster image by encircling it with a path.

Figure 2: Various types of Bezier Curves.

In both Photoshop and Illustrator, vector objects are created by depositing points and line segments with the Pen tool or by dragging over an area with the Shape tools. These paths are called Bezier Curves after Pierre Bezier, who invented them for the European Automobile industry in the 1960s. Bezier curves can take the form of any combination of straight, curved or scalloped paths and can be open ended or closed to form a shape (see Figure 2), but what makes them different from pixels is that that they are composed of points and segments.

When vector objects are RIPed and printed, they produce clean, sharp lines and edges because they are resolution-independent. They can conform to the highest resolution of the device on which they are output, whether it’s a desktop ink-jet printer, a laser printer, large-format printer or a high-resolution imagesetter, and that is one of the big advantages of creating vector graphics.

Figure 3: Vector programs like Adobe Illustrator produce images with intense colors and smooth gradients.

The other attribute is that vectors produce crisp, well-defined artwork with intense solid colors and sumptuous gradients (see Figure 3). Vector software is ideal for illustrators and graphic designers who work with type because typographical characters in both Illustrator and Photoshop are actually vector objects. When printed, they produce razor-sharp edges with no “jaggies” (stair-step edges caused by a series of right angles trying to represent a curve) no matter what the resolution of the document.

RASTER IMAGES
Photoshop is primarily a raster-based program, therefore most of the commands, tools and filters are designed to alter the color of pixels. Pixels are usually square areas of colored light and are the building blocks of raster images. Pixels are usually so small that when seen on a monitor, the colors blend into what appears to be a continuous-tone image. Each pixel is assigned a color based on the numerical configuration of binary information. It is the variation of colored pixels within the grid matrix of a digital image that produces variations of tone and color.

Figure 4: In Illustrator’s Envelope Distort feature, a distortion grid can be altered by dragging the horizontal or vertical lines with Direct Selection tool.

RASTER-VECTOR OBJECTS
The best way to understand how each software program manages these two types of images is to look closely at combination techniques. In Adobe Illustrator, a shape is generated with one of the Pen or Shape tools and edited with a variety of shape altering tools and commands that directly bend the Bezier curves to the will of the artist. By dragging an anchor point, a direction handle or a segment, the shape can be modified. Some of Illustrator’s features however, perform alterations that convert the shape into a temporary “unexpanded” state. The unexpanded state is a kind of a hybrid vector-raster object. For example, Illustrator’s Envelope Distortion feature (Object > Envelope Distort > Make with Mesh) creates a distortion grid that can be altered with the Direct Selection tool. Dragging points or lines on the grid distort areas on the object (see Figure 4). The paths that normally encircle the object are not visible during this process until the object is expanded (Object > Expand) when it returns to its vector status with reshaped Bezier curves.

Figure 5: A Gaussian Blur effect in Illustrator shares characteristics of both raster and vector images.

EFFECTS
Applying many of Illustrator’s special features from the Effects menu also creates a hybrid object. A drop shadow, a Gaussian Blur or a feather for example share characteristics of both raster and vector objects (see Figure 5) as do many other features that extend Illustrators capabilities. Photoshop’s path tools are slightly different in both the method of application and the ultimate results. For example, Photoshop can generate a shape as a path, a shape layer and even as solid colored pixels. A path can also be used to encircle an area and be converted to a selection marquee. This process enables the artist to use Bezier curves to select long clean edges of a raster image, enhancing the accuracy of the selection.

Figure 6: A vector mask in Photoshop can be edited with the Path selection tools. It conceals everything on the outside of the path.

Photoshop also has the capability of using paths to conceal a region by generating a vector mask (as in Figure 6). All content outside the mask is concealed so that the content on the layer below the vector mask is visible.

VECTOR TO RASTER
Okay, I hear you ask, “So what about converting files? Can raster images become vectors and visa versa?”

Of course! It’s easy to convert vector objects into pixels in Photoshop using the Rasterize command in the Layers menu. Vector Masks, Shape Layers, Type and Smart Objects can all be converted. Naturally, this process discards all vector information so it can no longer be edited with the Path tools.

Placing, pasting or drag-and-dropping an Illustrator file into Photoshop automatically rasterizes it unless its converted to a Smart Object during the process. (When the image is placed, it appears as a layer. In the Options bar, click the checkmark to make the layer a Smart Object). A Smart Object is another hybrid image that displays a rasterized version of the image in Photoshop that is directly linked to the Illustrator file that maintains the vector data. When the image is edited using the vector tools in Illustrator, the Photoshop image is automatically updated.

Another vector control feature in Photoshop is the Paths to Illustrator command that exports paths created in Photoshop directly to Illustrator. These paths though unfilled or un-stroked with color, can be altered just like any vector object directly generated in Illustrator.

Figure 7: The Live Trace command converts raster images into a multitude of vectors.

RASTER TO VECTOR
Placing a TIFF, PSD, JPG, PNG or EPS raster image file in Illustrator (File > Place) preserves its pixel information. Once placed, any of a number of filters can be applied that can alter the color and apply a number of cool artistic effects to the image, quite similar to Photoshop’s Filter Gallery.

But the real conversion engine that that converts a raster image placed in Illustrator into paths that can be edited with any of the paths editing features is the Live Trace command. Here’s how it works:

• Save the Photoshop document as a TIFF.
• Create a new Illustrator document. Choose File > Place to place the TIFF in Illustrator.
• Select Edit > Live Trace > Tracing Options to display the dialog box (see Figure 7).
• Set the specifications in the dialog box. You can choose a mode (Black and White, Grayscale or Color) and the number of colors. You can play with the other settings to simplify or refine the image as you preview the results by checking the Preview box.
• Click the Trace button to convert the areas of color on the raster image into vectors.

I find that this feature works better if the image is simplified by reducing the number of colors. Overly complex images are better left as rasters, but simplified images have a nice posterized look that can be modified with Illustrator’s many color application tools.