Foundations of Digital Image

02. Vector Images

Vector graphics is the use of geometrical primitives such as points, lines, curves, and shapes or polygon(s), which are all based on mathematical expressions, to represent images in computer graphics. “Vector,” in this context, implies more than a straight line.

Vector Files by Allamn McCollum

Paintings by Ellsworth Kelly

Sol Le Witt

All of these images could easily be reproduced as vector images in the digital realm. A vector image is a set of geometric instructions or a list of properties.

For example, consider a circle of radius r. The main pieces of information a program needs in order to draw this circle are an indication that what is to be drawn is a circle, specifically:

  • The radius r
  • The location of the center point of the circle
  • Stroke line style and color (possibly transparent)
  • Fill style and color (possibly transparent)

Unlike raster images, which contain a matrix of color information for each pixel, a very small amount of descriptive information is needed to reproduce a vector digital image. This small amount of information translates to a much smaller file size compared to large raster image files. Since this descriptive information is not pixel based, the image can be reproduced in any size. This means that scaling the size up or down does not change the definition of the image. The image can be infinitely zoomed in on a circle arc, for example, and it will remain smooth.

As stated in the beginning of this chapter, all digital images are displayed through digital displays. Even if the image is initially produced, edited or reproduced as a vector image, it should be rasterized before it is shown on a digital display or printed with a printer. This process is called rasterization. The process relies on various algorithms or techniques, but a very common parameter is for the user to decide on the size of the output. Since the vector images are “sizeless,” if the converter software or algorithm does not have a default output size, the size of the raster output is decided by the user.

The reverse of rasterization process is not so common, but still used for various practical or aesthetic purposes. It is called vectorization or image tracing. Unlike rasterization, vectorization is not well defined, meaning that there is not a single correct method. Many different algorithms exist and each gives different results, as vector representations are more abstract than pixels.

The iconic Barack Obama “Hope” poster, designed by artist Shepard Fairey, which came to represent the 2008 Obama presidential campaign, can be considered a good example of image tracing. In this case, low definition tracing works as a way of abstracting the image.

On left, original AP photograph of Barack Obama taken by Manny Garcia in April 2006. On right, Obama “Hope” poster designed by Shepard Fairey.

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