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Seeing between the Pixels

Brian Hayes

The French painter Georges Seurat had a lively interest in the sciences. His technique of pointillism—painting with tiny flecks of pure colors—was inspired by theories of light and perception that trace back to James Clerk Maxwell and Hermann von Helmholtz, among others. Unfortunately for Seurat, the scientific underpinnings of pointillism did nothing to soften criticism of his paintings when he exhibited them in the 1880s, but his ideas have certainly been vindicated since then. A century later we are all pointillists. We tend to see every image as a collection of little dots.

Figure 1. <em>La Parade de Cirque</em>Click to Enlarge Image

Decomposing a picture into dots seems natural today because that's the way images are displayed on a computer monitor or a television screen—by lighting up spots of phosphor that glow red, green and blue. Printed images are just as dotty. Under a magnifying lens, the pictures in this magazine dissolve into dots of cyan, magenta, yellow and black. Ink-jet printers spray similar arrays of microscopic colored droplets.

From printing and displaying pictures as dots, it's an easy step to storing them that way inside the computer—as rows and columns of "pixels," or picture elements. Photographs made with a digital camera are born as pixel arrays; satellite imagery and other kinds of scientific data also arrive in this format; conventional photographs can be converted into pixels by a scanner or even by a fax machine. In the case of a monochrome image, each pixel is represented in computer memory by a number that gives the pixel's brightness on a scale from black to white. For color images, a pixel typically consists of three or four numbers, encoding the intensities of the component colors. A snapshot-size color image might have a million pixels and fill up a few megabytes of computer memory.

Figure 2. Detail of <em>La Parade</em>Click to Enlarge Image

With the prevalence of digital imaging, the rectangular array of pixels begins to seem like the one and only way of representing graphic information; you can't look at a picture without seeing spots before your eyes. But there are many alternatives to pixels, with interesting and useful properties. Some of the pixelless representations are more compact; some preserve information about the structure and meaning of an image; some are easier to revise or edit; some can be displayed at various resolutions without loss of quality. Perhaps most intriguing, some of the alternatives could give hints about the way the brain stores and interprets images.

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