Bitmaps à la mode

Most users aren’t concerned about the precise nature of bitmaps. Like car drivers, they are more interested in getting from A to B than how the engine works. To digital camera owners in particular, bitmaps are just the way their computer handles photographs. However, if you are serious about graphic design, it is well worth taking a closer look at how bitmaps work, exploring the many different types and what they can be used for. Such an exploration will offer a number of surprises. For a start, while representing high-resolution colour photos is now the main function of bitmaps, their origin was very different. Back in the pioneering days of computing, high-resolution was merely a dream and even basic colour (let alone continuous-tone, full colour) was out of the question. The first bitmaps weren’t designed for handling graphics at all, but for handling text.

Bitmaps à la mode

The core problem was how to convert computer data into characters and numbers for display on the first computer screens, and the solution couldn’t be simpler. A CRT screen is coated with phosphor that emits light when hit by an electron beam, and the smallest addressable unit of this display is called a pixel (a contraction of ‘picture element’). These pixels are arranged in a rectangular grid and, by subdividing this into smaller grids of, say, 8 x 8 pixels each, the basic shape of any onscreen character can be constructed as a pattern of dots simply by switching each of these 64 pixels on or off. These on and off pixel states can be represented by a single data bit (0 or 1), and such bits are stored in a simple addressable grid or map. Put these two concepts together and you have your originial ‘bitmap’, a simple matrix of on/off values.

1-bit monochrome

These first 1-bit-per-pixel bitmaps were devised for drawing low-resolution screen fonts, but the simplicity of their grid-based architecture can easily be extended to handle any number of rows and columns of pixels. By using more pixels and more resolution, customisable bitmap files could be created that describe the on/off pattern for the whole screen, hence moving beyond text to produce monochromatic graphic images. In fact, there was no reason for the resolution of the bitmap to be tied to that of the physical screen. Crucially, breaking this link allowed high-resolution bitmaps to control the output to printers in the same way that they controlled output to the screen, via more detailed bitmapped printer fonts and high-quality bitmapped graphics files.

These days, printed type is no longer restricted to fixed-size, pre-built bitmapped fonts, but instead is handled via scalable outlines that get rasterized – that is, converted to a bitmap – on the fly. Nevertheless, 1-bit bitmaps still play an underappreciated but crucial role as a graphics format in professional design. When it comes to printing, their major limitation – namely, that they can only describe monochromatic images – becomes a major strength. Without the need for halftoning to simulate the effect of shades, such images can be printed at maximum resolution, with each image bit controlling precisely one printed dot. This makes the 1-bit bitmap format ideal for capturing the pin-sharp detail in black-and-white drawings, so long as it is then output to a single printing plate – usually black or a spot colour.

To take best advantage of 1-bit bitmaps, they need to be sampled at the highest resolution the output device can natively support – that is, without a halftone screen. However, there is little point going over 1,000dpi size-for-size even for typeset output, as it would exceed the limited resolution of the human eye. By comparison, when scanning for colour or greyscale output that does involve printing halftones, there is no need to go beyond 300dpi, so it is really only for 1-bit bitmaps (and for resizing) that you need a scanner capable of more than 300dpi.

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