Digitizing Images: Terminology
Pixel: A pixel is a single dot on a computer monitor. Depending on the bit depth of a computer monitor, each pixel can be displayed as anywhere from two to millions of colors. Each pixel is assigned a tonal value (black, white, shades of gray or color), which is represented in binary code (zeros and ones).
Resolution: A measure of the quality of a digital image; as the resolution increases, the quality of the image increases. Resolution is measured in terms of dots per inch (dpi) or pixels per inch (ppi). Monitors measure in ppi; flatbed scanners, drum scanners, and printers measure in dpi.
Screen Resolution: Resolution measured in pixels per inch (PPI). It helps to think of the image as a grid. As the resolution increases, the size of the grid cells get smaller, in effect increasing the number of cells (pixels) per inch.
Bit Depth: Bit depth concerns the number of bits used to convey tonality for each pixel; that is, bitonal (black and white), grayscale, or color. In general, the more bits per pixel, the larger the file size.
- 1-bit Bitonal: A 1-bit pixel has two possible values, black or white. The scanned image has no shading or gray. Bitonal scanning produces the smallest file. It is best used for printed text, or for distinct line drawings with no variation in tone.
- 8-bit Grayscale: Provides 256 shades of gray ranging from pure white to pure black. It is often used for printed black-and-white drawings, since these may have shading but no color.
- 24-bit Color: Provides a tonal range of about 16 million different colors. Color scanning produces quite large files, but it is the best choice for documents containing essential color information.
Tonality: Pixel depth or bit depth.
Halftone Frequency (or) Lines Per Inch (LPI): The number of lines per inch stored by the digital file. Used for printing screens.
Bitonal Image: An image consisting only of a foreground colour and a background colour.
Monochrome: Literally "one color". Usually used for a black and white (or sometimes green or orange) monitor as distinct from a color monitor.
Line art: Perfect for use either in print or on the web, these images can be emailed to you on whatever platform and graphics suite is currently used.
- The best resolution for lineart is 1200 ppi. Halftoning is the process of turning continuous tone grayscale or color images into a series of dots for printing that fool the eye. Learn the ins and outs of halftones, screens, traditional and digital halftoning techniques.
DPI: Number of pixels per inch stored by the digital file. Used for: printing screens and laser printers.
PPI: Number of pixels per inch stored by the digital file. Used for: monitor display resolution.
SPI: Number of samples per inch stored by the digital file. Used for: scanners; optical resolution vs. interpolated resolution.
LPI: Number of lines per inch stored by the digital file. Used for: printing screens.
Archival or Master image: An uncompressed, unedited rendition of data which serves as a long term storage form. Archival data storage is high quality and requires a large file size. Also called preservation format.
Presentation or Access image: A compressed, easily accessible rendition of data which serves as a general access working form primarily for the web. Presentation data storage is of medium quality and constitutes a reasonable file size for fast download time. Also called access format and display format.
Thumbnail image: A reduced-size version of an image, used to present a faster loading, albeit lower quality, alternative to larger image formats such as a GIF or a JPEG. This format is not recommended for archival purposes, but can be useful when a large number of images must be displayed on the same webpage.
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