Definitions!

Definitions

BIT DEPTH is determined by the number of bits used to define each pixel. An image with a high bit depth allows a greater number of tones to be represented. Digital images may be produced in black and white, grayscale, or color. A bitonal image is represented by pixels consisting of 1 bit each, which can represent two tones (typically black and white), using the values 0 for black and 1 for white or vice versa.

A grayscale image is composed of pixels represented by multiple bits of information, typically ranging from 2 to 8 bits or more. In a 2-bit image, there are four possible combination's: 00, 01, 10, and 11. If "00" represents black, and "11" represents white, then "01" equals dark gray and "10" equals light gray. The bit depth is two, but the number of tones that can be represented as 4. At 8 bits, 256 different tones can be assigned to each pixel.

A color image is typically represented by a bit depth ranging from 8 to 24 or higher. With a 24-bit image, the bits are often divided into three groupings: 8 for red, 8 for green, and 8 for blue. Combination's of those bits are used to represent other colors. A 24-bit image offers 16.7 million color values. Increasingly scanners are capturing 10 bits or more per color channel and often outputting 8 bits to compensate for "noise" in the scanner and to present an image that more closely mimics human perception.

Binary calculations for the number of tones represented by common bit depths:

1 bit = 2 tones
2 bits = 4 tones
3 bits = 8 tones
4 bits = 16 tones
8 bits = 256 tones
16 bits = 65,536 tones
24 bits = 16.7 million tones

COMMON DIGITAL FILE FORMATS

COMPRESSION is used to reduce image file size for storage, processing, and transmission. The file size for digital images can be quite large, slowing some computing and networking systems. Compression techniques abbreviate the string of binary code in an uncompressed image to a form of mathematical shorthand. There is considerable debate in the library and archival community over the use of compression in master image files. Compression schemes as they are called, can characterized as either lossless or lossy.

Lossless schemes, like ITU-T.6, abbreviate the binary code without discarding any information, so that when the image is "decompressed" it is bit for bit identical to the original. Lossy schemes, like JPEG, utilize a means for averaging or discarding the least significant information, based on an understanding of visual perception. However, it may be extremely difficult to detect the effects of lossy compression.

DIGITAL IMAGES are electronically coded snapshots of a subject or scanned from document form like photographs, printed material and art. A digital image is a representation of a two-dimensional image using ones and zeros (binary). Each digital image is a sample and is mapped in a grid of picture elements (pixels). Each pixel has a specific tonal value for each color represented (black, white, shades of gray or color), which is a mathematical representation in binary code as bits, ones and zeros.

These bits are interpreted by the computer to produce an analog version for display or print. Depending on whether or not the image resolution is fixed, it may be of vector or raster type. Without qualifications, the term "digital image" usually refers to raster images also called bitmap images.

Pixel values are shown in this simple two-tone image example, each pixel is assigned a tonal value, 0 for black and 1 for white.

DIGITAL IMAGING A field of computer science covering digital images - images that can be stored on a computer, particularly bit-mapped images. Digital imaging is a wide field that includes digital photography, scanning, and composition and manipulation of bit-mapped graphics.

DYNAMIC RANGE is the tonal difference or range between the lightest light and darkest dark of an image. The higher the dynamic range, the more potential shades can be represented. Dynamic range also describes a digital system's ability to reproduce tonal information. This capability is most important for continuous-tone documents and photograph reproductions; this may be the single most important aspect of image quality.

FILE FORMATS consist of bits that comprise the image and header information on how to read and interpret the file. File formats can vary in terms of resolution, bit-depth, color capabilities, and support for compression and metadata.

FILE SIZE is calculated by calculating the surface area of a document (height x width) divided by the bit depth and the dpi squared. Because image file size is represented in bytes, which are made up of 8 bits, divide this figure by 8.

File Size = (height x width x bit depth x dpi squared) / 8
If the pixel dimensions are given, multiply them by each other and the bit depth to determine the number of bits in an image file.

Example: 24-bit image is captured with a digital camera with pixel dimensions of 2,048 x 3,072, then the file size equals (2048 x 3072 x 24)/8, or 18,874,368 bytes.

File Size = (pixel dimensions x bit depth) / 8
Because digital images often result in very large files, the number of bytes is usually represented in increments of 210 (1,024) or more:

1 Kilobyte (KB) = 1,024 bytes
1 Megabyte (MB) = 1,024 KB
1 Gigabyte (GB) = 1,024 MB
1 Terabyte (TB) = 1,024 GB

GICLEE (pronounced "zhee-clay") A French word meaning "nozzle" or possibly a derivative of the French verb "gicler" meaning "to squirt". The process of making fine art prints. Giclee prints are images generated from high resolution digital scans or files, printed with archival quality inks onto various substrates including canvas, fine art, and photo-base paper.

This process produces better color accuracy and a far superior depth and range of colors than other means of reproduction such as that of lithography prints. Giclees also have a higher resolution than any other prints and uses ink-jet printers employing fade resistant "archival" inks. The quality of the giclee print rivals traditional silver-halide and gelatin printing processes and is commonly found in museums, art galleries, and photographic galleries.

RASTER Most users come into contact with raster images through digital cameras. Some digital cameras give access to almost all the data captured by the camera, using a raw image format. The Universal Photographic Imaging Guidelines (UPDIG) suggests this format be used when possible since raw files produce the best quality images.

These file formats allow the photographer and the processing agent the greatest level of control and accuracy for output. Unfortunately, there is an issue of proprietary information [trade secrets] for some camera makers, but organizations are attempting to influence the manufacturers of them to avail these records publicly. An alternative may be a Digital Negative (DNG) a proprietary Adobe product described as "the public, archival format for digital camera raw data".

RESOLUTION is the ability to distinguish fine spatial detail. The spatial frequency at which a digital image is sampled is often a good indicator of resolution. Dots-per-inch (dpi) or pixels-per-inch (ppi) are common terms used to indicate resolution for digital images.

PIXELS can be seen individually by zooming in on an image.

PIXEL DIMENSIONS are the horizontal and vertical measurements of an image expressed in pixels. The pixel dimensions may be determined by multiplying both the width and the height by the dpi. A digital camera will also have pixel dimensions, expressed as the number of pixels horizontally and vertically that define its resolution (e.g., 2,048 by 3,072).

Calculate the dpi achieved by dividing a document's dimension into the corresponding pixel dimension against which it is aligned. An 8" x 10" document that is scanned at 300 dpi has the pixel dimensions of 2,400 pixels (8" x 300 dpi) by 3,000 pixels (10" x 300 dpi).

VECTOR images may be created from scratch with illustration software, or by converting a raster image to vector form. Often, both raster and vector elements will be combined in one image, for example, in the case of a billboard with text (vector) and photographs (raster).

QUALITY CONTROL (QC) is an integral component of a digital imaging initiative which ensures that quality expectations have been met. It encompasses policies, procedures and techniques to verify the quality, accuracy, and consistency of digital products. Quality control strategies are implemented at different levels. Initial, Ongoing, and Final Evaluation are conducted to ensure quality throughout the digital imaging process.