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Image file
formats are standardized
means of organizing and storing images. This entry is about digital image
formats used to store photographic and other images; (for disk-image file
formats see Disk
image). Image files are composed of either pixel or vector
(geometric)
data that are rasterized to pixels when
displayed (with few exceptions) in a vector graphic display. The pixels that compose an
image are ordered as a grid (columns and rows); each pixel consists of numbers
representing magnitudes of brightness and colour.
Image file size—expressed as the number of
bytes—increases with the number of pixels composing an image, and the colour depth of the pixels. The greater
the number of rows and columns, the greater the image
resolution, and the larger the file. Also, each pixel of an image
increases in size when its colour depth increases—an
8-bit pixel (1 byte) stores 256 colours, a 24-bit
pixel (3 bytes) stores 16 million colors, the latter known as truecolor.
Image compression uses algorithms to
decrease the size of a file. High resolution cameras produce large image files,
ranging from hundreds of kilobytes to megabytes, per the camera's resolution
and the image-storage format capacity. High resolution digital
cameras record 8 megapixel (1MP = 1,000,000
pixels / 1 million) images, or more, in truecolor.
For example, an image recorded by an 8 MP camera; since each pixel uses 3 bytes
to record truecolor, the uncompressed image would
occupy 24,000,000 bytes of memory—a great amount of digital storage for one
image, given that cameras must record and store many images to be practical.
Faced with large file sizes, both within the camera and a storage disc, image
file formats were developed to store such large images. An overview of the major graphic file formats follows below.
There are two types of image
file compression algorithms: lossless and lossy.
Lossless compression algorithms
reduce file size without losing image quality, though they are not compressed
into as small a file as a lossy compression file.
When image quality is valued above file size, lossless algorithms are typically
chosen.
Lossy compression algorithms take advantage of
the inherent limitations of the human eye and discard invisible information.
Most lossy compression algorithms allow for variable
quality levels (compression) and as these levels are increased, file size is
reduced. At the highest compression levels, image deterioration becomes
noticeable as "compression artifacting".
The images below demonstrate the noticeable artifacting
of lossy compression algorithms; select the thumbnail
image to view the full size version.
There are many graphic file
formats, if we include the proprietary types. The PNG, JPEG, and GIF formats
are most often used to display images on the Internet. These graphic formats
are listed and briefly described below, separated into the two main families of
graphics: raster and vector.
These formats store images as bitmaps (also known
as pixmaps). For a description of the technology
aside from the format, see Raster
graphics.
JPEG (Joint
Photographic Experts Group) files are (in most cases) a lossy
format; the DOS filename extension is JPG (other OS might
use JPEG). Nearly every digital camera can save images in the JPEG
format, which supports 8 bits per color (red, green, blue) for a 24-bit total,
producing relatively small files. When not too great, the compression does not
noticeably detract from the image's quality, but JPEG files suffer generational
degradation when repeatedly edited and saved. Photographic images may be better
stored in a lossless non-JPEG format if they will be re-edited, or if small
"artifacts" (blemishes caused by the JPEG's compression algorithm)
are unacceptable. The JPEG format also is used as the image compression
algorithm in many Adobe PDF files.
The TIFF (Tagged Image File
Format) is a flexible format that normally saves 8 bits or 16 bits per
color (red, green, blue) for 24-bit and 48-bit totals, respectively, using
either the TIFF or the TIF filenames. The TIFF's
flexibility is both blessing and curse, because no single reader reads every type
of TIFF file. TIFFs are lossy
and lossless; some offer relatively good lossless compression for bi-level
(black&white) images. Some digital cameras
can save in TIFF format, using the LZW compression algorithm for lossless storage. The TIFF image
format is not widely supported by web browsers. TIFF remains widely accepted as
a photograph file standard in the printing business. The TIFF can handle
device-specific colour spaces, such as the CMYK
defined by a particular set of printing press inks.
RAW refers to a family of raw
image formats that are options available on some digital cameras. These
formats usually use a lossless or nearly-lossless compression, and produce file
sizes much smaller than the TIFF formats of full-size processed images from the
same cameras. The raw formats are not standardized or documented, and differ
among camera manufacturers. Many graphic programs and image editors may not accept
some or all of them, and some older ones have been effectively orphaned
already. Adobe's Digital Negative specification is an attempt at
standardizing a raw image format to be used by cameras, or for archival storage
of image data converted from proprietary raw image formats.
The PNG (Portable Network Graphics) file format was created as the free,
open-source successor to the GIF. The PNG file format supports truecolor (16 million colours)
while the GIF supports only 256 colours. The PNG file
excels when the image has large, uniformly coloured
areas. The lossless PNG format is best suited for editing pictures, and the lossy formats, like JPG, are best for the final
distribution of photographic images, because JPG files are smaller than PNG
files. Many older browsers currently do not
support the PNG file format, however, with Internet Explorer 7, all contemporary web
browsers now support all common uses of the PNG format, including full 8-bit
translucency (Internet Explorer 7 may display odd colors on
translucent images ONLY when combined with IE's opacity filter). The Adam7-interlacing
allows an early preview, even when only a small percentage of the image data
has been transmitted.
GIF (Graphics Interchange Format) is limited to an 8-bit palette, or 256
colors. This makes the GIF format suitable for storing graphics with relatively
few colors such as simple diagrams, shapes, logos and cartoon style images. The
GIF format supports animation and is still widely used to provide image
animation effects. It also uses a lossless compression that is more effective
when large areas have a single color, and ineffective for detailed images or dithered images.
The BMP
file format (Windows bitmap) handles graphics files within the Microsoft
Windows OS. Typically, BMP files are uncompressed, hence they are large;
the advantage is their simplicity, wide acceptance, and use in Windows programs.
See also: Vector
graphics
As opposed to the raster
image formats above (where the data describes the characteristics of each
individual pixel), vector image formats contain a geometric
description which can be rendered smoothly at any desired display size.
Vector file formats can contain
bitmap data as well. 3D graphic file formats are technically vector formats
with pixel data texture mapping on the surface of a vector virtual
object, warped to match the angle of the viewing perspective.
At some point, all vector
graphics must be rasterized in order to be displayed
on digital monitors. However, vector images can be displayed with analog CRT
technology such as that used in some electronic test equipment, medical
monitors, radar
displays, laser shows and early video
games. Plotters
are printers that use vector data rather than pixel data to draw graphics.