Image file formats are standardized means of organizing and storing digital images.
Image files are composed of digital data in one of these formats that can be rasterized
for use on a computer display or printer.
An image file format may store data in uncompressed, compressed, or vector formats.
Once rasterized, an image becomes a grid of pixels, each of which has a number of bits
to designate its color equal to the color depth of the device displaying it.
Image file sizes Generally speaking, in raster images, Image
file size is positively correlated to the number of pixels in an image and the color
depth, or bits per pixel, of the image.
Images can be compressed in various ways, however.
Compression uses an algorithm that stores an exact representation or an approximation
of the original image in a smaller number of bytes that can be expanded back to its
uncompressed form with a corresponding decompression algorithm.
Considering different compressions, it is common for two images of the same number of
pixels and color depth to have a very different compressed file size.
Considering exactly the same compression, number of pixels, and color depth for two
images, different graphical complexity of the original images may also result in very
different file sizes after compression due to the nature of compression algorithms.
With some compression formats, images that are less complex may result in smaller compressed
file sizes.
This characteristic sometimes results in a smaller file size for some lossless formats
than lossy formats.
For example, graphically simple images may be losslessly compressed into a GIF or PNG
format and result in a smaller file size than a lossy JPEG format.
Vector images, unlike raster images, can be any dimension independent of file size.
File size increases only with the addition of more vectors.
For example, a 640 * 480 pixel image with 24-bit color would occupy almost a megabyte
of space: 640 * 480 * 24 = 7,372,800 bits = 921,600
bytes Image file compression
There are two types of image file compression algorithms: lossless and lossy.
Lossless compression algorithms reduce file size while preserving a perfect copy of the
original uncompressed image.
Lossless compression generally, but not always, results in larger files than lossy compression.
Lossless compression should be used to avoid accumulating stages of re-compression when
editing images.
Lossy compression algorithms preserve a representation of the original uncompressed image that may
appear to be a perfect copy, but it is not a perfect copy.
Often lossy compression is able to achieve smaller file sizes than lossless compression.
Most lossy compression algorithms allow for variable compression that trades image quality
for file size.
Major graphic file formats Including proprietary types, there are hundreds
of image file 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.
In addition to straight image formats, Metafile formats are portable formats which can include
both raster and vector information.
Examples are application-independent formats such as WMF and EMF.
The metafile format is an intermediate format.
Most Windows applications open metafiles and then save them in their own native format.
Page description language refers to formats used to describe the layout of a printed page
containing text, objects and images.
Examples are PostScript, PDF and PCL.
Raster formats JPEG/JFIF
JPEG is a lossy compression method; JPEG-compressed images are usually stored in the JFIF file
format.
The JPEG/JFIF filename extension is JPG or JPEG.
Nearly every digital camera can save images in the JPEG/JFIF format, which supports 8-bit
grayscale images and 24-bit color images.
JPEG applies lossy compression to images, which can result in a significant reduction
of the file size.
Applications can determine the degree of compression to apply, and the amount of compression affects
the visual quality of the result.
When not too great, the compression does not noticeably affect or detract from the image's
quality, but JPEG files suffer generational degradation when repeatedly edited and saved.
JPEG 2000 JPEG 2000 is a compression standard enabling
both lossless and lossy storage.
The compression methods used are different from the ones in standard JFIF/JPEG; they
improve quality and compression ratios, but also require more computational power to process.
JPEG 2000 also adds features that are missing in JPEG.
It is not nearly as common as JPEG, but it is used currently in professional movie editing
and distribution.
Exif The Exif format is a file standard similar
to the JFIF format with TIFF extensions; it is incorporated in the JPEG-writing software
used in most cameras.
Its purpose is to record and to standardize the exchange of images with image metadata
between digital cameras and editing and viewing software.
The metadata are recorded for individual images and include such things as camera settings,
time and date, shutter speed, exposure, image size, compression, name of camera, color information.
When images are viewed or edited by image editing software, all of this image information
can be displayed.
The actual Exif metadata as such may be carried within different host formats, e.g. TIFF,
JFIF or PNG.
IFF-META is another example.
TIFF The TIFF format is a flexible format that
normally saves 8 bits or 16 bits per color for 24-bit and 48-bit totals, respectively,
usually using either the TIFF or TIF filename extension.
The tagged structure was designed to be easily extendible, and many vendors have introduced
proprietary special-purpose tags – with the result that no one reader handles every
flavor of TIFF file..
TIFFs can be lossy and lossless; some offer relatively good lossless compression for bi-level
images.
Some digital cameras can save images in TIFF format, using the LZW compression algorithm
for lossless storage.
TIFF image format is not widely supported by web browsers.
TIFF remains widely accepted as a photograph file standard in the printing business.
TIFF can handle device-specific color spaces, such as the CMYK defined by a particular set
of printing press inks.
OCR software packages commonly generate some form of TIFF image for scanned text pages.
RAW RAW refers to raw image formats that are available
on some digital cameras, rather than to a specific format.
These formats usually use a lossless or nearly lossless compression, and produce file sizes
smaller than the TIFF formats.
Although there is a standard raw image format,, the raw formats used by most cameras are not
standardized or documented, and differ among camera manufacturers.
Most camera manufacturers have their own software for decoding or developing their raw file
format, but there are also many third-party raw file converter applications available
that accept raw files from most digital cameras.
Some graphic programs and image editors may not accept some or all raw file formats, and
some older ones have been effectively orphaned already.
As far as videocameras are concerned, ARRI's Arriflex D-20 and D-21 cameras provide raw
3K-resolution sensor data with Bayer pattern as still images in a proprietary format.
Red Digital Cinema Camera Company, with its Mysterium sensor family of still and video
cameras, uses its proprietary raw format called REDCODE, which stores still as well as audio+video
information in one lossy-compressed file.
GIF GIF 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.
Its LZW lossless compression is more effective when large areas have a single color, and
less effective for photographic or dithered images.
BMP The BMP file format handles graphics files
within the Microsoft Windows OS.
Typically, BMP files are uncompressed, and therefore large; their advantage is their
simple structure and wide acceptance in Windows programs.
PNG The PNG file format was created as a free,
open-source alternative to GIF.
The PNG file format supports 8 bit paletted images and 24 bit truecolor or 48 bit truecolor
with and without alpha channel - while GIF supports only 256 colors and a single transparent
color.
Compared to JPEG, PNG excels when the image has large, uniformly colored areas.
Even for photographs – where JPEG is often the choice for final distribution since its
compression technique typically yields smaller file sizes – PNG is still well-suited to
storing images during the editing process because of its lossless compression.
PNG provides a patent-free replacement for GIF, and can also replace many common uses
of TIFF.
Indexed-color, grayscale, and truecolor images are supported, plus an optional alpha channel.
The Adam7 interlacing allows an early preview, even when only a small percentage of the image
data has been transmitted.
PNG can store gamma and chromaticity data for improved color matching on heterogeneous
platforms.
PNG is designed to work well in online viewing applications like web browsers and can be
fully streamed with a progressive display option.
PNG is robust, providing both full file integrity checking and simple detection of common transmission
errors.
Animated formats derived from PNG are MNG and APNG.
The latter is supported by Mozilla Firefox and Opera and is backwards compatible with
PNG.
PPM, PGM, PBM, and PNM Netpbm format is a family including the portable
pixmap file format, the portable graymap file format and the portable bitmap file format.
These are either pure ASCII files or raw binary files with an ASCII header that provide very
basic functionality and serve as a lowest common denominator for converting pixmap,
graymap, or bitmap files between different platforms.
Several applications refer to them collectively as PNM.
WEBP WebP is a new open image format that uses
both lossless and lossy compression.
It was designed by Google to reduce image file size to speed up web page loading: its
principal purpose is to supersede JPEG as the primary format for photographs on the
web.
WebP is based on VP8's intra-frame coding and uses a container based on RIFF.
HDR Raster formats Most typical raster formats cannot store HDR
data, which is why some relatively old or complex formats are still predominant here,
and worth mentioning separately.
Newer alternatives are showing up, though.
RGBE is the format for HDR images originating from Radiance and also supported by Adobe
Photoshop.
Other raster formats JPEG XR
SGI PCX, obsolete
Sun Raster is an obsolete format TGA, obsolete
ILBM DEEP
IMG ILBM - Interleaved Bitmap, used by Electronic
arts PLBM - Planar Bitmap, proprietary Amiga format
ECW IMG
SID CD5
FITS PGF
PAM is a late addition to the Netpbm family XCF
PSD PSP
VICAR file format Vector formats
As opposed to the raster image formats above, vector image formats contain a geometric description
which can be rendered smoothly at any desired display size.
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.
CGM CGM is a file format for 2D vector graphics,
raster graphics, and text, and is defined by ISO/IEC 8632.
All graphical elements can be specified in a textual source file that can be compiled
into a binary file or one of two text representations.
CGM provides a means of graphics data interchange for computer representation of 2D graphical
information independent from any particular application, system, platform, or device.
It has been adopted to some extent in the areas of technical illustration and professional
design, but has largely been superseded by formats such as SVG and DXF.
Gerber format The Gerber format was developed by Gerber
Systems Corp., now Ucamco, and is a 2D bi-level image description format.
It is the de facto standard format used by printed circuit board or PCB software.
It is also widely used in other industries requiring high-precision 2D bi-level images.
SVG SVG is an open standard created and developed
by the World Wide Web Consortium to address the need for a versatile, scriptable and all-purpose
vector format for the web and otherwise.
The SVG format does not have a compression scheme of its own, but due to the textual
nature of XML, an SVG graphic can be compressed using a program such as gzip.
Because of its scripting potential, SVG is a key component in web applications: interactive
web pages that look and act like applications.
Other 2D vector formats AI
CDR DrawingML
GEM metafiles Graphics Layout Engine
HPGL, introduced on Hewlett-Packard plotters, but generalized into a printer language
HVIF MathML
NAPLPS ODG
!DRAW, a native vector graphic format for the RISC-OS computer system begun by Acorn
in the mid-1980s and still present on that platform today
POV-Ray markup language PPT
Precision Graphics Markup Language, a W3C submission that was not adopted as a recommendation.
PSTricks and PGF/TikZ are languages for creating graphics in TeX documents.
ReGIS, used by DEC computer terminals Remote imaging protocol
VML WMF / EMF
Xar format used in vector applications from Xara
XPS 3D vector formats
AMF - Additive Manufacturing File Format Asymptote - A language that lifts TeX to 3D.
.blend - Blender COLLADA
.dwf .dwg
.dxf eDrawings
.flt - OpenFlight HSF
IGES IMML - Immersive Media Markup Language
IPA JT
OpenGEX - Open Game Engine Exchange PRC
STEP SKP
STL - A stereolithography format U3D - Universal 3D file format
VRML - Virtual Reality Modeling Language XAML
XGL XVL
xVRML X3D
.3D 3DF
.3DM .3ds - Autodesk 3D Studio
3DXML X3D - Vector format used in 3D applications
from Xara Compound formats
These are formats containing both pixel and vector data, possible other data, e.g. the
interactive features of PDF.
EPS PDF
PostScript, a page description language with strong graphics capabilities
PICT SWF
XAML User interface language using vector graphics for images.
Stereo formats MPO The Multi Picture Object format consists
of multiple JPEG images.
PNS The PNG Stereo format consists of a side-by-side image based on PNG.
JPS The JPEG Stereo format consists of a side-by-side image format based on JPEG.