Image file format

Image file formats provide a standardized method of organizing and storing image data. This article deals with digital image formats used to store photographic and other image information. Image files are made up of picture elements, called pixels. The pixels that comprise an image are in the form of a grid of columns and rows. Each of the pixels in an image stores digital numbers representing brightness and color.

Image file sizes

Image file sizes, expressed in bytes, increase with the number of pixels in the image, and the color depth of the pixels. The more rows and columns, the greater the image resolution and the greater the file size. Also, each pixel making up the image increases in size as color depth is increased. An 8-bit pixel (1 byte) can store 256 colors and a 24-bit pixel (3 bytes) can store 16 million colors. The latter is known as truecolor.

Image compression is a method of using algorithms to decrease file size. High resolution cameras lead to large image files. Files sizes may range from hundreds of kilobytes to many megabytes depending on the cameras resolution and the format used to save the images. High resolution digital cameras record 8 megapixels (MP) images, or more, in truecolor. Consider an image taken by an 8 MP camera. Since each of the pixels uses 3 bytes to record true color, the uncompressed image would occupy 24,000,000 bytes of memory. That is a lot of storage space for just one image, and cameras must store many images to be practical. Faced with large file sizes, both within the camera, and later on disc, image file formats have been developed to address the storage problem. An overview of the major graphic file formats is given below.

Image file compression

There are two types of image file compression algorithms: lossy and lossless.

Lossless compression

Lossless compression algorithms reduce file size with no loss in image quality, although compression ratios are generally weak. Most images destined for print, or when image quality is valued above file size, are compressed using lossless algorithms.

Lossy compression

Lossy compression algorithms take advantage of the inherent limitations of the human eye and discard information that cannot be seen. Most lossy compression algorithms allow for variable levels of quality (compression) and as these levels are decreased, file size is also reduced. At the highest compression levels, image deterioration becomes noticeable. This deterioration is known as compression artifacting.

The images linked below demonstrate the noticeable artifacting associated with lossy compression algorithms. Click on the thumbnail image to view the full size version.

Lossless Image Compression

Lossy Image Compression

NOTE: The two images above are designed to be illustrative only. Both were converted from a losslessly compressed RAW image in Adobe Photoshop 9.0. The "lossy" image was repeatedly opened and resaved using a low JPEG quality setting to increase visual artifacting and then exported as a PNG, while the "lossless" image was saved once as a PNG to minimize visual artifacting.

Exif

The Exif (Exchangeable image file) format is an algorithm incorporated in the JPEG software used in most cameras. Its purpose is to record and to standardize the exchange of data between digital cameras and editing and viewing software. The data are recorded for individual images and includes such things as; camera settings, time and date, shutter speed, exposure, image size, compression, name of camera, color information, etc. When images are viewed or edited by image editors, such as Paint Shop Pro, all of this image information can be displayed.

Major graphic file formats

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.

Raster formats

JPEG

The JPEG (Joint Photographic Experts Group) image files are a lossy format. The DOS filename extension is JPG, although other operating systems may use JPEG. Nearly all digital cameras have the option to save images in JPEG format. The JPEG format supports 16-bit color and produces relatively small file sizes. Fortunately, the compression in most cases does not detract noticeably from the image. But JPEG files do suffer generational degradation when repeatedly edited and saved. Photographic images are best stored in a lossless non-JPEG format if they will be re-edited in future, or if the presence of small "artifacts" (blemishes), due to the nature of the JPEG compression algorithm, is unacceptable. JPEG is also used as the image compression algorithm in many Adobe PDF files.

TIFF

The TIFF (Tagged Image File Format) is a flexible image format that normally uses a filename extension of TIFF or TIF. TIFF's flexibility is both a feature and a curse, with no single reader capable of handling all the different varieties of TIFF files. TIFF can be lossy or lossless. Some types of TIFF offer relatively good lossless compression for bi-level (black and white, no grey) images. Some high-end digital cameras have the option to save images in the TIFF format, using the LZW compression algorithm for lossless storage. The TIFF image format is not widely supported by web browsers, and should not be used on the World Wide Web. TIFF is still widely accepted as a photograph file standard in the printing industry. TIFF is capable of handling device-specific color spaces, such as the CMYK defined by a particular set of printing press inks.

RAW

The RAW image format is a file option available on some digital cameras. It usually uses a lossless compression and produces file sizes much smaller than the TIFF format. Unfortunately, the RAW format is not standard among all camera manufacturers and some graphic programs and image editors may not accept the RAW format. The better graphic editors can read some manufacturer's RAW formats, and some (mostly higher-end) digital cameras also support saving images in the TIFF format directly. Adobe's Digital Negative Specification is a recent (September 2004) attempt at standardizing the various "raw" file formats used by digital cameras.

PNG

The PNG (Portable Network Graphics) file format is regarded and was made as the free and open successor to the GIF file format. The PNG file format supports true color (16 million colors) whereas the GIF file format only allows 256 colors. PNG excels when the image has large areas of uniform color. The lossless PNG format is best suited for editing pictures, and the lossy formats like JPG are best for final distribution of photographic-type images because of smaller file size. Many older browsers do not yet support the PNG file format. The Adam7-interlacing allows an early preview even when only a small percentage of the data of the image has been transmitted.

GIF

GIF (Graphic 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 and cartoon style images. The GIF format supports animation and is still widely used to provide image animation effects.

BMP

The BMP (bit mapped) format is used internally in the Microsoft Windows operating system to handle graphics images. These files are typically not compressed resulting in large files. The main advantage of BMP files is their wide acceptance and use in Windows programs. Their large size makes them unsuitable for file transfer. Desktop backgrounds and images from scanners are usually stored in BMP files.

XPM

The XPM format is the default X Window System picture format (very popular in the Linux world). Its structure is based on the string format of the C programming language. Because XPM was designed to be human-readable, and is stored as uncompressed plain-text, the file size of these pictures can be more than twice as large as uncompressed binary bitmap files (such as BMP, uncompressed TIFF, MacOS-PICT, or Irix-RGB formats). This format is unsupported by most non-Unix software and operating systems (though many web-browsers retain display support for the XBM subset, which was the minimal image format in the early days of the WWW).

MrSID

The MrSID (Multiresolution Seamless Image Database) format is a wavelet compression format used mostly by Geographic Information Systems to store massive satellite imagery for map software.

Vector formats

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.

SVG

SVG (Scalable Vector Graphics) is an open standard created and developed by the World Wide Web Consortium to address the need (and attempts of several corporations) 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.

See also

• Comparison of graphics file formats
• Graphics file format summary

External links

• Description of Bitmap graphics file
• Educational treatment of GIF, PNG, and JPG file formats
• Image File Formats and Extensions
• When to Save Web Images in JPEG Format