JPG

JPG (or JPEG) stands for the Joint Photographic Experts Group. It uses 24 bits to represent a color pixel. It uses a lossy compressed graphics format that is designed to support photographs rather than line art. See also M-JPEG and JXR.

Description

JPG (or JPEG) stands for the Joint Photographic Experts Group. It uses 24 bits to represent a color pixel (often called True Color). It uses a lossy compressed graphics format that is designed to support photographs rather than line art. It was developed in 1992 and issued as the ISO 10918-1 standard in 1994, the quality depends directly on the amount of compression employed. It is widely used on the Internet and by most digital camera manufacturers.

The JPEG File Interchange Format (JFIF) is a minimal version of the JPEG Interchange Format that was deliberately simplified so that it could be widely implemented and thus has become the de-facto standard. Another standard format is the Exchangeable image file format (Exif). This is the specification used for the image files on digital cameras. (Taken from Graphics#JPG)

The losses in JPG can be both in detail and color. For example large areas of the same color will be modified with slightly different variations of that color over the surface. Copies made from copies will have even more losses that the original.

JPEG LS

JPEG-LS is a nearly lossless implementation of JPG. The recommended version is available from HP LOCO. This is a new addition to the JPG standard.

IrfanView has a plugin for JPG-LS that can read/write this format. It uses a JLS extension. Note the file size is fairly large compared to other compressed formats. It can be useful as an intermediate format during editing to avoid successive resolution reduction.

The term "near-lossless compression" refers to a lossy algorithm for which each reconstructed image sample differs from the corresponding original image sample by not more than a pre-specified value, the (usually small) "loss." Lossless compression corresponds to loss=0.

JPEG 2000

This is a 21st century JPG format from the JPEG group. It will normally have a JP2 or JPG2 extension. It can be lossy or lossless depending on the options used with the file is saved. Some PDF files may contain images in this format. It is sometimes written as JPEG2000.

JPEG 2000 is a new image coding system that uses state-of-the-art compression techniques based on wavelet technology. Its architecture should lend itself to a wide range of uses from portable digital cameras through to advanced pre-press, medical imaging and other key sectors. For information on this format see JPEG org demo. For an even newer format see JPEG XR format. Neither format has wide use in eBooks, however they can show up in PDF format using newer revisions of this standard.

Features

(From the above web site)

• Better efficiency in compression
• Possibility of lossless compression
• Decoding with different output resolutions
• A process to calculate the integrated bit rate (possibility of reaching an aimed bit rate)
• Dividing the image into smaller parts to be coded independently from the others
• ROI coding
• Improvement in noise resilience
• Access to the compressed bit rate at any point in order to access the image directly
• Better performances in coding/decoding through many different cycles
• More flexible file format

Exif

Exchangeable image file format (officially Exif) is a standard that specifies the formats for images (Photographs), sound, and ancillary tags used by digital cameras and smartphones. It is a subset of Jpeg for photos and will also have a JPG extension. It is also used in scanners and other systems handling image and sound files. The specification uses the following existing file formats with the addition of specific metadata tags:

• JPEG discrete cosine transform (DCT) for compressed image files,
• TIFF Rev. 6.0 (RGB or YCbCr) for uncompressed image files
• and RIFF WAV for audio files
  • Linear PCM for uncompressed audio data, and
  • IMA-ADPCM for compressed audio data

Exif also defines the file system and Metadata (covered below)

Metadata

Metadata is supported in the JPG format. It comes in three forms, Comments, IPTC info, and for photographs EXIF data. There are also image properties stored in the file such as size, color info, and resolution.

Comments are free form and are stored at the very top of the file for easy reading by viewing the binary.

IPTC, International Press Telecommunications Council, is a standard which allows full meta including a description that identifies a Title, the author, copyright, caption, Headline and special instructions. Keywords, categories, and credits can also be stored. IPTC - Photo Metadata, core data is exchangeable with Adobe XMP and can be edited with IrfanView. On cameras that also have a GPS the IPTC data may include location data.

Photographs generally include a third type of metadata stored by the camera itself. This is EXIF data (EXchangeable Image File format) and includes information about the camera and settings used to make the picture such as exposure. It may also include GPS location data if the camera has this data or some tools can add it later. Some software editing tools, such as photoshop, create and store EXIF data about the editing parameters.

JPG has some defined references for this data.

Tools

• The JPEGTran program has many jpg manipulation features that can losslessly modify a JPG image. It can resize the image, rotate the image and crop the image.
• http://jpegclub.org/ contains many tools to manipulate JPG images.
• IrfanView is a general viewing editing program that has lossless rotate and crop commands for JPG. It can also view and edit metadata
• See also Editing graphics.

For more information

• http://en.wikipedia.org/wiki/JPEG
• http://www.ijg.org/ the independent JPEG group that makes free tools for jpeg.
• http://www.jpeg.org/ The jpeg standards web site.
• https://en.wikipedia.org/wiki/Exchangeable_image_file_format Exif.