- Image compression
Image compression is the application of
Data compressionon digital images. In effect, the objective is to reduce redundancy of the image data in order to be able to store or transmit data in an efficient form.
Image compression can be lossy or lossless. Lossless compression is sometimes preferred for artificial images such as technical drawings, icons or comics. This is because lossy compression methods, especially when used at low
bit rates, introduce compression artifacts. Lossless compression methods may also be preferred for high value content, such as medical imagery or image scans made for archival purposes. Lossy methods are especially suitable for natural images such as photos in applications where minor (sometimes imperceptible) loss of fidelity is acceptable to achieve a substantial reduction in bit rate.
Methods for lossless image compression are:
Run-length encoding– used as default method in PCXand as one of possible in BMP, TGA, TIFF
* Adaptive dictionary algorithms such as
LZW– used in GIF and TIFF
* Deflation – used in PNG, MNG and
Methods for lossy compression:
* Reducing the color space to the most common colors in the image. The selected colors are specified in the color palette in the header of the compressed image. Each pixel just references the index of a color in the color palette. This method can be combined with
ditheringto avoid posterization.
Chroma subsampling. This takes advantage of the fact that the eye perceives brightness more sharply than color, by dropping half or more of the chrominance information in the image.
Transform coding. This is the most commonly used method. A Fourier-related transform such as DCT or the wavelet transformare applied, followed by quantizationand entropy coding.
The best image quality at a given
bit-rate(or compression rate) is the main goal of image compression.However, there are other important properties of image compression schemes:
Scalability generally refers to a quality reduction achieved by manipulation of the bitstream or file (without decompression and re-compression).Other names for scalability are "progressive coding" or "embedded bitstreams".Despite its contrary nature, scalability can also be found in lossless codecs, usually in form of coarse-to-fine pixel scans.Scalability is especially useful for previewing images while downloading them (e.g. in a web browser) or for providing variable quality access to e.g. databases.There are several types of scalability:
* Quality progressive or layer progressive: The bitstream successively refines the reconstructed image.
* Resolution progressive: First encode a lower image resolution; then encode the difference to higher resolutions.
* Component progressive: First encode grey; then color.
Region of interest coding. Certain parts of the image are encoded with higher quality than others.This can be combined with scalability (encode these parts first, others later).
Meta information. Compressed data can contain information about the image which can be used to categorize, search or browse images.Such information can include color and texture statistics, small
previewimages and author/copyright information.
Processing power. Compression algorithms require different amounts of
processing powerto encode and decode. Some high compression algorithms require high processing power.
The quality of a compression method is often measured by the
Peak signal-to-noise ratio. It measures the amount of noise introduced through a lossy compression of the image. However, the subjective judgement of the viewer is also regarded as an important, perhaps the most important, measure.
Graphics file formats
Digital signal processing
Standard test image
* Image Quality
* [http://video.google.com/videoplay?docid=-163332982698249627 MIT Linear Algebra Lecture on Image Compression] at Google Video, from MIT OpenCourseWare
* [http://videocodecs.blogspot.com/2007/05/image-coding-fundamentals_08.html Image Coding Fundamentals]
* [http://www.mvnet.fi/index.php?osio=Tutkielmat&luokka=Yliopisto&sivu=Image_compression A study about image compression] (Image compression basics and comparing different compression methods like JPEG2000, JPEG and JPEG XR / HD Photo)
* [http://dvd-hq.info/data_compression.php Data Compression Basics] (includes comparison of PNG, JPEG and JPEG-2000 formats)
* [http://datacompression.dogma.net/index.php?title=FAQ:What_is_the_state_of_the_art_in_lossless_image_compression%3F FAQ:What is the state of the art in lossless image compression?] from [news://comp.compression comp.compression]
Wikimedia Foundation. 2010.