What is compression by wpr1947


									What is compression?
During compression, data that is duplicated or which has no value is eliminated or saved in a
shorter form, greatly reducing a file's size. When the image is then edited or displayed, the
compression process is reversed.
There are two forms of compression, lossless and lossy, and digital photography uses both forms.
Lossless compression
Lossless compression uncompresses an image so its quality matches the original source.
Although lossless compression sounds ideal, it doesn't provide much compression. A leading
lossless compression scheme is LZW (Lempel-Ziv-Welch). This is used in GIF and TIFF files
and achieves compression ratios of 50 to 90%
Lossy compression
Although it's possible to compress images without loosing some quality, it's not practical in
many cases. Therefore, all popular digital cameras use a lossy compression (rhymes with bossy)
that degrades images to some degree and the more they're compressed, the more degraded they
become. In many situations, such as posting images on the Web, the image degradation isn't
obvious. However, enlarged prints reveal it quite clearly.
Although lossy compression does not uncompress images to the same quality as the original
source image, the image remains visually lossless and appears normal if not enlarged too much.
The trick is to remove data that isn't obvious to the viewer. For example, if large areas of the sky
are the same shade of blue, only the value for one blue pixel needs to be saved along with the
locations of the other identical pixels in the image. The leading lossy compression scheme is
JPEG (Joint Photographic Experts Group) used in JFIF files (JPEG File Interchange Format).
This scheme allows you to select the degree of compression. Compression Ratios between 10:1
and 40:1 are common.

Digital camera file formats
When images are stored in your camera, they are stored in one of the available formats. Over the
years, a number of different formats have been developed to store photographs and other bit-
mapped images. Each format has its own unique characteristics which determine when and
where you might choose it over the others. Whichever format you choose you are not
permanently locked into that format. There are programs that will convert it to any of the other
commonly used formats.
Raw image file formats
Since compression degrades images to some degree, some cameras allow you to use an
uncompressed format to preserve as much of the captured image quality as possible. In some
cases, these raw formats are proprietary and not supported by many other programs. For
example, Canon uses the CCD Raw mode to record images using no compression. This format is
transferred to the computer right off the sensor and interpolated there. Other cameras use a more
universal format such as TIFF.
The JPEG (Joint Photographic Experts Group) format, pronounced "jay-peg," is by far the most
popular format for the storage of photographic images and displaying them on the Web.
The JPEG format is optimized for the display of photographs and doesn't work as well as GIF for
text or line drawings since GIF is optimized for those kinds of images. JPEG images have two
distinctive features:
Because JPEG compression affects the quality of the image, most cameras allow you to choose
between different levels of compression. This allows you to choose between lower compression
and higher image quality or greater compression and poorer quality. The only reason to choose
higher compression is to create smaller image files so you can store more images, send them by
e-mail, or post them on the Web; if you want to make prints, larger file sizes generally make
better quality prints. Most cameras give you two or three choices equivalent to Good, Better,
Best although the names change.

To top