Compression

Vincent Bockaert, 123di.com

Image files can be compressed in two ways: lossless and lossy.

Lossless Compression

Lossless compression is similar to what WinZip does. For instance, if you compress a document into a ZIP file and later extract and open the document, the content will of course be identical to the original. No information is lost in the process. Only some processing time was required to compress and decompress the document. TIFF is an image format that can be compressed in a lossless way.

Lossy Compression

Lossy compression reduces the image size by discarding information and is similar to summarizing a document. For example, you can summarize a 10 page document into a 9 page or 1 page document that represents the original, but you cannot create the original out of the summary as information was discarded during summarization. JPEG is an image format that is based on lossy compression.

A Numerical Example

The table below shows how, on average, a five megapixel image (2,560 x 1,920 pixels) is compressed using the various image formats which are discussed in this glossary. Please note that in reality, the compressed file sizes will vary significantly with the amount of detail in the image. For example, the table shows 1.3 MB as file size for an 80% Quality JPEG five megapixel image. However, if the image has a lot of uniform surfaces (e.g. blue skies), it could be only 0.8 MB at 80% JPEG quality, and if it has a lot of fine detail, it could be 1.7 MB. The purpose of this table is to give a ballpark estimate.

Image Format
Typical File Size in MB
Comment
Uncompressed TIFF
14.1
3 channels of 8 bits
Uncompressed 12-bit RAW
7.7
1 channel of 12 bits
Compressed TIFF
6.0
Lossless compression
Compressed 12-bit RAW
4.3
Nearly lossless compression
100% Quality JPEG
2.3
Hard to distinguish from uncompressed
80% Quality JPEG
1.3
Sufficient quality for 4" x 6" prints
60% Quality JPEG
0.7
Sufficient quality for websites[1]
20% Quality JPEG
0.2
Very low image quality

The JPEG topic in this glossary shows an example as to how image quality is affected by JPEG compression.

Footnote

  1. (1) For the web you would of course downsample the image to a lower resolution.
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
Click here to visit 123di.com