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.
