HTML and CSS Reference
In-Depth Information
Photorealistic images with partially transparent parts can be represented in 32 bits with an additional channel
called the
alpha channel
8
(that is, RGBA color space).
The color depth of logos, pictograms, and icons varies, depending on the complexity and image content.
Compression
To reduce file size, digital bitmap images are usually compressed. Compressed images are popular not only for storage
but also for web publishing because of the smaller bandwidth requirement.
Image compression can be lossy or lossless. In the case of
lossy compression
s, part of the original information
is lost and cannot be restored, which might affect image quality.
Lossless compression
algorithms allow the
reconstruction of the original data from the compressed data, which is a nice feature but not always needed. The
resulting files of lossy compression algorithms are generally smaller in size than the ones using lossless compression.
As you will see, JPEG is the most popular lossy image format, and PNG is the most popular lossless image format on
the Web.
Image-processing software tools usually provide the option to adjust the degree of compression, making it
possible to select a trade-off between storage size and image quality. For example, Adobe Photoshop has a scale for a
JPEG compression ratio with values
0
-
12
. The supported quality ranges are
Low
(
0
-
4
),
Medium
(
5
-
7
),
High
(
8
-
9
), and
Maximum
(
10
-
12
). The settings are available after determining the name and destination of the file in the File
➤
Save
As… dialog window (Figure
9-7
).
Figure 9-7.
Excellent quality with small file size
JPEG typically achieves about 10:1 compression with barely noticeable loss in image quality.
8
There are also other approaches to provide transparency, but for partially transparent photographs, it is the common solution.
Search WWH ::
Custom Search