|
Image Optimization
Getting the most
from your graphic images can make your web content effective and efficient.
Although Internet access is speeding up and graphics are becoming richer,
it is still important to optimize an image to the smallest possible
size without affecting how the image looks.
Optimization techniques
are different for each bitmap format. GIF files can be optimized by
removing colors from the image color palette. JPG can be optimized
by lessening the quality of the image.
GIF Optimization
The best way to reduce the file size of a GIF image is to reduce the
number of colors stored in the file. GIF images contain a maximum of
256 colors
and
the fewer colors an image has the smaller its file size.
Images suitable to GIFs are graphic files using flat areas of color
like an illustration or graphic image of text.
Color
Depth
Graphics programs refer to the number of colors in a file as color
depth or bit depth. Bit depth determines the number of colors a pixel
value can represent. In most cases the larger the bit depth, the more
colors, the better the graphics, and the larger the file size. Fortunately,
most graphics programs eliminate the guesswork by letting you preview
an image at a selected color depth before you export.
|
COLOR
DEPTH CHART
|
|
1
bit
|
=
|
2
colors |
|
2
bits
|
=
|
4
colors |
|
3
bits
|
=
|
8
colors |
|
4
bits
|
=
|
16
colors |
|
5
bits
|
=
|
32
colors |
|
6
bits
|
=
|
64
colors |
|
7
bits
|
=
|
128
colors |
|
8
bits
|
=
|
256
colors |
Each
GIF is annotated in what color depth it was exported and the resulting
file size. You can see there is very little difference between the
256, 128, 64, and 32 color images. This graphic's optimum color depth
would be 32 colors (5bit) or 16 colors (4bit).
256 colors
3.45k |
128 colors
2.98k |
64 colors
2.81k |
32 colors
2.30k |
16 colors
1.81k |
8 colors
1.4k |
4 colors
1.02k |
2 colors
.7k |
JPG Optimization
JPG, useful for photographs or other complex images with continuous
color, can compress and reduce significantly in file size. The greater
the compression the smaller the file size.
Quality Control
Graphics programs usually refer to the rate of JPG compression as Quality.
By making adjustments down in quality the image lessens in color (or
quality) and thereby reduces the file size. Some programs express this
quality as a percentage of the original image and other programs as
a 1-10 rating system where 1 is the lowest quality and 10 the highest.
For the best results use your graphics program preview options to experiment
with quality.
Each
JPG is annotated in what quality rate it was exported and the resulting
file size. JPGs (particularly this small) reduce very well. Photographic
images represent many more colors than the human eye can perceive
so information sometimes lost through compression is beyond the range
of vision.
This graphic's optimum Quality would be around 60% of the original
saving a great deal in file size.
100 %
11.74k |
90%
5k |
80%
3.55k |
70%
2.89k |
60%
2.51k |
50%
2.23k |
40%
1.99k |
30%
1.73k |
True
vs. Web Formats
JPG and GIF compression compromises the quality of the original image.
These formats actually lose information when saving and compressing
a file. That information can never be recovered. When you need to save
or keep a copy of an image for further editing or to use in other circumstances
save the original in a format that records the image correctly without
losing any details. These are sometimes called "true formats".
Most graphics programs offer an Export or Save As command
to let you safely export separate JPG and GIF images for posting on
the Internet, saving the original as a true image format. Images can
often be saved in the proprietary format of your graphics program. As
an example, Photoshop files (files ending in a .psd extension) can retain
all the information of original artwork and are readily read by a variety
of other programs.
|
