Image Processing Reference
In-Depth Information
If you want to appropriate a color from an existing image as the background
color for a web page, you will need to specify hexadecimal numbers (base
16). Convert the decimal numbers (see above) into hexadecimal numbers,
which are simply denoted by adding a # symbol in front of the number. You
can use any tool for this conversion, including the Windows Calculator ( Start >
Programs > Accessories > Calculator > View > Scientific). ).
GIMP conveniently performs this conversion for you. Its Color Picker tool
will provide you with the hexadecimal number value for every color.
Indexed Colors
Many image file formats used on the Internet use indexed colors rather than
RGB. Indexed color images don't save the color values in the pixels themselves,
but add a defined color palette. The number of colors in this indexed color
palette is limited to 256. Indexed images are usually smaller than RGB images
since they possess a color depth of 8 bits instead of 24 bits. When an image
is converted to indexed color, a predefined color palette or a set of colors
derived from the image itself will automatically be formulated. The palette
can contain a maximum of 256 colors. File formats that automatically create
images with their own color palettes include the compressed GIF format
as well as the 8-bit PNG format. Indexed images can also include gray-level
images (with a maximum of 256 shades of gray).
However, you may find using an indexed palette cumbersome because
it won't allow you to access all of GIMP's editing options. Indexed images
are normally edited in RGB mode. After editing, the indexed palette can be
selected and attached to the image before saving and exporting the file for
use on the Internet.
The CMYK Color Model—Cyan, Magenta, Yellow, Key (Black)
Digital pre-press in four-color printing uses the CMYK color model . The
CMYK model behaves quite differently than the RGB model. For one thing,
CMYK has four color channels rather than three like RGB, so the nominal
number of colors increases in CMYK. Nevertheless, the color range of CMYK is
smaller than RGB. Thus, when you convert an image from RGB to CMYK, it may
appear paler or darker due to the loss of image information or the insertion
of additional black. To avoid fading or darkening, edit your image in the RGB
mode before converting it to CMYK mode. Also, because changes often result
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