Glossary
Antialiased
A way of making jagged lines appear smoother by filling the edges with colors that are averaged between the line color and background color. Similar to a blurring effect.
Blur
A type of filter that softens an image, making it looked blurred. A gaussian
average can be used to reduce detail and create a hazy effect.
Clipboard
Copying an image takes that information and writes it into a portion of
RAM, called the clipboard, that serves as temporary storage. Pasting the
image transfers it back into a document. To preserve all the information,
it usually has to be in the same application, though sometimes information
(like text) can be transferred, using the clipboard, from one application
to another.
Color
8, 16, 24 or 32-bit color, which is equal to 256, thousands of colors and millions of colors respectively. 8 bit color can be called indexed color because it uses a specific color table (the "palette") of only 256 colors. 16, 24, and 32-bit color is commonly called RGB (red-green-blue) color. File types like GIF, Pixelpaint and Macpaint can only be 8-bit color, while other file types like JPEG and TIFF are all-inclusive.
Crop
Using a tool (as in Photoshop or other painting program) to draw a line around the area of an image that you want, and "cropping" by clicking inside of the area. This procedure varies from program to program, but most of them have an option to adjust the canvas size or area of the image, which has the same effect.
Dithering
Dithering is the process of approximating a color that cannot be displayed
with uniformly dispersed dots of other colors. Unless the resolution is
high, this leads to a noticable degradation of the image. Using the web-safe
color palette for your GIF images helps to avoid dithering on your
web pages.
DPI
Dots (pixels) per inch - the number of pixels per inch in an image. The
more pixels there are per inch, the higher the resolution of the image
will be. If this is changed proportionally with the size (e.g. increase
size/decrease dpi) then there is no change in quality. If this is changed
independent of the size, then there is a change in quality.
Grayscale
8 bit black and white, where there is a progression from black to white with 254 grays in between. Gets very good results from LZW and JPEG Compression. Grayscale images can be converted to 1 bit line art using a pattern dither, a random dither or a 50% threshold.
HSB
Hue-Saturation-Brightness. The hue is the color tone, saturation is the presence or lack of white (lots of white produce "washed-out" colors) and brightness is the presence or lack of black (lots of black produce opaque, dull colors).
JPEG
Joint Photographers' Expert Group - a type of compression that minimizes
the amount of color information in your image that is actually not distinguishable
by the human eye. JPEG compression rates can be very high, although using
it too many times on the same image will result in a loss of quality.
Line Art
1 bit black and white (bitmapped), where there are only two colors possible
in your image. Normal line art may consist of a diagram or line drawing.
Diagonal lines viewed onscreen have jagged edges due to the screen's low
resolution.
LZW Compression
A type of compression that can be used automatically when saving TIFF
files. Using LZW compression will not result in a loss in quality, but
the compression rate decreases as the amount of information (number of
bits) increases - very good for line art images.
Sharpen
A type of filter that finds changes in color and contrast and makes them
more defined.
Web-safe color palette
A palette of 216 colors contained in Windows' 256 default colors (VGA)
and in the Macintosh system palette, so using it avoids dithering
on both platforms. The colors it contains are the combinations of 0%,
20%, 40%, 60%, 80%, and 100% for each of the channels Red, Green and Blue
(hex-codes 00, 33, 66, 99, CC, FF).
Last revision March 9, 1999 VK
An example of using JPEG compression on the same file four times and the composite result.
An example of different types of dithering schemes for conversion to
2 bit black and white
(clockwise from upper left: grayscale original, 50% threshold, random
dither, pattern dither).
An example of different types of dithering schemes for conversion to
8 bit Indexed color
(clockwise from upper left: color original, adaptive palette / no dither,
adaptive palette / diffusion dither, system palette / diffusion dither).