Image Processing Reference
The manner in which you can edit an image is influenced by the structure
of its pixels. Basically, each image dot can be edited in terms of brightness and
color. GIMP 2.6 supplies appropriate and easy-to-use tools for editing single
dots as well as groups of dots.
When you make a general change to a pixel image, usually the whole
image will be affected. Therefore, if you wish to manipulate only a specific
area of an image, you should use a selection tool to designate that area. You
may even want to cut a desired selection from the image so you can work
with layers (transparent “foils” containing distinct image objects that can be
manipulated separately and are layered one on top of the other).
Selections, masks, and layers are advanced tools that are provided by
image editing programs like GIMP for detail work. These topics will be dealt
with extensively in the hands-on exercises that follow.
In contrast to pixel images, vector graphics are used when creating
original graphics and logos. Rather than editing image pixels, you can use
vectors to create novel image elements. Vector images are made up of lines,
curves, circles, rectangles, and fills. The size of each of these elements can be
scaled; the contour can be filled with color or gradients. For graphics, this is
less data intensive. Vector or contour shapes can also be selected and edited
individually. At any time, you can tweak the shape or change the color of a
fill. However, this requires another type of image editing program, a so-called
vector graphics program. For instance, Inkscape is the best-known free, open
source vector graphics program ( http://www.inkscape.org/ ). Commercial
programs for this type of graphics work are Corel Draw and Adobe Illustrator.
You should know: Vector graphics are almost boundlessly scalable.
However: Editing vector graphics images requires different techniques
and specifications than editing pixel images.
Bottom line: Photos and other pixel images can be converted to vector
graphics only in an extremely simplified form, and sometimes not at all.
Comparing pixel and vector images
Problems with Pixel Images
You can add text or graphic elements to pixel images. These are also displayed
using pixels, but they have a disturbing element. In text for example, all but
horizontal or vertical edges of the letters appear serrated. This is called aliasing .
Anti-aliasing is a countermeasure used to smooth the border of the pixilated,
and therefore serrated, letters. Anti-aliasing adds pixels at the border of a
letter, which are colored in the color of the text, but fading to transparency. In
this way, a kind of blending is achieved. The edges of the letters lose definition
and appear smoother (see figure 1.3d ).
You can smooth the edges of pixilated graphic elements by choosing
the feathered edges. Feathered edges of selections will be dealt with in great
detail later in the topic.
Text without and with anti-aliasing