If you were to look at a computerized image with a
magnifying glass, you would see that the image is made up of
tiny squares. These squares are called 'picture elements',
or 'pixels' for short. Picture 'resolution'
refers to the number of pixels that make up the picture, and
thus the level of detail that the picture contains. The
images that we will use in these lessons are usually about
800 pixels wide and 600 pixels tall (note that this gives a
ratio of 4:3 which corresponds to the shape of your computer
screen). The resolution of such a picture is 800 x 600 = 480
000 pixels or about 0.5 megapixels (1 megapixel = 1 million
pixels). This is a good size for learning to use the GIMP, but this
might be a bit big to include
on a web page. Pictures on web pages should not
normally exceed about 400 000 pixels or about 200KB.
Pictures that are destined to be printed, on the other
hand, need to have a much higher resolution, for printers
are capable or reproducing much finer detail than computer
monitors. Many digital cameras have a resolution of 8
megapixels or more (file size more than 3 MB), and this will give a
good result when printed even up to A4 size.
There are many different formats for saving digital
images. Some are 'lossless' formats which can produce very
large files, others are 'lossy' formats which make the file
size much smaller at the cost of a (usually small) loss of
image quality. Here are some of the most important
formats for you as a GIMP user:
- XCF The 'native' file format of the GIMP is
the XCF format which is a lossless format storing all
the data in the image, including layer data. You should
usually save your unfinished images in this format
unless you are very short of disk space.
- PSD is 'Photoshop document' format - this
format has large file sizes, conserves most of the image
data and is useful if you want to move an image from the
GIMP and continue working on it in Photoshop.
- JPEG is a compressed format that is very
popular for digital photographs. You can set the
compression level yourself, making a compromise between
file size and image quality.
- GIF format is popular for 'artificial' images
such as logos or line art which contain no more than 256
colors (the maximum that GIF can handle). For images
with no more than 256 colors this format is compressed
- PNG combines some of the advantages of both
JPEG and GIF and is set to become popular but it is
fairly new and some programs such as browsers may not
handle this format well.
For compatibility with web pages images should usually be
saved in the JPEG, GIF or PNG formats.
'Color' is a key concept in graphics. Computer graphics
programs need to describe colors with numbers - the RGB
(red, green, blue) convention is the most common way of
doing this. The GIMP can reproduce 256 levels of brightness
(numbered 0 to 255) for each of these 3 primary colors (red,
green, blue), giving a total of 256 x 256 x 256 = 16777216
or about 16 million different colors. (0,0,0) represents
black, (255,0,0) represents bright red, (255,255, 255)
represents white. Adding primary colors gives the secondary
colors: red + blue = magenta, red + green=yellow, green +
blue=cyan. This is demonstrated in the following diagram
which shows what happens when circular red, green and blue
light beams are projected onto a white screen and made to
Such a diagram is sometimes called a color wheel. Colors
on opposite sides of the color wheel, such as green and
magenta, are called complementary colors.
Some of the above statements, such as green + red =
yellow, may surprise you, for your experience mixing paints
may seem to contradict this. The explanation is that paint
pigments absorb colors, leading to color subtraction,
whereas the above text and the color wheel describe color
addition. Color addition corresponds to the way your
computer screen works - if the screen's tiny red and green
dots are lit simultaneously then you will see yellow. It
also corresponds to the way the human eye works - the eye
has color-sensing cells only for red, green and blue - if
the red and green sensors are stimulated simultaneously then
you see yellow. Look again at the diagram, and make sure
that you can tell the difference between red and magenta,
and between blue and cyan. Note how the diagram correctly
shows that when red, green and blue lights (not pigments)
are added together, the result is white.
Can you tell what color the RGB combination (0,255,255)
would refer to? This would be a combination of no red +
bright green + bright blue, in other words CYAN.
When you are working in the GIMP you can click any part
of the image with the Eye Dropper tool
to 'pick up' the color of that pixel and make it the new
foreground color, as displayed in the toolbox (top left
Instead of using the eye dropper tool you can also simply
Alt-click the image at any time. The foreground color is
used by various tools, such as the airbrush, paintbrush,
pencil, paint bucket etc. You can also change the foreground
or background color by clicking their respective squares.
Clicking the small black and white squares sets the
foreground color to black and the background color to white.
Clicking the double-headed arrow swaps the foreground and