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Low end digital cameras do not at all deliver image quality comparable to scanning photographic
slides or prints. So the main purpose for using them may be the need to get images fast
at moderate quality, for example when producing id-cards for public transportation with photographs.
Imaging devices usually use their own file formats. Sometimes they allow to export the image
in some common format like TIFF. In medicine many different imaging devices are used [2],
for example ultrasound, and computer tomography. Very important for these is the standard
ACR/NEMA 2 and its successor DICOM ("Digital Imaging and Communications in Medicine")
[3], which explains file formats and communication mechanisms. However, to explain
DICOM is beyond the scope of this tut orial.
2.2 Manipulation
We call every change of the source data a manipulation. There are different purposes to manipulate
an image, for example enhancement, compression, cropping, or change of resolution.
Compression will be discussed in a separate section due to its essential importance in networking.
Most commonly used techniques for image enhancement are color correction, adjustment of
brightness, contrast and gamma, labeling, and the usage of filters like sharpening. They can be
performed from every state of the art image processing program, for example Photoshop.
There are also very powerful free- and shareware programs for this like Paintshop Pro or
Lview Pro.
Note: Manipulation should always be done before down scaling an image.
Lossy compression should be applied only after all changes have been
made.
Lossy compression should be applied after all changes have been made, because repeated decompression-plus-lossy-compression
cycles will decrease the image quality. However there is
one exception to this rule [4]. You can compress an image with JPEG, decompress it, make
local changes and compress it again with exactly the same parameters as before. This will only
affect the area close to the change.
To decide, in which resolution in space and color an image should be stored, have a look at the
intended usage: Will it be display on a computer screen with about 70 dpi or is it print at 1200
dpi, is zooming required? How many colors are needed? With many image manipulation programs
you can count the number of colors used in a specific image. Would you like to use
greyscale images or color images?
Greyscale images usually use 1, 4, 8, 10, 12, and (16) bits per pixel (bpp). More than 8 bpp
(corresponding to more than 256 levels of grey) are only used in specialized application areas
like digital photography and radiology. For color images usually 4, 8, 15 (direct color), 16
(high color), 24 (true color), 36, and (48) bpp are used.
It is difficult to display a colored image realistically with 8 bpp or less. Due to this, color palettes
have been invented which select the best suitable 256 colors out of 16,8 million for a
given image. These color values will be stored for every image and the bit value for every pixel
is just a reference to the color selection. Using an optimized color palette there will be hardly
any difference visible between the 8 bpp image and the true color original. The quality of the
image depends crucially on the selection of good palette entries, which is not always straight
forward.
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