User Guide

GNU Image Manipulation Program
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Now you should be able to read PostScript files with GIMP. Please note that you must not move the Ghostscript directories
once the installation is complete. The installation creates registry entries which allow Ghostscript to find its libraries.
(These instructions courtesy of http://www.kirchgessner.net.)
PNG
PNG is the acronym of ‘Portable Network Graphic’ (pronounce ‘ping’. This recent format offers many advantages and a
few drawbacks: it is not lossy and gives files more heavy than the JPEG format, but it is perfect for saving your images
because you can save them several times without losing data each time (it is used for this Help). It supports True Colors
(several millions of colors), indexed images (256 colors like GIF), and 256 transparency levels (while GIF supports only
two levels). Unfortunately, Microsoft Internet Explorer recognizes only two transparency levels).
PSD
PSD is Adobe Photoshop’s native file format, and it is therefore comparable to XCF in complexity. GIMP’s ability to
handle PSD files is sophisticated but limited: some features of PSD files are not loaded, and only older versions of PSD
are supported. Unfortunately, Adobe has now made the Photoshop Software Development Kit which includes their file
format specifications proprietary, and only available to a limited set of developers approved by Adobe. This does not
include the GIMP development team, and the lack of information makes it very difficult to maintain up-to-date support for
PSD files.
PSD is Adobe Photoshop’s native file format, and it is therefore comparable to XCF in complexity. GIMP’s ability to
handle PSD files is sophisticated but limited: some features of PSD files are not loaded, and only older versions of PSD
are supported. Unfortunately, Adobe has now made the Photoshop Software Development Kit which includes their file
format specifications proprietary, and only available to a limited set of developers approved by Adobe. This does not
include the GIMP development team, and the lack of information makes it very difficult to maintain up-to-date support for
PSD files.
Q
Quantization
Quantization is the process of reducing the color of a pixel into one of a number of fixed values by matching the color to
the nearest color in the colormap. Actual pixel values may have far more precision than the discrete levels which can be
displayed by a digital display. If the display range is too small, then abrupt changes in colors (false contours, or banding)
may appear where the color intensity changes from one level to another. This is especially noticeable in Indexed images,
which have 256 or fewer discrete colors.
One way to reduce quantization effects is to use
Dithering. The operations in GIMP which perform dithering are the Blend
tool (if you have enabled the dithering option) and the Convert to Indexed command. However, they only work on RGB
images and not on Indexed images.
R
RGB
Figure 17.2: Additive color model