Originally, GIMP was a Linux/UNIX program. However, it has been ported to the Microsoft Windows platform -- that effectively makes GIMP a cross-platform (XP) program.
GIMP v Photo$hop
GIMP looks and feels much like Adobe Photoshop (GIMP = GNU Image Manipulation Program). Unlike Photoshop, the GIMP is free (as in beer) and does not need to be registered or activated.
There are desktop and user interface (UI) differences between GIMP and Photoshop. That might take some getting use to if you already are familiar with Photoshop.
Traditionally, Photoshop has had an easier to use desktop and user interface. However, the GIMP 1.3 desktop and UI changes all that. GIMP now is as easy to use as is Photoshop -- perhaps even easier.
GIMP does not have all the advanced, commercial, pre-press features that Photoshop does have. However, it comes pretty close to Photoshop with actual photo editing and image manipulation. Moreover. GIMP has some features that Photoshop does not have.
In short, unless you are a professional photographer or image editor who needs Photoshop's prepress features, you likely can do just about everything that you need or want to do with GIMP instead of Adobe Photoshop. Moreover at Photoshop's $649 price tag ($169 for upgrade from a licensed copy of Photoshop 7 or earlier) there are 649 more good reasons to use GIMP instead of Photoshop.
Photoshop has a greedy and consumer-unfriendly end-user license agreement (EULA). GIMP is free and has a very consumer-friendly license known as the General Public License (GPL). Please see the Adobe Photoshop & GIMP Licensing Note in the right-hand sidebar.
Additionally, Photoshop has a horrible and very anti-consumer Product Activation requirement. GIMP has no such crap! Please see the Adobe Product Activation Note in the sidebar.
A very nice thing about GIMP is that you can try it without paying a cent. Moreover, if you try the GIMP and like it, you do not have to pay a cent to keep on using it. If you are a Linux user, chances are that you already have GIMP installed on your Linux-based computer.
If you are a Microsoft Windows user, chances are that you do not already have the GIMP installed. Nevertheless you can download a Windows version of GIMP, free, and easily install it yourself. If your Linux distribution did not come with the GIMP you also can download a free Linux version of the GIMP. Download links are in the Resources section at the end of this article on page 2.
Gimp User's Manual
Karin and Olof S Kylander have a very nice chapter, Gimp for Photoshop users, in their book, Gimp User's Manual, that quickly will get you up to speed if you are a Photoshop user migrating to the GIMP. It's Chapter 5. Gimp User's Manual is online and there is a free-download link in the Resources section at the end of this article on page 2.
Version-wise, Gimp User's Manual is somewhat outdated. It is about GIMP 1.0.x. The current stable version of GIMP is 1.2.5 and the current developer version is 1.3.x. Moreover, GIMP 2.0 is expected to be released soon.
Nevertheless, you may freely download the Gimp User's Manual. That makes it a very good deal and well worth using to learn how to use the GIMP. The main point of this versions discussion is to let you know that GIMP has lots more features and many user-interface improvements since Gimp User's Manual was published.
The GIMP Desktop and Canvas
Figure 1, below, is a screen shot of GIMP version 1.3 running on SUSE Linux 9.0 and the KDE desktop for Linux. Figure 2, on page 2, is a screen shot of GIMP version 1.2.5 running on Microsoft Windows 98 SE.
Not all the dialog boxes or palettes are shown in these figures. However, please notice the overall similarity of the GIMP and Photoshop collections of tools, dialog boxes, and palettes. Please see the Dialogs and Palettes Note in the right-hand sidebar.
Some of the differences in appearance of the GIMP desktop in Figures 1 and 2 are because SUSE uses a different theme in its implementation of the GIMP than the default Windows GIMP theme. However, some differences are version-based differences.
For example, the menu-bar over the canvas screen in Figure 1 (GIMP 1.3) is not part of the canvas screen in GIMP 1.2.5 (Figure 2).
In GIMP 1.3, some dialogs/palettes are toggled in the same dialog box. If you look at the bottom left dialog box in Figure 1 you should see the brushes dialog/palette. Just above the line Circle (11) (11x11) on that brushes dialog/palette there are five icons; tools, brush, gradient, paint bucket, and layers. You can toggle that dialog/palette to display either the Tool Options, Brush Grid, Gradient List, Pattern Grid, or Layer List dialog/palette by clicking on the appropriate icon.
You can add icons to toggle that bottom left dialog box to display any of the dialogs that are available in the GIMP. Or, you can add icons to toggle any dialog/palette to include any of the other dialogs.
This is a very handy feature of GIMP 1.3. By making one or more such combination dialog/palette boxes, you can keep dialog/palette clutter from creeping all over your monitor screen. That leaves much more room for image canvases.
In Figure 3, on page 2, that bottom-left dialog box from Figure 1 has been resized and icons for many other dialogs/palettes have been added. Figure 3 shows the combination dialog box opened in the Pattern Grid (paint bucket icon) dialog.
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