Gimp is open-source and cross-platform. It comes with a built-in help. Once you started the program, context-sensitive help is available by pressing F1. The user manual for the GIMP 2.8 release is available in 17 languages.
GIMP requires limited hardware and memory resources. That is, when working on large images, each one needing megabytes, perhaps a lot more, GIMP can gracefully manage multiple images opened at the same time. My opinion is that GIMP has an efficient tile-based images' memory management that avoid consuming too much resources. To configure the tile cache in the toolbox 'File/Preference', state how GIMP must deal with memory for images. Recommendations on how to set the tile cache:
GIMP allows one or more toolbox and dockable dialogs. It is possible to set up a toolbox with one or several dockable dialogs in it, at your convenience. You can place them in one or multiple columns. From version 2.8, there is an optional single-window mode. Any tool opens by clicking on it or using its shortcut key (in lower case). There are nice features, such as the zoom value and parameter at the bottom of the screen when displaying an image. An easy way to discover GIMP's functions is to check what each menu contains.
GIMP does not offer support for image library management. For some designers, GIMP is not intuitive. As with all features-rich software, there is a learning curve, be it only to discover the many features. Starting from the main menu (in version 2.8.16), GIMP offers the following group of functions, some of which are common and others less:
New, Create, Open (as layer, location or URL, recent), Save (save as, save a copy - Saving an image can only be done in XCF, GIMP's native file format, to save all kinds of information necessary for works in progress), Revert, Export (export is a separate job, distinct from save, for all kind of file formats). Create Template, Page setup, Print, Properties, Close (view, all) and Quit.
Undo, Redo, Fade, History, Cut, Copy (copy visible), Paste (paste into, paste as), Buffer, Clear, Fill (with foreground colour, with background colour), Stroke selection, Stroke path, Preferences, Input devices, Keyboard shortcuts, Modules (including gamma color display filter, watercolor style color selector, CMYK color selector, ICC color profiles and more), Units.
All, None, Invert, Float, by Color, from Path, Selection editor, Feather (" The Feather command feathers the edges of the selection. This creates a smooth transition between the selection and its surroundings"), Sharpen, Shrink, Grow, Border, Distort, Rounded Rectangle, Toggle Quick Mask, Save to Channel, To Path.
New View, Dot for Dot ("a pixel on screen represent an image pixel"), Zoom, Shrink Wrap ("reduce an image window to the size of the image"), Fullscreen, Navigation Window ("The Navigation dialog is designed to offer easy movement around the active image if the zoom is set higher than what the image window can display"). Display Filters, Show Selection, Show Layer Boundary, Show Guides, Show Grid, Show Sample points ("While the Color Picker can display color information about one pixel, the 'Sample Points' dialog can display the data of four pixels of the active layer or the image"). Snap to Guide, Snap to Grid, Snap to Canvas Edge, Snap to Active Path, Padding Color (The Padding Color command permits to change the color of the canvas that surrounds the image), Show Menubar, Show Rulers, Show Scrollbars, and Show Statusbar.
Duplicate, Mode (to assign RGB Color Profile), Transform (flip, rotate, guillotine), Canvas Size, Fit Canvas to Layer, Fit Canvas to Selection, Print Size, Scale Image (change the size of the image). Crop to Selection, Autocrop (removes empty border of the image), Zealous Crop (crops an image using a single solid color as a guide. It crops the edges, as does Autocrop, but it also crops the areas in the middle of the image that have the same color; it crops all layers and is to use with care). Merge Visible Layers (in one), Flatten Image (merge all layers in one), Align Visible Layers, Guides, Configure Grid, and Image Properties.
New Layer, New from Visible, New Layer Group ("For complex compositions, a flat layer structure is very limiting." GIMP 2.8 lets users organize their compositions through the introduction of layer groups, which allow organizing layers in tree-like structures. Layer groups are also fully scriptable through the GIMP plug-in API), Anchor Layer, Merge Down, Delete Layer. Stack (select, raise, lower layer), Mask (add, apply, delete, show, edit, disable layer mask, mask to selection, add, substract, intersect with selection), Transform (flip, rotate, offset), Autocrop Layer.
Color Balance, Hue-Saturation, Colorize, Brightness-Contrast, Threshold, Levels, Curves, Posterize, Desaturate, Invert (colors), Value Invert (brightness of each pixel), Auto (functions to equilize, enhance, etc.). Components (channel mixer, compose, decompose, recompose), Map (functions to alter colors in various ways), Info (histogram, border average, color cube analysis, smooth palette). Color to Alpha, Colorify, Filter Pack, Hot, Maximum RGB, Retinex (filter improving visual rendering).
This menu includes a wealth of tools in each section. We do not detail all but they are worth discovering one by one. Selection (rectangle, ellipse, free, foreground, fuzzy, by color, intelligent scissors to select shape using intelligent edge-fitting), Paint Tools, Transform Tools (align, move, crop, rotate, scale, shear, perspective, flip, cage transform to deform a selection with a cage). Color Tools, Paths (to create and edit paths), Color Picker, Zoom, Measure (distance and angles). Text (the Text function now performs Text editing on-canvas instead of a separate window; apart from the usual text formatting features like font family, style and size selectors, you get numeric control over baseline offset and kerning, as well as the ability to change text color for a selection). GEGL operation ('Generic Graphical Library' is a graph-based image-processing library designed to handle various image-processing tasks needed in GIMP). Toolbox, Default Colors and Swap Colors.
Repeat Print, Re-show Print, Recently Used, Reset All Filters, more than 14 standard filters, and Alpha to Logo includes another 19 helpful pre-set filters.
Export file formats
GIMP can export a large array of image file formats. This is the list in version 2.8.16 (ordered alphabetically):
Alias Pix image (PIX, MATTE, MASK, ALPHA,ALS),
AUTODESK FLIC animation (FLI, FLC),
BZIP archive (XCF, BZ2, XCFBZ2),
C sourcode (C) and C source code header (H)
Coloured XHTML (HHTML)
Digital imaging and communication in medicine image (DCM, DICOM)
Encapsulated postscript image (EPS)
Flexible image transport system (FIT, FITS)
GIF image (GIF)
GIMP brush (GBR)
GIMP brush animated (GIH)
GIMP pattern (PAT)
GZIP archive (XCF.GZ, XCFGZ)
HTML table (HTM, HTML)
JPEG image (JPG, JPEG, JPE)
KISS CEL (CEL)
Microsoft Windows icon (ICO)
MNG animation (MNG)
Open Raster (ORA)
PBM image (PBM)
PGM image (PGM)
Photoshop image (PSD)
PNG image (PNG)
PNM image (PNM)
Portable document format (PDF)
Postscript document (PS)
PPM image (PPM)
Raw image data (DATA)
Silicon Graphics IRIS image (SGI, RGB, RGBA, BW, ICON)
Sun Raster file image (IM1, IM8, IM24, IM32, RS, RAS)
TarGA image (TGA)
TIFF image (TIF, TIFF)
Windows BMP image (BMP)
X bitmap image (XBM, ICON, BITMAP)
X PixMap image (XPM)
X Windows Dump (XWD)
Zsoft PCX image (PCX, PCC).
Open file formats
Apart from the above formats, GIMP also opens SVG images, which is practical to export as PNG, and Paint Shop Pro images (PSP, TUB, PSPIMAGE), G3 fax images (G3) and Desktop link (DESKTOP).
These factual data are meant for users already familiar with image creation and processing; I suggest a list of resources for beginners, to familiarise with GIMP and image editing before learning GIMP's rich features.
Tutorials For beginners
Book Beginning Photo Retouching & Restoration Using GIMP (version 2.8), print and eBook.
e-Book 'How-to GIMP' by Kat Landreth (GIMP version 2.8.4).
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