GIMP
Started in 1995, GIMP is an open-source image editing and manipulation program that can edit digital graphics and photographs. GIMP has similar features as Adobe Photoshop but is not a clone of the popular image editor.
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1 Using GIMP
2 Brush, Pattern, Gradient
3 Color History
4 Hex Color Changes
5 Foregrounds and Backgrounds
6 Dodge or Burn
7 The Smudge Tool
8 Blur and Sharpen
9 Clone Tool
10 Ink Tool
11 Eraser
12 The Gradient Tool
13 Bucket Fill
14 Text Tool
15 Flip
16 Transform
17 Shear
18 Scale
19 Rotate
20 Pencil, Paint, Airbrush
21 Resize
22 Crop
23 Move Tool
24 Measure
25 Magnify
26 Color Picker
27 Paths
28 Selection by Color Tool
29 The Fuzzy Selection Tool
30 Free-Hand Selection Tool
31 The Elliptical Tool
32 Shape Selection
33 Lighting
34 Layers and Text Tool
35 Using Grayscale
36 Using Filters
37 Crop An Image
38 Gimp Installation
39 Rectangular Selection Tool