Use Gimp to improve digital photos

For many of us, editing digital images means using the automated-correction tools in common photography applications. These work very well but often offer only general changes. Adjusting brightness or contrast levels, for example, is likely to affect the whole photo, rather than adding colour to an overcast skyline, say.

If you want more control over how your photos look, then you will need to investigate some more involved image-editing techniques. In this Masterclass, we’ll show you how to use some of the most useful advanced photo-improving tools.

Getting started
We are going to be using an application called the GNU Image Manipulation Program (or Gimp), a comprehensive image-editing application that can be downloaded at no cost. It has a wide variety of useful features to fix and enhance photos including red-eye removal, cloning and healing.

These tools exist in other popular image-editing programs, such as Photoshop and Paint Shop Pro, though the precise method of their operation will differ. So, feel free to follow along using your preferred software or, if you want to follow our instructions precisely, then Gimp can be downloaded here.

The basics
As with all other image-editing tools, when Gimp launches it displays a bunch of commonly used tools in a toolbar. It calls this the Toolbox and it is displayed at the top-left corner of the screen.

Included in the Toolbox are selection tools (Rectangle Select, Fuzzy Select and Free Select among others), common paint tools (Pencil, Paintbrush, Clone and Heal) and transform tools (Align, Crop, Rotate and so forth). Colour tools (Levels, Curves etc) are accessed via the Colours menu, found in another floating window, and all tools can be accessed in the Tools menu.

Hovering the mouse pointer over a tool displays a tooltip and the keyboard shortcut. Underneath the Toolbox is the Tool Options dialogue box, showing the options for the selected tool.

In the bottom right-hand corner of the Tool Options is the ‘Reset to Default Values’ button (a yellow arrow), which will return the tool options to their original settings.

The main work area automatically resizes when an image is opened. Each opened image is displayed in a new window with a menu bar located across the top. The layout of Gimp is flexible. Instead of placing each setting or tool in its own window, they can be grouped together using ‘docks’. A dock is a window that holds a collection of settings.

On the right is the Layers, Channels, Paths and Undo dock (showing the layer options by default), with the options for managing Brushes, Patterns and Gradients docked underneath. In a dock, each set of options has its own tab.

Using the Magic Wand
The Fuzzy Select tool (or Magic Wand) is one of a number of selection tools available in Gimp. It is used to select a colour range within an image, which can then be changed using Gimp’s colour tools, such as Hue-Saturation or Brightness-Contrast. It can be used to select a burned-out or washed-out sky to amend the colour of just that area, for example.

To show how this works we are going to change the colour of a vase that appears in our photo. Whenever you amend an image it is good practice to work on a new ‘layer’ because changes are separated into different elements that can then be edited individually. If you think of a layer as a sheet of transparent film over the original image you will get the idea.

Click on Layer on the menu bar, then choose Duplicate Layer. A new layer called ‘Background Copy’ will appear in the Layer on the right-hand side of the screen. If the Layer options are not visible, hold down the Control (Ctrl) key on the keyboard and press L.

Click on the Fuzzy Select tool in the Toolbox (or press U on the keyboard) and set the Threshold slider in the Tool Options to approximately 30. The threshold setting determines the range of colour that the tool will select. You will need to experiment with this to find the best setting for your image.

Now click on the part of the image you want to select – in our example, the orange vase. The pixel that is clicked determines the selected colour, the tool assesses the surrounding area and, if it matches, it is added to the selection.

Setting a threshold of 30 means that the Fuzzy Select tool also selects all pixels that are 30 shades lighter and 30 shades darker that the selected pixel. Although the vase is orange, it is many different shades, so it hasn’t all been selected.

Amending a selection
To add to the selection, hold down the Shift key on the keyboard, and click on an area that hasn’t been selected (a ‘+’ symbol will be displayed next to the mouse pointer). Or, in the Fuzzy Select options under the Toolbox, choose ‘Add to the Current Selection’ from the Mode area. Keep clicking until the area you want is selected. Use the + (plus) key on the keyboard to zoom in, and the – (minus) key to zoom out.

Navigate around the image by holding down the spacebar on the keyboard and dragging with the mouse. (Alternatively click and hold down on the Navigate button – the crossed arrows – in the bottom right-hand corner of the Image Window.) Keep adding to the selection until the whole object is selected.

If part of the background becomes selected, hold down the Control (Ctrl) key on the keyboard and click on it (holding down Shift and clicking adds to a selection, holding down Control and clicking takes away from it). Alternatively, choose Edit on the menu bar, then Undo. To start again, click on Select on the menu bar, then click None.

To change the colour of the selection, choose Colours on the menu bar, then click on Hue-Saturation. Use the Hue slider to change the colour, then click OK. Finally, click on Select on the menu bar, and choose None to get rid of the line around the selected area.

To compare the edited image to the original, click on the Layer Visibility icon (the eye) next to the Background Copy layer in the Layers dialogue. Only the original layer will be visible. Click on the visibility icon again to see both of the layers.