Photo Manipulation with GIMP 2.6

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Introduction

When working with photos we need to be sure that colors always look the way we want. Monitors, LCDs, and laptop screens, all render colors differently, and they don’t look the same. There’s no way you can control how people see their own screen, but you can calibrate your own using some kind of color profiling. The problem extends to different devices like scanners, printers, and cellphones, among others, because each device has its own color characteristics. Also, each device can handle different ranges of color.

To avoid these problems, color profiles were implemented. It’s a table that matches a specific color of a device to a device-independent color. Color profiles are created by each manufacturer, but the ICC (International Color Consortium) created its own standard.

Calibration, image modes, and colour profiles

This is one of the first things to do before working with photos. The Color Management menu (Edit | Preferences | Color Management) enables you to load any preset for your monitor. The presets can be created using any hardware calibration and measurement tool. For Linux, you can use LPROF and ARGYLL. For more information on color management in Linux, check the Linux color management page on Wikipedia.

Apart from the color profile in use, you should set up your monitor and video card to render colors as close as possible to the standard. For Linux you can use XCALIB or DISPWIN.

You can also simulate how an image will look when printed by selecting Print Simulation from the Mode of Operation in the Color Management menu.

Scaling an image

Image scaling is one of the easiest tasks, and also one of the most useful. In just a few seconds you can reduce an image’s size, making e-mail transfers quicker.

How to do it…

To scale an image follow these steps:

  1. Open a file, go to Image | Scale Image.

  2. Change the size to whatever you want:

    Tutorial on Photo Manipulation with GIMP 2.6

    As you change one value, you can see the other changing too. That’s to keep the original file aspect ratio. If you click on any of the chain icons you can change any value independently.

How it works…

Image scaling works by changing the number of pixels of an image. The way pixels are created/deleted can be changed by choosing a different interpolation method in the Quality section of the Scale Image window. The best (but slowest) method is the Cubic interpolation, which calculates the color of each pixel by estimating an average of the eight closest ones. On the opposite corner, if you choose None as the interpolation method, each pixel gets its own color from its closest neighbor. This is the fastest method, but it also generates unwanted aliasing that can give you headaches if you are working with transparencies and highcontrasts.

The resolution fields are useful when printing the image, they say how many pixels are per inch. If your image is too big and the resolution too low, the printed image can look pixelated. You can change the print size of an image without changing its resolution or size by going to Image | Print Size.

Cropping an image

There’s another quick way to rescale your image and select just a section of it. Use the Crop Tool.

How to do it…

The following steps will show you how to crop an existing image:

  1. Open an image, and select the Crop Tool:

  2. Quickly click and drag around the area you want. Then, just press Return (Enter in Windows), and anything around the selection area will be cropped.

Flipping or rotating an image

Sometimes you scan and old photo or take a picture with the camera tilted. Following is how to quickly rotate or flip them.

How to do it…

To rotate or flip an image easily, carry out the following steps:

  1. Open an image, for example the following:

  2. Select the Flip Tool from the Toolbox:

    Tutorial on Photo Manipulation with GIMP 2.6

    Use the left-mouse button along with the Ctrl key to change between horizontal and vertical flipping.

  3. Now, to rotate the image, select the Rotate Tool from the Toolbox:

    Tutorial on Photo Manipulation with GIMP 2.6

    Click and drag your image to rotate it. Use the Ctrl key to constrain rotation to 15 degrees steps. Press the Return key when you’ve finished:

How it works…

Flip and rotate are basic tools. You can use the mouse to perform the operations after selecting any of them, but using their respective tool options gives you a lot more control. In both cases, you can control what to rotate (a layer, a selection, or a path).

Also, in the Rotate tool, you can change the middle point around which the image rotates. This allows you to rotate the image with more freedom. To change it, select the Rotate Tool, and click and drag the small circle that appears in the center of the image:

You can also set its coordinates from the Rotate window:

Tutorial on Photo Manipulation with GIMP 2.6

You can also flip and rotate an image by using the menu items in Image | transform.

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