# Common Tasks

This tutorial is based on text Copyright © 2004 Carol Spears. The original tutorial can be found online: bibliography-online-tutorial02.

## Intention

GIMP is a powerful image editing program with many options and tools. However, it is also well suited for smaller tasks. The following tutorials are meant for those who want to achieve these common tasks without having to learn all the intricacies of GIMP and computer graphics in general.

Hopefully, these tutorials will not only help you with your current task, but also get you ready to learn more complex tools and methods later, when you have the time and inspiration.

All you need to know to start this tutorial, is how to find and open your image. ( File Open from the Image window).

## Change the Size of an Image for the screen

You have a huge image, possibly from a digital camera, and you want to resize it so that it displays nicely on a web page, online board or email message.

### Example Image for Scaling

The first thing that you might notice after opening the image, is that GIMP opens the image at a logical size for viewing. If your image is very large, like the sample image, GIMP sets the zoom so that it displays nicely on the screen. The zoom level is shown in the status area at the bottom of the Image window. This does not change the actual image.

The other thing to look at in the title-bar is the mode. If the mode shows as RGB in the title bar, you are fine. If the mode says Indexed or Grayscale, read the gimp-tutorial-quickie-change-mode.

### GIMP Used for Image Scaling

Use Image Scale Image to open the Scale Image dialog. You can right click on the image to open the menu, or use the menu along the top of the Image window. Notice that the Scale Image menu item contains three dots, which is a hint that a dialog will be opened.

### Dialog for Image Scaling in Pixels

The unit of size for the purpose of displaying an image on a screen is the pixel. You can see the dialog has two sections: one for width and height and another for resolution. Resolution applies to printing only and has no effect on the image's size when it is displayed on a monitor or a mobile device. The reason is that different devices have different pixels sizes and so, an image that displays on one device (such as a smartphone) with a certain physical size, might display on other devices (such as an LCD projector) in another size altogether. For the purpose of displaying an image on a screen, you can ignore the resolution parameter. For the same reason, do not use any size unit other than the pixel in the height / width fields.

If you know the desired width, enter it in the dialog at the top where it says Width. This is shown in the figure above. If you don't have such a number in mind, choose an appropriate width for the desired use. Common screen sizes range between 320 pixels for simpler phones, 1024 pixels for a netbook, 1440 for a wide-screen PC display and 1920 pixels for an HD screen. for the purpose of displaying an image on-line, a width of 600 to 800 pixels offers a good compromise.

When you change one of the image's dimensions, GIMP changes the other dimension proportionally. To change the other dimension, see gimp-tutorial-quickie-crop. Bear in mind that when you change the two dimensions arbitrarily, the image might become stretched or squashed.

## Change the Size of an Image for print

As discussed before, pixels don't have a set size in the real world. When you set out to print an image on paper, GIMP needs to know how big each pixels is. We use a parameter called resolution to set the ratio between pixels and real-world units such as inches.

By default, most images open with the resolution set to 72. This number was chosen for historical reasons as it was the resolution of screens in the past, and means that when printed, every pixel is 1/72 of an inch wide. When printing images are taken with modern digital cameras, this produces very large but chunky images with visible pixels. What we want to do is tell GIMP to print it with the size we have in mind, but not alter the pixel data so as not to lose quality.

To change the print size use Image Print Size to open the Print Size dialog. Select a size unit you are comfortable with, such as inches. Set one dimension, and let GIMP change the other one proportionally. Now examine the change in resolution. If the resolution is 300 pixels per Inch or over, the printed image's quality will be very high and pixels will not be noticeable. With a resolution of between 200 and 150 ppi, pixels will be somewhat noticeable, but the image will be fine as long as its not inspected too closely. Values lower than 100 are visibly coarse and should only be used for material that is seen from a distance, such as signs or large posters.

### Dialog for Setting Print Size

## Compressing Images

### Example Image for JPEG Saving

If you have images that take up a large space on disk, you can reduce that space even without changing the image dimensions. The best image compression is achieved by using the JPG format, but even if the image is already in this format, you can usually still make it take up less space, as the JPG format has an adaptive compression scheme that allows saving in varying levels of compression. The trade-off is that the less space an image takes, the more detail from the original image you lose. You should also be aware that repeated saving in the JPG format causes more and more image degradation.

To save you image as a JPG file, therefore, use File Save As to open the Save As dialog.

### Save As Dialog

The dialog opens with the file name already typed in the Name box. If the image is not a JPG image, delete the existing extension and type JPG instead, and GIMP will determine the file type from the file extension. Use the file extension list, circled in the figure above, to see the types supported by GIMP. The supported extensions change depending on your installed libraries. If GIMP complains, or if JPEG is grayed out in the Extensions menu, cancel out of everything and step through the gimp-tutorial-quickie-change-mode. Once you have done this, click Save. This opens the Save as" JPEG dialog that contains the quality control.

The Save as JPEG dialog uses default values that reduce size while retaining good visual quality; this is the safest and quickest thing to do.

### Save as JPEG dialog with poor quality.

Reduce the image Quality to make the image even smaller. Reduced quality degrades the image, so be certain to check Show preview in image window to visually gauge the degradation. As shown in the figure above, a Quality setting of 10 produces a very poor quality image that uses very little disk space. The figure below shows a more reasonable image. A quality of 75 produces a reasonable image using much less disk space, which will, in turn, load much faster on a web page. Although the image is somewhat degraded, it is acceptable for the intended purpose.

### Dialog for Image Saving as JPEG

Finally, here is a comparison of the same picture with varying degrees of compression:

### Example for High JPEG Compression

Quality: 10; Size: 3.4 KiloBytes

Quality: 40; Size: 9.3 KiloBytes

### Example for Moderate JPEG Compression

Quality: 70; Size: 15.2 KiloBytes

Quality: 100; Size: 72.6 KiloBytes

## Crop An Image

### Example Image for Cropping

Source image

Image after cropping

There are many reasons to crop an image; for example, fitting an image to fill a frame, removing a portion of the background to emphasize the subject, etc. There are two methods to activate the crop tool. Click the button in the Toolbox, or use Tools Transform Tools Crop in the image window. This changes the cursor and allow you to click and drag a rectangular shape. The button in the toolbox is the easiest way to get to any of the tools.

### Select a Region to Crop

Click on one corner of the desired crop area and drag your mouse to create the crop rectangle. You don't have to be accurate as you can change the exact shape of the rectangle later.

### Dialog for Cropping

After completing the click and drag motion, a rectangle with special regions is shown on the canvas. As the cursor is moved over the different areas of the selected crop area, the cursor changes. You can then drag the rectangle's corners or edges to change the dimensions of the selected area. As shown in the figure above, as the crop area is resized, the dimensions and ratio are shown in the status bar. See gimp-tool-crop for more information on cropping in GIMP. If you would like to crop the image in a specific aspect ratio, such as a square, make sure the tool options are visible ( Windows Dockable Dialogs Tool Options ). In the Tool Options dockable, check the mark next to Fixed and make sure the drop-down box next to it is set to Aspect Ratio. You can now type the desired aspect ratio on the text box below, such as 1:1. You also have controls to change the aspect from landscape to portrait. After you set the aspect ratio, drag one of the corners of the crop rectangle to update it. The rectangle changes to the chosen ratio, and when you drag it should maintain that ratio.

## Find Info About Your Image

### Finding Info

When you need to find out information about your image, Use Image Image Properties to open the Image Properties dialog, which contains information about the image size, resolution, mode and much more.

### Image Properties Dialog

## Change the Mode

As with anything else, images come in different kinds and serve different purposes. Sometimes, a small size is important (for web sites) and at other times, retaining a high color depth (e.g., a family portrait) is what you want. GIMP can handle all of this, and more, primarily by converting between three fundamental modes, as seen in this menu. In order to switch your image to one of these modes, you open it and follow that menu and click the mode you want.

### Dialog for changing the mode

RGB- This is the default mode, used for high-quality images, and able to display millions of colors. This is also the mode for most of your image work including scaling, cropping, and even flipping. In RGB mode, each pixel consists of three different components: R->Red, G->Green, B->Blue. Each of these in turn can have an intensity value of 0-255. What you see at every pixel is an additive combination of these three components.

Indexed- This is the mode usually used when file size is of concern, or when you are working with images with few colors. It involves using a fixed number of colors (256 or less) for the entire image to represent colors. By default, when you change an image to a palleted image, GIMP generates an optimum palette to best represent your image.

### Dialog Change to Indexed Colors

As you might expect, since the information needed to represent the color at each pixel is less, the file size is smaller. However, sometimes, there are options in the various menus that are grayed-out for no apparent reason. This usually means that the filter or option cannot be applied when your image is in its current mode. Changing the mode to RGB, as outlined above, should solve this issue. If RGB mode doesn't work either, perhaps the option you're trying requires your layer to have the ability to be transparent. This can be done just as easily via Layer Transparency Add Alpha Channel .

### Add Alpha Channel

Grayscale- Grayscale images have only shades of gray. This mode has some specific uses and takes less space on the hard drive in some formats, but is not recommended for general use as reading it is not supported by many applications.

There is no need to convert an image to a specific mode before saving it in your favorite format, as GIMP is smart enough to properly export the image.

## Flip An Image

Use this option when you need the person in the photo looking in the other direction, or you need the top of the image to be the bottom. Use Tools Transform Tools Flip , or use the button on the toolbox. After selecting the flip tool from the toolbox, click inside the canvas. Controls in the Tool Options dockable let you switch between Horizontal and Vertical modes.

### Dialog Flip an Image

After selecting the flip tool from the toolbox, click inside the canvas. The tool flips the image horizontally. Use the options dialog to switch between horizontal and vertical. If it is not already displayed in the dock under the toolbox, double click the toolbox button. You can also use the Ctrl key to switch between horizontal and vertical.

In the images below, all possible flips are demonstrated:

### Example Image to Flip

Source image

Horizontal flipped image

Vertical flipped image

Horizontal and vertical flipped image

## Rotate An Image

### Menu for Rotate An Image

Images that are taken with digital cameras sometimes need to be rotated. To do this, use Image Transform Rotate 90° clockwise (or counter-clockwise). The images below demonstrate a 90 degrees CCW rotation.

### Example for Rotate An Image

Source Image

Rotated image 90 degree CCW

## Separating an Object From Its Background

### Object with Background

Sometimes you need to separate the subject of an image from its background. You may want to have the subject on a flat color, or keep the background transparent so you can use it on an existing background, or any other thing you have in mind. To do this, you must first use GIMP's selection tools to draw a selection around your subject. This is not an easy task, and selecting the correct tool is crucial. You have several tools to accomplish this.

The Free Select Tool allows you to draw a border using either freehand or straight lines. Use this when the subject has a relatively simple shape. Read more about this tool here: gimp-tool-free-select

### Free Select Tool

The Intelligent Scissors Select Tool lets you select a freehand border and uses edge-recognition algorithms to better fit the border around the object. Use this when the subject is complex but distinct enough against its current background. Read more about this tool here: gimp-tool-iscissors

### Intelligent Scissors Select Tool

The Foreground Select Tool lets you mark areas as Foreground or Background and refines the selection automatically. Read more about this tool here: gimp-tool-foreground-select

### Foreground Select Tool

Once you have selected your subject successfully, use Select Invert . Now, instead of the subject, the background is selected. What you do now depends on what you intended to do with the background.

- To fill the background with a single color:

  Click the foreground color swatch (the top left of the two overlapping colored rectangles) in the toolbox and select the desired color. Next, use gimp-tool-bucket-fill to replace the background with your chosen color.

  ### Result of Adding a Plain Color Background

- To make a transparent background:

  Use Layer Transparence Add Alpha Channel to add an alpha channel. Next, use Edit Clear or hit the Del key on the keyboard to remove the background. Please note that only a small subset of file formats support transparent areas. Your best bet is to save your image as PNG.

  ### Result of Adding a Transparent Background

- To make a black-and-white background while keeping the subject in color:

  Use Colors Desaturate . In the dialog that opens, cycle between the modes and select the best-looking one, then click OK.

  ### Result of Desaturating the Background