Priya Kapoor
GIMP (GNU Image Manipulation Program) is a cross-platform image editor available for GNU/Linux, macOS, Windows, and other operating systems. It is free software that allows users to change its source code and distribute changes. When operating on an image in GIMP, users can select a specific part of the image they want to edit, with most operations affecting only the selected portions. GIMP's selection tools can be challenging when creating complex selections, and this is where the QuickMask feature comes in handy. QuickMask allows users to paint a selection instead of tracing its outline, providing a more detailed view of the selected areas. This guide will explore how to paint out parts of a selection in GIMP, utilizing tools like QuickMask and Bucket Fill, to achieve precise and creative image manipulations.
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What You'll Learn
Using the QuickMask
The QuickMask is a convenient way to modify selections using pixel-changing tools such as the paintbrush. It allows you to paint a selection instead of just tracing its outline. Normally, when you create a selection in GIMP, you see it represented by the "marching ants" that trace along its outline. However, there may be a lot more to a selection than the marching ants show you. In GIMP, a selection is actually a full-fledged grayscale channel, covering the image, with pixel values ranging from 0 (unselected) to 255 (fully selected). The marching ants are drawn along a contour of half-selected pixels. Thus, what the marching ants show you as either inside or outside the boundary is really just a slice through a continuum. The QuickMask is GIMP's way of showing you the full structure of the selection.
To activate the QuickMask, click on the small red-outlined button at the lower left of the image window. The button is a toggle, so clicking it again will return you to the normal marching-ant mode. You can also activate the QuickMask by selecting in the image window menu Select → Toggle QuickMask, or by using the Shift-Q shortcut. Activating the QuickMask shows you the selection as though it were a translucent screen overlying the image, whose transparency at each pixel indicates the degree to which that pixel is selected. By default, the mask is shown in red, but you can change this if another mask color would be more convenient. The less a pixel is selected, the more it is obscured by the mask. Fully selected pixels are shown completely clear. When you are in QuickMask mode, many image manipulations act on the selection channel rather than the image itself. This includes, in particular, paint tools. Painting with white selects pixels, and painting with black unselects pixels. You can use any of the paint tools, as well as the bucket fill and gradient fill tools, in this way.
The QuickMask can be used to modify any existing selection, and you can even use it to create new selections. For example, you can use Select → Invert and Edit → Fill with BG Color. The result, a soft white fade into the background, is shown in the final step.
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How to create a selection
GIMP has a variety of selection tools, each with its own unique properties, but they also share some features in common. The Path tool, for example, can be used as a selection tool by converting any closed path into a selection.
To create a selection, you can use the Selection Tool, which can be found in the Tool Options dialog box. This should always be visible when you are using tools. Here, you can also see the current settings for the selection tools. The selection you create can be combined with any pre-existing selection in a number of ways. The Replace mode, for instance, will destroy or replace any existing selection when a new one is created. The Add mode will add a new selection to an existing one. The Subtract mode will subtract a new selection from an existing one, as long as they share common pixels.
You can also use keyboard shortcuts to create and modify selections. Using Ctrl+A will select the entire image on the selected layer. Ctrl+Shift will switch the selection mode to intersection, so that the selection will consist of the intersection of the region traced out with the pre-existing selection. Ctrl+Alt+Left-click-and-drag and Shift+Alt+Left-click-and-drag are used to move selections.
If you want to come back to a selection later, you can save it using the selection menu and restore it using the channels dialogue. To move, scale, or rotate an existing selection, choose the appropriate tool (e.g. the Scale Tool), tick Transform Selection in the tool's options tab, and then drag the corners or input fixed values for width or height.
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Selection as a channel
Selections in GIMP are implemented as channels. Internally, they are identical to the red, green, blue, and alpha channels of an image. Each pixel in the image has a value defined for it, ranging from 0 (unselected) to 255 (fully selected). This allows for smooth transitions between selected and unselected regions, with intermediate values between 0 and 255 representing partially selected pixels. This is particularly useful when creating complex selections that require smooth edges, such as selecting a tree to cut out from its background.
The "marching ants" selection outline in GIMP only shows the boundary between selected and unselected pixels, but the selection itself is a full-fledged grayscale channel covering the entire image. The QuickMask feature in GIMP allows you to view and edit the selection as a translucent overlay, where the transparency at each pixel indicates the degree of selection. Activating QuickMask can be done by clicking the small red-outlined button in the lower-left corner of the image window or by using the Shift-Q shortcut.
The Channels dialog in GIMP allows you to edit, modify, and manage your channels. It is divided into two parts: one for colour channels and the other for selection masks. You can create a new channel by specifying its name, colour tag, attributes, opacity, and colour used for the mask. The colour button opens the GIMP colour selector for further customization.
Additionally, GIMP offers selection tools such as the Rectangle Select tool and the Foreground Select Tool, which enable you to isolate specific parts of an image for editing. By understanding the concept of selections as channels and utilizing the available tools and features, you can achieve precise and complex selections in your image editing workflow with GIMP.
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The Bucket Fill tool
If you Shift+click and use the Bucket tool, it will use the background colour instead. Depending on how the tool options are set, the Bucket Fill tool will either fill the entire selection or only parts whose colours are similar to the point you click on. The amount of fill depends on what Fill Threshold you have specified. The fill threshold determines how far the fill will spread, starting at the point where you click and spreading outward until the colour or alpha value becomes "too different".
The Fill by option allows you to choose which component of the image GIMP will use to calculate the similarity and determine the borders of filling. The components you can choose from are Composite, Red, Green, Blue, HSV Hue, HSV Saturation, HSV Value, LCh Lightness, LCh Chroma, and LCh Hue.
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The Foreground Select Tool
To use the Foreground Select Tool, start by roughly selecting the object you want to extract, including a little bit of the background around it. You can adjust the size of the brush used for selection in the Tool Options area. Once you have the initial selection, you can refine it by drawing a continuous line over the selected foreground area, moving over the colours you want to keep. Be careful not to paint background pixels. It's recommended to use a colour that is different from the foreground for easier visibility.
During the refinement process, you can toggle the preview button or press Enter to verify the result. If you need to remove unwanted background areas, switch to "Draw background" mode and use a brush to mark those areas for exclusion from the selection. You can also switch between Grayscale and Colour preview modes to better distinguish between similar foreground and background colours.
Once you are satisfied with the selection, click the "Select" button or press Enter to finalize it. This will allow you to copy-paste or click-and-drag the selected object to another image, or you can invert the selection to make changes to the background. Remember that the Foreground Select Tool doesn't have a default shortcut, but you can create one by going to Edit → Keyboard Shortcuts and following the necessary steps.
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Frequently asked questions
To paint out parts of a selection in GIMP, you can use the Bucket Fill tool. This tool fills a selection with the current foreground colour. You can also use the Ctrl-click function and the Bucket tool to fill with the background colour.
To get a crisp line, you need a partial selection on the edge pixels. This can be done by using the 'path to select' function and then colouring in the sections.
To see the selection in complete detail, click the QuickMask button in the lower-left corner of the image window. This will show the selection as a translucent overlay, with selected areas unaffected and unselected areas reddened.
QuickMask mode is a function in GIMP that allows you to paint a selection instead of tracing its outline. It shows the full structure of the selection as a translucent screen overlying the image.
The Foreground Select Tool is used for separating a shape in an image from everything else.
