Pulling objects out of an image

Quick Masks

Quick masks provide a means by which you can easily correct existing selections. You activate this mode by clicking with the mouse in the lower left corner of the work area (Figure 5). Alternatively, you can select the mode from the Selection | Toggle quick masks menu, or by pressing the Shift+Q keys.

Figure 5: The quick masks mode lets you edit selections with drawing tools. The button takes the user to a pop-up menu that contains various options.

Afterwards, GIMP overlays a red layer on the image for which no selection has yet been made. You work on this layer with a drawing tool such as a pencil or paintbrush . Gimp will then create a selection in areas that are painted with a white color. When you use black, this removes the existing selection. Gray tones are used to create a detailed gradient.

If the color of the virtual film clashes with the image that is being edited, it can be adjusted via the pop-up menu found under the quick mask button. As soon as this "tracing paper" matches what you need to select, you click on the button with the mouse, and Gimp will once again display a normal view. Since the editing program when operating in this mode only displays the 50-percent line of the selection, the detailed scaling does not appear any more on the screen. It does persist, however, as becomes apparent with subsequently copying.

After completing work on the selection, you should not forget to transfer the relevant, or selected, parts of the layer to a new layer. This has the advantage that you create a transparent layer. One of the special characteristics of these types of layers is that they contain all of the information that was found in the original layer. Therefore the layer that has been copied partly consists of pixels positioned on spots where the layer appears to be transparent. At this point, the erase method becomes handy. This method will be discussed later in the article.

One version of this selection method consists of applying paths. In Gimp, the procedures for converting paths into selections and selections into paths along the 50-percent line are very similar to one another. You first create the paths along the lines intended as selections. Then, you take the path tool and create a closed linear shape in Design mode.

The advantage of this tool is that the control points can be moved at any time. Likewise, it is possible to adjust the segments, which are the lines running between the control points, in terms of position and shape. However you pay a relatively high price for these freedoms. Creating a selection in this way takes a lot of time. Even so, this tool offers elegant solutions for many selection problems, especially when an asymmetrical shape is involved. Figure 4 shows good examples with the sail and the hull.

The Eraser

It is much easier to perform an extraction with the eraser. This tool offers suitable erasers for removing all of the undesired parts of an object from the current layer. In order for this to work at least partially well, you first have to prepare the tool in accordance with the task at hand. You should select a suitable softness, specify a proper brush size, and then check to see whether the other parameters are correct (Figure 6).

Figure 6: Normally, erasing works by first moving from the outside to the inside. If the user removes too much from the image, the action can be undone with Ctrl+Z.

Now you work from the outside moving to the inside. Sometimes it happens that too much material is removed. At this point, the transparency property becomes useful. The anti-erase mode can be activated via holding down the Alt key. In this mode, Gimp restores the pixels that were previously present in the area. Unerasing is typically performed by moving from inside the selected object to an outside border that is no longer needed.

With the soft brush points (Hardness … ), the brush size also influences the soft area. It makes sense therefore to define a key combination that can be used to quickly change the size of the brush. The coverage achieved by the erasing tool also influences the strength of the effect. It can be helpful to work with less covering power. However, there are only a few cases in which this technique is actually advantageous. You can usually achieve a faster result with layer masks.

In order to make the work somewhat easier in this mode, you should lay a monochromatic layer under the work layer that was previously copied. This is done in order to create a better contrast and increase visibility of the object. Then you can immediately figure out where too much or too little has been erased. If necessary, each layer can be converted into a transparent layer via the Layer | Transparency | Add alpha channel menu or via the pop-up menu for the relevant layer.