Selection tools: Part III, the Magic Wand
- — 22 June, 2004 08:08
Compared to many other types of software, graphics programs have some of the simplest or most descriptive terms to describe various functions. It is not hard to guess the purpose of the Crop or Text tools. You may not immediately understand the roles of the Magic Wand, but after seeing what it can do, it’s not hard to see why it has this name.
The Magic Wand bases its selections primarily on colour. The advantage of this technique is that it can find natural edges in a flash, so there is less need for painful edge-tracing.
The process behind the Magic Wand is simple. When you click on part of your image, anything nearby that is a similar colour will also be selected (where you initially click is known as the ‘sampled area’ or ‘sampled pixel’). For example, click part of a blue sky with the Magic Wand and you’ll find that the most of the sky will be selected.
The Magic Wand’s tolerance value will determine how similar a colour has to be to the sampled pixel in order to be included in the selection. Set a value of zero and only that precise colour will be selected; conversely, if the tolerance is set too high, it may start to include unwanted parts of the image, particularly at borders. Trying different values will help select the maximum desired area, but don’t get obsessed with trying to catch everything in one click. As discussed later in this column, there are ways to fine-tune selections.
Depending on your program, there may-be an option for a ‘contiguous’ selection. This means that in order to be included with the selection, pixels must be touching other pixels than fall within the tolerance range — and connect to the sampling point. For example, if you have a blue sky that it is cut down the middle by a white cloud, then using the Magic Wand setting on the left side will select only the left section of sky (with the contiguous setting turned on). This is because there is no connecting blue area between both halves. If there was just a small sliver of blue connecting both halves, then the entire blue sky would be highlighted.
If you turn off the contiguous option, everything in the picture that falls with the tolerance value will be highlighted. If someone is wearing a blue shirt and you sample the sky without the contiguous setting, the shirt will also be selected (assuming it falls with the same tolerance range). Both techniques have their uses, but the non-contiguous option features more in colour management and high-end techniques. It can also be used to remove purple fringing (purple artefacts introduced by digital cameras). By default, most Magic Wands are set to contiguous.
Tricks to use
One overlooked but exceptionally useful aspect of any selection tool is inversion. In other words, you exchange the selected sections with those that were not selected. Most programs have this feature listed under the Selection menu or similar (such as Select-Inverse or Selection-Invert). The reason this option is so powerful is that frequently it is easier to highlight everything you do not want and then use the invert option to highlight the desired object.
Consider the picture of the polar bear shown here. It was far easier to select the water (which is mainly blue) than to select the bear, which is black, yellow, white and other colours. Even using this technique, the Magic Wand does not make a perfect selection on the first attempt. A small part of the left side of the bear’s head ended up in the wrong selection zone, and smaller dots in the water were not included, see here. There are many solutions to these problems, but one of the easiest is to use the Freehand lasso tool.See understanding selection tools part one. In programs such as Paint Shop Pro and Photoshop, if you hold down the <Shift> key while using any selection tool, the new area you highlight will be added to the overall selected area. Likewise, <Alt> will subtract the new area. In the bear image, most of the hard work had been done with the Magic Wand, so the Freehand lasso was used in a flash to tidy up the stragglers. The ‘add’ option was used for the small dots in the water and the subtract option for the small area on left hand side of the bear’s head. Finally Selection-Invert highlighted only the bear.