Hands on: Make good selections with image-editing software

Selections are often the first step in the editing process and knowing how to make them quickly and accurately will save you time and frustration, while improving your image-editing skills.

Making good selections isn’t always easy ­ if it were, software developers wouldn’t keep introducing new tools for the job.

Most photo-editing applications provide a dozen different ways to isolate a bunch of pixels depending on similarities of position, tone, colour, or your ability to draw a very wiggly line around them using the mouse.

On the understanding that most people know how to use geometric selection tools and the lasso tool including straight line, magnetic and the other variants your editing program offers, I’ll skip straight to the more useful and interesting stuff.

With the obvious exception of the Quick Selection tool described below the usual caveat applies ­ although I talk about Photoshop you can do most of this using any photo editor that supports selections and masks.

Quick Selection tool
First, a few words about The new Quick Selection tool in Photoshop CS3 which has pretty much taken over from the Magic Wand tool as the first choice for making pixel value-based selections.

It’s innovative tools like this that keep Photoshop well ahead of image editing competition and make it the choice of professionals.

If you’re not a professional, or don’t do sufficient image editing to justify the outlay for Photoshop there’s some good news for you. Adobe has announced the release of Photoshop Elements 6, which also includes the Quick Selection tool.

This tool is as simple to use as it is effective. It’s a brush, but the only parameter you need to set is the size; there’s no tolerance setting, the tool just has a knack of knowing where the edge of your selection is. You simply paint over the bits you want, getting reasonably close to the edges, and the tool does everything else.

The best results are obtained with short strokes, adding small chunks to the overall selection as you go. Like the other Photoshop selection tools, you can use keyboard modifiers or the Tool Options palette to add to or subtract from existing selections.

On its own, the Quick Selection tool is a godsend, but combine it with the new Refine Edge Controls, which can be used on any selection, and it can go where no selection tool has gone before.

As well as providing a variety of view options that make assessing the selection edge easier, the Refine Edge dialogue box provides sliders to smooth, feather, contract and expand the selection, as well as contrast and radius sliders for fine tuning and re-introducing lost detail.

These new Photoshop tools take a lot of the effort out of making first rate selections, but if you’re working with an older version of Photoshop, or another image editor, don’t be too disheartened as there are plenty of other ways you can produce accurate selections without too much effort.

The Magic Wand
I’m not going to say too much about this one on the same grounds as I ignored the lasso. The key with the Magic Wand is the tolerance setting, which determines how closely adjacent pixels must match the pixel you click on in order to be included in the selection. If you get more than you bargained for, reduce the tolerance and vice versa.

Few people appreciate the importance of the exact area you click with this tool. Even in an area that appears similar in colour there can be a relatively wide variation in pixel values, so if the first click produces poor results deselect and try again in the vicinity of the first attempt, before experimenting with a different tolerance setting.

Grow and Similar
When you’ve got most of what you need with the Magic Wand, use Grow and Similar to expand your selection. Grow adds contiguous pixels, only adding regions that are connected to the original selection. Similar adds pixels in the image that match, within the set tolerance, the pixels in the existing selection.

Make a difference
The Magic Wand and other tone and colour-based selection tools select groups of pixels on the basis of the similarity of pixel values within user-defined tolerances. This works well when the subject contains pixels that are similar, yet different from the things you don’t want to select.

If the subject and background are similar it’s not so straightforward, but you can skew the odds in your favour by applying some image adjustments to accentuate small differences and make the job of the selection tools easier.

Other than for the purpose of making the selection, the adjustments won’t be required, so start by duplicating the layer ­ when the selection is made you can delete it. Use a Levels adjustment to accentuate tonal differences. Drag the black and white sliders in from the ends of the histogram, or use the central gamma slider to increase the image contrast.

If you can’t differentiate between like pixels inside and outside of your required selection using Levels, take a look at the individual RGB channels ­ in Photoshop you can do this by selecting the channel in the Channels palette, in Paint Shop Pro you’ll have to separate the image to its component channels. You’ll find that what you want to select stands out more in one of the channels than in the composite image.

Saving and loading selections
Most applications will allow you to save and load selections as alpha channels ­ greyscale images in which selected pixels are shown as white and non-selected pixels black.

Grey pixels indicate partial selection ­ a feathered selection would have a grey border fading to black. One advantage of this is that it makes the selection permanent. You can also work on the alpha channel using the editing tools, but if you plan on doing this it’s easier to convert the selection into a mask.

I briefly explained how masks work here: like alpha channels they hide or reveal detail in the layers below (or, in the case of Photoshop layer masks, the layer to which they belong) depending on the greyscale value of mask pixels ­ white shows underlying detail, black hides it.

Editing masks
I do a lot of beach photography and one thing I’m constantly having to do is add yellow and increase saturation to make the sand more natural-looking. This process almost always involves making a selection and converting it to a mask to adjust the beach colour without affecting the sky.

It demonstrates how the selection tools are often just the first step and how, once you have a rough selection, it’s simpler to turn it into a mask and edit it than to try to get it perfect using the selection tools.

I start by using the Magic Wand to select the sky. I Shift & click until I’ve got most of it including clouds.
I then invert the selection and apply a colour balance adjustment layer with increased yellow in the highlights and midtones ­ Photoshop automatically converts the selection into a layer mask.

Next, I Alt & click the layer mask to display it and use a soft-edged brush to paint in any holes, black in the sky region and white on the land.

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