Selection Areas

It's an almost universal constant in 2D graphics editors that you'll have the ability to select only part of an image to work with. This selected area will almost always be denoted by a dotted / dashed line around said area.

In image editing the phrase "selection" refers to a user-defined area which have three main uses.

  • While a selection is active any action taken will be constricted to the selected area. Further brush strokes, filters, or movements will affect only those areas.
  • Selected areas can be moved around a canvas seperate from the rest of the image.
  • The edges of a selected area can be used to create other objects such as shapes and masks. (These operations are discussed in layers and masks sections)

Imagine the box above is a (blank) canvas in an image editor like Photoshop. In this case two selections have been made in the form of simple circle shapes. A green line has been drawn across the canvas horizontally. Because we have a selection in the form of two circles the stroke is confined to those two areas.

Now regard the box below and again imagine it to be an empty canvas. In this case you'll notice how there is now a rectangular selection around the two circles. This indicates that the area selected is NOT the two circles but the area around them and within that rectangle. We know this because you can never have selection "overlap". A selection may be made up of separated pieces - but it will always be a single selection.

Because the selected area is outside the circles any edits or additions made to the canvas after the selection has been made is constrained to that new area. In this case the addition of a green stroke from a brush tool results in the green filling the selected area within the dashed line but no where else.

Selection Tool Types

Most image editors make use of multiple tools to let you select the areas you want in the most effecient manner.

Rectangular & Ellipse

Every paint program will allow you to make selections with two primitive shapes; the rectangle and the ellipse.

We don't refer to them as "square" and "circle" or even have tools for those shapes because a square and a circle are simply rectangle and ellipse shapes with equal length and width measurements.

Lasso & Polygon

Tools like "lasso" and "polygon" selection allow you to "draw" more freeform shapes than simple rectangles or ellipse shapes.

The lasso tool is generally regarded as "draw and select" meaning that once you've pressed the left mouse button down the edge of the selection will be formed by wherever you drag the mouse and will automatically close with the beginning when you release the left mouse button.

Generally you can use the polygon tool to define a selection area made up of straight lines that begin and end where ever you click the mouse.

Be aware that Programs such as GIMP have begun to merge the lasso and polygon tool into a single use tool.

Some programs have unique tools, such as Adobe Photoshop's "magnetic" lasso tool. See the Photoshop specific page for instructions on using this tool.

Selection Tool Options

Even though there are multiple tools the options associated with those tools are typically the same. Here are a few options that you can change no matter which selection tool you are using.

Feathering

"Feathering" of a selection describes the fuzziness or "blurring" of the edge around a selection. This allows you to make "soft" selections where the effect applied, whether it's a color fill, or moving the entire selection (as seen below) gradually falls off from the selection perimeter.

Keep in mind that no matter what your chosen program you'll typically have to change the "feathering" amount before you make the selection.

  • On the left you can see a selection that was made while the feathering amount was set to "0".
  • On the right you can see a selection that was made while the feathering amount was set to "5" (in this case the feathering was measured in pixels, this amount will change depending on your personal needs).

Anti-aliasing

Just as in other contexts the option to enable "anti-aliasing" refers to whether or not the edge of the selection is made by either selecting all or none of a pixel (with no opacity variation) or selecting pixels while allowing for variation in opacity.

Let's look at an image to make that clearer.

  • On the left you can see a selection that was made while the anti-aliasing option was turned on.
  • On the right you can see a selection that was made while the anti-aliasing option was turned off.

Anti-Aliasing can give the appearance of a "smoother" selection edge but this may not always be what you want. If you try to fill an anti-aliased selection with a solid color for instance the fill might leave a slight "space" at the edges where the color starts to change. The aliased cut-out with hard edges will allow a fill to completely cover the selected or removed area all the way to the edge.

For most photo editing where you won't be editing images on a per-pixel level then you can probably leave the "anti-aliasing" option on for your selections. Conversely artists creating works from scratch will probably leave it off.

Selection Operations

As said selections should be thought of as *temporary* and therefore operations on them are fairly simple. But those simplistic operations can be extremely helpful. For instance you are not usually going to be limited to the shaped made by the basic selection tools. Most selection tools let you change their shape via combination operations or other selection refinement functions.

Addition & Subtraction

Any decent image editing program will allow you to perform selection operations such as adding to a currently existing selection or subtracting from a currently existing selection. These can change between programs (Photoshop will subtract from currently selected areas when the "ALT" key is pressed while GIMP will do so when "CTRL" is pressed) so it's actually a subject best left to the program-specific section.

Seleciton Refinement

Selection refinement is something specific to Adobe Photoshop at this point in time. See the Photoshop subsection for information on how to utilize it.