Unlike vector images which consist of discrete objects each of which can be selected and manipulated independently of the rest of the drawing, a bitmapped image is one continuous whole consisting of a matrix of pixels. Making selections in a bitmapped image like Photoshop is therefore much more complex and difficult than simply clicking on an object as you would do in a vector drawing. In fact, selections are such a big deal in Photoshop that there is a whole menu dedicated to selection options and techniques: the Select menu.
There are a good many reasons why we may need to make selections in an image. Perhaps we want to cut out a product or person and place them on a different background. Perhaps we want to blur part of an image to add emphasis to the remainder.
Selections are an essential component within Photoshop and can be manipulated independently of the pixels they include. Selections almost make you forget the absence of objects within bitmapped images. Once part of an image is selected, it can be targeted and manipulated in such a way that the pixels inside the selection can almost be treated as an object.
On our Adobe Photoshop courses, we always show users how Photoshop allows you to build selections gradually and contains a number of sophisticated tools for modifying them. One of the simplest techniques is to add to or subtract from an existing selection. This can be done either by using modifier keys (hold down Shift to add or Alt to subtract) or by clicking on icons to enter Add or Subtract mode. Any selection made using one of these methods will take into account the pixels that are already selected.
Photoshop also allows you to transform selections in a manner not dissimilar to the way that objects are transformed in the vector environment. One key thing to remember is that in order for this transformation to work, one of the selection tools has to be active. If the Move tool is active, the pixels inside the selection will also be transformed.
The most fundamental transformation is probably movement. This can be done either by placing the cursor inside the selection and dragging or by using one of the cursor keys on the keyboard. Each time a cursor key is pressed, the selection will move one pixel in the direction specified. If the Shift key is held down while a cursor key is pressed, the selection will move 10 pixels in the given direction.
For other forms of transformation, choose Transform Selection from the Select menu. A bounding rectangle will then be displayed around the selection with handles similar to those found in vector drawing programs. You can drag the handles to resize the selection or drag just outside the handles to rotate. You can even hold down the Control key and drag the handles to distort your selection.
The writer of this article runs training courses for an independent computer training company offering Adobe Photoshop CS4 tuition in London and all over the UK.
