Sometimes the need will arise to make a precise selection. Suppose you wanted to exactly select a square starting at 10 pixels from the top and left of your image with sides 100 pixels in length. The easiest way to do this is to use the Exact Position dialog as follows:
Select Tools/Exact Position
Enter 10 in the X position and 10 in the Y position.
Set Radius to 20
Leave Lock Mouse checked.
Repeat 1 to 4 but enter 110 for each of the X and Y values.
You have now created two, exact, mouse capture points – you can see them on your image as small circles. Move the mouse over the first point – note that the mouse pointer "falls-into" the pre-defined point. It has been snapped to exactly the coordinate 10,10. The size of the "fall into" zone is governed by the Radius set in step 3 – in the above example, 20 pixels.
Now draw the mouse towards the second mouse capture point – again as you get within 20 pixels, the mouse will a fall into the exact coordinate 110, 110 and the selection is made.
You can add as many (or just one) mouse capture points as you want, depending on what shape you want to create. You can also use exact position to govern the movement of other tools, for example the paint brush.
The Lock Mouse checkbox ensure the mouse is snapped to an exact point. Without this, exact position is a visual guide only.
For general information about selections, see Selections