Your application determines the size of the pick aperture using GpiQueryPickApertureSize, and determines the page space coordinates of the center of the aperture with GpiQueryPickAperturePosition.
Because your objective is to retrieve a single detectable object for each correlation operation, you have to obtain a balance between the number of detectable objects in a picture and the size of the pick aperture. The more detectable objects you define, the smaller the pick aperture must be to return a manageable amount of information to the application. However, the pick aperture usually is not a 1 pel-by-1 pel rectangle. A larger rectangle makes better sense because the pick aperture can be difficult to hit with a single pel; but a larger sampling area, for example 4 pels-by-4 pels, increases the probability of an intersection between a lighted pel and the pick aperture.
The pick aperture can be specified in any of the units available to the presentation space. However, the type of unit must be identical to the type specified for the presentation page. The default size of the pick aperture is square, with sides equal to the default character cell height.