Another important task for computer-based image analysis is counting, measuring and identifying the features present in images. This applies both to microscope images of sections through structures, and to other classes of microscopic or macroscopic specimens such as particles dispersed on a substrate. It is important to know if the features seen in the images are projections or sections through the objects. Projections show the external dimensions of the objects, and are usually easier to interpret. Some section measurements can be used to calculate size information about the three-dimensional objects as described in Chapter 1, subject to assumptions about object shape.