FIGURE 23.

Counting frames that are not fully contained in the region of interest must be used. An object located fairly centrally in the region of interest (a) can be contained in many differently positioned counting frames (only two examples are drawn). If only counting frames that are fully located in the region of interest are used (colored frames), while partial frames (gray frames) are excluded, the number of frames that could contain a more peripheral object decreases (b) until the object cannot be contained in any frame placed on the region (c). Number would consequently be underestimated [Color figure can be viewed at wileyonlinelibrary.com]