Global measures are computed on the grain to make a first estimation about the possible type, and to guide which grain characteristics to look for.

We compute measures like the diameter, the mean colour (RGB), the shape, ...on the central image of the grain.

For example, we can compute the diameter (in microns) vs the mean blue colour for each grain to see that already we can identify some specific types (*Parietaria* for example). For other types, more knowledge-based techniques will have to be used.

Cupressaceae can appear as broken on the microscopic slide. It is possible to detect the broken grains easily using some shape criteria:

- Compacity:
*4 surface / perimeter*^{2} - Convexity ratio:
*grain surface / convex hull surface*

Broken grain | Convex hull |

2001-03-06