The planimetric method counts the detected grains inside the reference area and returns the Grain Size value. The approach to counting of grains inside the reference (measured) area differs according to the selected standard:
The reference area is created by the grains placed only inside the measurement mask. All grains touching the mask boundary are excluded.
The reference area equals the measurement mask. All grains entirely inside the mask have weight 1, grains which touch the mask boundary have weight 0.5 and grains in corners (using a rectangular mask of ISO 643) have weight 0.25.
Figure 351. ASTM E1382-97
Figure 352. ASTM E112-13, JIS G0551, ISO 643
In contrast to a common thresholding in NIS-Elements, the Grain Size module detection automatically transforms the binary objects (grains) keeping 1 pixel boundary thickness between two neighbours (according to the Norm / Standard instruction). That´s why the binary image changes a little bit after the basic threshold interval setting.
Note
After the detection parameters are set, the algorithm transforms all the boundaries between grains to be 1 pixel wide as required by the standard. This is why the binary image changes a little.