(b).

Clustering	Distance of samples
	Criterion: the G value in RGB color space
Categories	I	II	III	IV
Number of clustering centers	31	2	33	3
Samples in the class	18, 23, 24, 25, 27, 32	1, 10, 21, 22, 28, 34	4, 5, 9, 13, 14, 20, 29	6, 7, 8, 11, 12, 15, 16, 17, 19, 26, 30
Scope of feature values	[41,66]	[70,83]	[95,120]	[126,203]
Feature thresholds	G < 70	70 < G < 120		G > 120