Via the option '-adaptive 0.2' lasthin will allow "adaptive thinning" based on a vertical tolerance of, for example, 20 centimeters. Here are the command lines that are available for this in the current release of LAStools.
Here an example:
lasthin -i ..\data\fusa.laz ^
-ignore_class 1 3 4 5 6 7 ^
-adaptive 0.2 ^
-classify_as 8 ^
-o fusa_classified_keypoints.laz
will produce a new version of the venerable "fusa.laz" where the "most important" ground points (classification code 2) are reclassified as keypoints (classification code 8). Here th same but flagging those points as keypoints instead of reclassifying them:
lasthin -i ..\data\fusa.laz ^
-ignore_class 1 3 4 5 6 7 ^
-adaptive 0.2 ^
-flag_as_keypoint ^
-o fusa_flagged_keypoints.laz
Re can get a coarser version of only ground and buildings using
lasthin -i ..\data\fusa.laz ^
-keep_class 2 6 ^
-adaptive 0.2 ^
-o fusa_thinned_ground_buildings.laz
I have attached two screenshots for the result that the last command line is getting.