pts:=[[16,3],[12,17],[0,6],[-4,-6],[16,6],[16,-7],[16,-3],[17,-4],[5,19],[19,-8],
[3,16],[12,13],[3,-4],[17,5],[-3,15],[-3,-9],[0,11],[-9,-3],[-4,-2],[12,10]]:

with(geometry):
map(coordinates,convexhull([seq(point(P||i,pts[i]),i=1..nops(pts))]));
# [[-9, -3], [-3, -9], [19, -8], [17, 5], [12, 17], [5, 19], [-3, 15]]

with(ComputationalGeometry):
pts[ConvexHull(pts)];
# [[12, 17], [5, 19], [-3, 15], [-9, -3], [-3, -9], [19, -8], [17, 5]]

simplex:-convexhull(pts);
# [[-9, -3], [-3, -9], [19, -8], [17, 5], [12, 17], [5, 19], [-3, 15]]