function [Ms, xs, ys] = lp_thin(M, x, y, maxn)
% LP_THIN  Subsample a field so that a picture of it fits in a JSON message.
%
% Keeps at most maxn points in each direction, always including both ends.
nx = numel(x);
ny = numel(y);
ix = round(linspace(1, nx, min(nx, maxn)));
iy = round(linspace(1, ny, min(ny, maxn)));
Ms = M(iy, ix);
xs = x(ix);
ys = y(iy);
end