% B-spline Tutorial
% by Ashish Myles (marcianx@gmail.com)

% Plot the knots as vertical black dotted/dashed lines, and the control
% points and polygon in red.
function plot_knots_and_cpts(knots, cpts, draw_flags)
hold on;

nk = length(knots);
deg = nk - length(cpts) - 1;
grev = greville(knots, deg);

dy = ( max(cpts) - min(cpts) ) * 0.20;
range_x = [ min(knots), max(knots) ];
if (deg == 0)
    range_x = [ 2*knots(1)-knots(2), 2*knots(end)-knots(end-1) ];
end
range_y = [ min(cpts)-dy, max(cpts)+dy ];

% Draw the control points
if bitand(draw_flags, 1)
    plot(grev, cpts, 'r*');
    plot(grev, cpts, 'ro');
end

% Draw the control polygon
if bitand(draw_flags, 2)
    plot(grev, cpts, 'r');
end

% Draw the knots
if bitand(draw_flags, 4)
    uknots = unique(knots);
    mult = histc(knots, sort(uknots));
    mult2stipple = { ':', '-.', '--', '-' };
    for i = 1:length(uknots)
        c = strcat('k', mult2stipple{min(mult(i), length(mult2stipple))});
        plot([uknots(i), uknots(i)], [range_y(1), range_y(2)], c);
    end
end

axis([range_x, range_y]);

hold off;