NAME

       exspline - Constructing smooth movement paths from spline curves. Allegro game programming library.

SYNOPSIS

       #include <allegro.h>

       Example exspline

DESCRIPTION

       This  program  demonstrates  the  use of spline curves to create smooth paths connecting a number of node
       points. This can be useful for constructing realistic motion and animations.

       The technique is to connect the series of guide points p1..p(n) with spline  curves  from  p1-p2,  p2-p3,
       etc.  Each spline must pass though both of its guide points, so they must be used as the first and fourth
       of the spline control points. The fun bit is coming up with sensible values  for  the  second  and  third
       spline control points, such that the spline segments will have equal gradients where they meet. I came up
       with the following solution:

       For  each  guide  point p(n), calculate the desired tangent to the curve at that point. I took this to be
       the vector p(n-1) -> p(n+1), which can easily be calculated with the inverse tangent function, and  gives
       decent  looking results. One implication of this is that two dummy guide points are needed at each end of
       the curve, which are used in the tangent calculations but not connected to the set of splines.

       Having got these tangents, it becomes fairly easy to calculate the spline control points.  For  a  spline
       between  guide  points p(a) and p(b), the second control point should lie along the positive tangent from
       p(a), and the third control point should lie along the negative tangent  from  p(b).  How  far  they  are
       placed  along these tangents controls the shape of the curve: I found that applying a 'curviness' scaling
       factor to the distance between p(a) and p(b) works well.

       One thing to note about splines is that the generated points are not all equidistant. Instead  they  tend
       to  bunch  up  nearer to the ends of the spline, which means you will need to apply some fudges to get an
       object to move at a constant speed. On the other hand, in situations where the  curve  has  a  noticeable
       change  of  direction  at each guide point, the effect can be quite nice because it makes the object slow
       down for the curve.

SEE ALSO

       END_OF_MAIN(3alleg4), SCREEN_W(3alleg4), acquire_screen(3alleg4), alert(3alleg4), allegro_error(3alleg4),
       allegro_init(3alleg4),     allegro_message(3alleg4),      calc_spline(3alleg4),      circlefill(3alleg4),
       clear_keybuf(3alleg4),      clear_to_color(3alleg4),     desktop_palette(3alleg4),     fixatan2(3alleg4),
       fixcos(3alleg4), fixed(3alleg4),  fixmul(3alleg4),  fixsin(3alleg4),  fixsqrt(3alleg4),  fixtof(3alleg4),
       fixtoi(3alleg4),   font(3alleg4),   ftofix(3alleg4),  install_keyboard(3alleg4),  install_mouse(3alleg4),
       install_timer(3alleg4),    itofix(3alleg4),     key(3alleg4),     keypressed(3alleg4),     line(3alleg4),
       makecol(3alleg4),    mouse_b(3alleg4),    mouse_x(3alleg4),   mouse_y(3alleg4),   palette_color(3alleg4),
       poll_mouse(3alleg4), readkey(3alleg4), release_screen(3alleg4),  screen(3alleg4),  set_gfx_mode(3alleg4),
       set_palette(3alleg4),       show_mouse(3alleg4),       spline(3alleg4),       textout_centre_ex(3alleg4),
       textprintf_centre_ex(3alleg4), textprintf_ex(3alleg4), vsync(3alleg4), xor_mode(3alleg4)

Allegro                                           version 4.4.3                                exspline(3alleg4)