Here's a snippet I did for a challenge, might be of some use.
rem catmullrom vector order: control, pt1, pt2, control
rem hermite vector order: pt1, control, pt2, control
set display mode 800,600,32
randomize timer()
rem 2d vector type
type vector
x as float
y as float
w as float
endtype
rem 2 vectors as line segment's endpoints, 2 vectors for control points
type hermiteline
a as vector
b as vector
c as vector
d as vector
endtype
rem number of points defining hermite spline
spline_total = 5
rem get screen dimensions
width = screen width()
height = screen height()
rem array of hermite splines
dim spline(spline_total) as hermiteline
for t = 1 to spline_total
if t > 1
spline(t).a.x = spline(t-1).c.x
spline(t).a.y = spline(t-1).c.y
spline(t).b.x = spline(t-1).d.x
spline(t).b.y = spline(t-1).d.y
else
spline(t).a.x = rnd(width)
spline(t).a.y = rnd(height)
spline(t).b.x = (rnd(200)-100) + spline(t).a.x
spline(t).b.y = (rnd(200)-100) + spline(t).a.y
endif
spline(t).c.x = rnd(width)
spline(t).c.y = rnd(height)
spline(t).d.x = (rnd(200)-100) + spline(t).c.x
spline(t).d.y = (rnd(200)-100) + spline(t).c.y
next t
rem vector points for hermite splines
null = make vector2(1)
null = make vector2(2)
null = make vector2(3)
null = make vector2(4)
rem result vector
null = make vector2(5)
null = make vector2(6)
rem more steps mean a smoother curve in the spline
steps = 20
rem colors of rainbow
dim colors(7) as dword
colors(1) = rgb(255,0,0)
colors(2) = rgb(213,0,213)
colors(3) = rgb(0,0,255)
colors(4) = rgb(0,255,255)
colors(5) = rgb(0,255,0)
colors(6) = rgb(255,255,0)
colors(7) = rgb(255,0,0)
p_total = 100
dim particles(p_total) as vector
for t = 1 to p_total
particles(t).x = rnd(width)
particles(t).y = 0
particles(t).w = rnd(5)+1
next t
DO
cls
for t = 1 to spline_total
rem allow user to click on spline point
if mouseclick() and movePoint = 0
if abs(mousex()-spline(t).a.x) < 4 and abs(mousey()-spline(t).a.y) < 4 then movePoint = t
endif
rem create hermite spline
for s = 1 to steps
rem get the 4 vectors
set vector2 1,spline(t).a.x,spline(t).a.y
set vector2 2,spline(t).b.x,spline(t).b.y
set vector2 3,spline(t).c.x,spline(t).c.y
set vector2 4,spline(t).d.x,spline(t).d.y
time# = s
hermite vector2 5,1,2,3,4,(time#-1.0)/steps
hermite vector2 6,1,2,3,4, time#/steps
rem make rainbow
offset = -6
c = 1
for r = 1 to 6
ink colors(c),0
line x vector2(5), y vector2(5)+offset,x vector2(6), y vector2(6)+offset
line x vector2(5), y vector2(5)+offset+1,x vector2(6), y vector2(6)+offset+1
inc offset,2
inc c
next r
rem draw circle to show spline points
ink rgb(255,255,255),0
circle spline(t).a.x,spline(t).a.y,4
circle spline(t).c.x,spline(t).c.y,4
next s
next t
rem rain particles
lock pixels
for p = 1 to p_total
particles(p).y = particles(p).y + particles(p).w
for t = 1 to spline_total
rem create hermite spline
for s = 1 to steps
rem get the 4 vectors
set vector2 1,spline(t).a.x,spline(t).a.y
set vector2 2,spline(t).b.x,spline(t).b.y
set vector2 3,spline(t).c.x,spline(t).c.y
set vector2 4,spline(t).d.x,spline(t).d.y
time# = s
hermite vector2 5,1,2,3,4,(time#-1.0)/steps
hermite vector2 6,1,2,3,4, time#/steps
x1 = x vector2(5)
y1 = y vector2(5)
x2 = x vector2(6)
y2 = y vector2(6)
rem keep low to high order of points
if x2 < x1
temp = x1
x1 = x2
x2 = temp
temp = y1
y1 = y2
y2 = temp
endif
if particles(p).x >= x1 and particles(p).x <= x2
if point_line(particles(p).x, particles(p).y, x1, y1, x2, y2) >= 0
particles(p).x = rnd(width)
particles(p).y = 0
endif
endif
if particles(p).y >= height
particles(p).x = rnd(width)
particles(p).y = 0
particles(p).w = rnd(5)+1
endif
next s
next t
rem draw raindrop
set_locked_pixel(particles(p).x, particles(p).y, rgb(255,255,255))
next p
unlock pixels
rem allow user to move spline points (control points stay in original locations)
if movePoint > 0
spline(movePoint).a.x = mousex()
spline(movePoint).a.y = mousey()
if movePoint > 1
spline(movePoint-1).c.x = spline(movePoint).a.x
spline(movePoint-1).c.y = spline(movePoint).a.y
endif
endif
if mouseclick() = 0 then movePoint = 0
if spacekey() then resetSplines(spline_total)
loop
REM give new random coordinates for points
function resetSplines(spline_total as integer )
for t = 1 to spline_total
if t > 1
spline(t).a.x = spline(t-1).c.x
spline(t).a.y = spline(t-1).c.y
spline(t).b.x = spline(t-1).d.x
spline(t).b.y = spline(t-1).d.y
else
spline(t).a.x = rnd(width)
spline(t).a.y = rnd(height)
spline(t).b.x = (rnd(200)-100) + spline(t).a.x
spline(t).b.y = (rnd(200)-100) + spline(t).a.y
endif
spline(t).c.x = rnd(width)
spline(t).c.y = rnd(height)
spline(t).d.x = (rnd(200)-100) + spline(t).c.x
spline(t).d.y = (rnd(200)-100) + spline(t).c.y
next t
endfunction
REM if point(px,py) is right or left of line segment (x1,y1),(x2,y2)
REM return negative if left, positive if right, 0 if on
function point_line(px#,py#, x1#,y1#,x2#,y2#)
dp# = (x2# - x1#) * (py# - y1#) - (px# - x1#) * (y2# - y1#)
endfunction dp#
REM function borrowed from Coding Fodder's Rain code
function set_locked_pixel(x as integer, y as integer, color_value as dword )
start = get pixels pointer()
repeat_number = get pixels pitch()
bits_per_pixel = bitmap depth(num)/8
pointer = start + y*repeat_number + x*bits_per_pixel
*pointer = color_value
endfunction
