Ellipse, rotated about an angle

[Henko]

The standard sB function is not capable of drawing an ellipse under an angle.
This function does.

A88CF834-44BF-4924-9053-F6E43606E154.png (1.98 MiB) Viewed 5006 times

Code: Select all

graphics ! graphics clear
get screen size sw,sh ! cx=sw/2 ! cy=sh/2
draw color 0,0,0 ! draw size 3
for phi=0 to 360 step 15
  ellipse(cx,cy,200,80,phi) ! pause .3
  next phi
stop

' draw ellipse rotated counterclockwise with phi degrees
' xf,yf = coordinates of left focal point
' ra,rb = horizontal and vertical radii (ra > rb supposed)
' phi   = rotation angle in degrees
' option angle in calling code segment is untouched
'
def ellipse(xf,yf,ra,rb,phi)
if sin(90)=1 then oa=1 else oa=0
option angle degrees
d_alfa=1
e=sqrt(1-rb*rb/ra/ra)
rho=ra*(1-e*e)/(1-e*cos(-phi))
x=xf+rho ! y=yf
draw to x,y
for alfa=d_alfa to 360 step d_alfa
  rho=ra*(1-e*e)/(1-e*cos(alfa-phi))
  x=xf+rho*cos(alfa) ! y=yf-rho*sin(alfa)
  draw line to x,y
  next alfa
if oa=1 then option angle degrees else option angle radians
end def

[matt7]

I always forget about the DRAW TO and DRAW LINE TO functions! Nice use of them.

Because of the difference in screen resolutions, I did not get those thin gaps between line segments on my iPhone like in your screenshot. But if I make the DRAW SIZE larger then I do start to get some and part of the ellipse can appear transparent. But this can be avoided if you set DRAW LINECAP ROUND before calling the ellipse function.

[rbytes]

Another cool drawing effect. Thanks, Henk!

[Henko]

Another cool drawing effect. Thanks, Henk!

Putting d_alfa=15 in the function (a very coarse ellipse) :

4D9190F9-D2D6-4126-A08F-1DEDBFD68310.png (1.92 MiB) Viewed 4986 times

[Henko]

To finalize the ellipse "library", fuctions are added to draw or fill an ellips which is rotated about the centre of the ellips in stead of rotation about a focal point.

CAC989EF-2C5C-4AC2-80E2-152FD671711E.png (1.74 MiB) Viewed 4983 times

Code: Select all

graphics ! graphics clear 
get screen size sw,sh ! cx=sw/2 ! cy=sh/2
draw color 0,0,0 ! draw size 3
for phi=45 to 315 step 90
  draw_ellipse(cx,cy,360,60,phi) ! pause .3
  fill_ellipse(cx,cy,300,80,phi+45,.5,.5,.5,.3)
  next phi
stop


' draw ellipse rotated counterclockwise with phi degrees
' xc,yc = coordinates of the centre of the ellipse
' ra,rb = horizontal and vertical radii (ra > rb supposed)
' phi   = rotation angle in degrees
' option angle in calling code segment is untouched
'
def draw_ellipse(xc,yc,ra,rb,phi)
if sin(90)=1 then oa=1 else oa=0
option angle degrees
dt=6 ! sp=sin(phi) ! cp=cos(phi)
draw to xc+ra*cos(phi),yc-ra*sin(phi)
for t=dt to 360 step dt
  st=sin(t) ! ct=cos(t)
  draw line to xc+ra*cp*ct-rb*sp*st,yc-ra*sp*ct-rb*cp*st
  next t
if oa=1 then option angle degrees else option angle radians
end def

' fill ellipse rotated counterclockwise with phi degrees
' xc,yc = coordinates of the centre of the ellipse
' ra,rb = horizontal and vertical radii (ra > rb supposed)
' phi   = rotation angle in degrees
' R,G,B,A = color and alpha info
' option angle in calling code segment is untouched
'
def fill_ellipse(xc,yc,ra,rb,phi,R,G,B,A)
if sin(90)=1 then oa=1 else oa=0
option angle degrees
fill color R,G,B ! fill alpha A
dt=6 ! np=int(360/dt) ! sp=sin(phi) ! cp=cos(phi)
dim x(np+2),y(np+2)
ip=0
for t=0 to 360 step dt
  ip+=1 ! st=sin(t) ! ct=cos(t)
  x(ip)= xc+ra*cp*ct-rb*sp*st ! y(ip)= yc-ra*sp*ct-rb*cp*st
  next t
fill poly x,y
if oa=1 then option angle degrees else option angle radians
end def

' draw ellipse rotated counterclockwise with phi degrees
' xf,yf = coordinates of left focal point
' ra,rb = horizontal and vertical radii (ra > rb supposed)
' phi   = rotation angle in degrees
' option angle in calling code segment is untouched
'
def ellipse(xf,yf,ra,rb,phi)
if sin(90)=1 then oa=1 else oa=0
option angle degrees
d_alfa=10
e=sqrt(1-rb*rb/ra/ra)
rho=ra*(1-e*e)/(1-e*cos(-phi))
draw to xf+rho,yf
for alfa=d_alfa to 360 step d_alfa
  rho=ra*(1-e*e)/(1-e*cos(alfa-phi))
  x=xf+rho*cos(alfa) ! y=yf-rho*sin(alfa)
  draw line to x,y
  next alfa
if oa=1 then option angle degrees else option angle radians
end def