arc via 3 points in specific direction

Question

I need to draw the arc from 3 points in specific direction.

Lets say i have 3 vec2 points P1, P2, P3;

I\'ve been manage to find arc center:

circ         


        

Answer 1

heh was there too while convert some specific vector data elliptic arcs to SVG ...

few if should do it. I see your case like this:

try this angle correction:

if (a2-a1>+180.0) a2-=360.0;
if (a2-a1<-180.0) a2+=360.0;
if (a3-a2>+180.0) a3-=360.0;
if (a3-a2<-180.0) a3+=360.0;

and after this is direction easy:

if (a2-a1< 0) dir = CW;
if (a2-a1> 0) dir = CCW;
if (a2-a1==0) dir = none;

The only problem is when your arc cover whole 360 degree circle or more ...

• then this is not enough

draw can be done like this:

for (i=0,a=a1;i<100;i++,a+=(a3-a1)/99.0) // 100 lines per arc or use GDI arc ...
 {
 x=C.x + R*cos(a*PI/180.0);
 y=C.y + R*sin(a*PI/180.0); // or - R*...  if your render device has opposite Y direction
 if (!i) ...->Canvas->MoveTo(x,y);
 else    ...->Canvas->LineTo(x,y);
 }

• R = |P1-C|