TRIANGLE_STRIP Mode by Ira Greenberg.

Generate a closed ring using vertex() function and beginShape(TRIANGLE_STRIP) mode. outerRad and innerRad variables control ring's outer/inner radii respectively. Trig functions generate ring.

size(200, 200);
background(204);
smooth();

int x = width/2;
int y = height/2;
int outerRad = 80;
int innerRad = 50;
float px = 0, py = 0, angle = 0;
float pts = 36;
float rot = 360.0/pts;

beginShape(TRIANGLE_STRIP); 
for (int i = 0; i < pts; i++) {
  px = x + cos(radians(angle))*outerRad;
  py = y + sin(radians(angle))*outerRad;
  angle += rot;
  vertex(px, py);
  px = x + cos(radians(angle))*innerRad;
  py = y + sin(radians(angle))*innerRad;
  vertex(px, py); 
  angle += rot;
}
endShape();