The basic shape primitive functions are triangle(), rect(), quad(), ellipse(), and arc(). Squares are made with rect() and circles are made with ellipse(). Each of these functions requires a number of parameters to determine the shape's position and size.
size(640, 360)
background(0)
noStroke()
fill(204)
triangle(18, 18, 18, 360, 81, 360)
fill(102)
rect(81, 81, 63, 63)
fill(204)
quad(189, 18, 216, 18, 216, 360, 144, 360)
fill(255)
ellipse(252, 144, 72, 72)
fill(204)
triangle(288, 18, 351, 360, 288, 360)
fill(255)
arc(479, 300, 280, 280, PI, TWO_PI)