The white triangles outside polygons is a known issue
size(500, 100, P3D); background(200); strokeWeight(3); beginShape(); stroke(255,0,0);vertex(20, 20); stroke(0,0,255);vertex(40, 20); stroke(0,255,0);vertex(40, 40); stroke(0,0,255);vertex(60, 40); stroke(128,0,255);vertex(60, 60); stroke(128,0,64);vertex(20, 60); endShape(CLOSE); beginShape(); stroke(255,0,0);vertex(120, 20); stroke(128,0,64);vertex(140, 20); stroke(128,0,255);vertex(140, 40); vertex(160, 40); stroke(128,64,0);stroke(0);vertex(160, 60); endShape(CLOSE); beginShape(); stroke(0,255,0);vertex(220, 20); vertex(240, 20); stroke(64,128,0);vertex(240, 40); stroke(255,0,255);vertex(260, 40); endShape(CLOSE); beginShape(); stroke(0,255,64);vertex(320, 20); stroke(64,64,0);vertex(340, 20); stroke(0,255,0);vertex(340, 40); endShape(CLOSE); beginShape(); stroke(128,64,255);vertex(420, 20); stroke(0,64,128);vertex(440, 20); endShape(CLOSE);