broken blend modes :/

gistfile1.pde

size(100, 100);

// drawing surface with no background
PGraphics pg = createGraphics(100, 100);
  pg.beginDraw();
  pg.blendMode(DIFFERENCE); // same for SUBTRACT!
  pg.fill(#ffffff);
  pg.noStroke();
  pg.ellipse(50, 50, 30, 30);
  pg.endDraw();

// display the surface
background(255);
image(pg, 0, 0);