Hi, I'm doing simple painting tool, where I wan't alpha-blend several layers. It's working quite well using GLGraphics, however because of compactibility with older computers (or not instaled drivers like on Linux) I wan't also to do it with standard Processing PGraphics. 

However, the result is not what I want. Instead of blending new layer with background pixel color, the image()
blend layer with black (Completly opaque)

See folowing simple example

1. int sx=500; int sy=500;
   
   PGraphics GUI, Canvas;
   PGraphics Layer1;
   PGraphics Layer2;
   
   void setup() {
     size(sx, sx,P2D);
     //smooth();
     GUI    = createGraphics(sx, sx,P2D);
     Layer1 = createGraphics(sx, sx,P2D);
     Layer2 = createGraphics(sx, sx,P2D);
   
     frameRate(60);
         Layer1.beginDraw();
         GUI.background(255,0);
         Layer1.fill(255,0,0,50); Layer1.noStroke();
         Layer1.beginDraw();
        
         Layer2.beginDraw();
         GUI.background(0,0);
         Layer2.fill(0,0,255,50); Layer2.noStroke();
         Layer2.beginDraw();
    
   }
   
   void draw() {
    
     GUI.beginDraw();
     GUI.background(255,0);
     GUI.noFill();
     GUI.ellipse(mouseX, mouseY,20,20);
     GUI.endDraw();
    
     if (mousePressed == true){
       if(mouseButton == LEFT){Canvas = Layer1; }else{Canvas = Layer2;};
       Canvas.beginDraw();
       Canvas.ellipse(mouseX, mouseY,20,20);
       Canvas.endDraw();
       {
      
       }
     }
    
     background(200);
     image(Layer1, 0, 0);
     image(Layer2, 0, 0);
     image(GUI, 0, 0);
   }