guys, i'm feeling really ashamed, asking such a stupid question, but I'm getting crazy on this problem.

i have this script:

for (int i = 0; i< width; i+=20) {
for (int j = 0; j< height; j+=20) {

rect (i,j,2,2);
}
}

as you can see i get sort of a grid when displaying that. when pressing 'k' i want it to appear, when pressing 'k' once again i want it to disappear....
sounds easy, but i don't know how to realize it.

in c++ i know a function named VISIBLE. (visible.true and visible.false). is there any equivalent in processing?

hope for your tips once again..sorry, for beeing newby.

awesome....
thank you thank you thank you
it works perfectly well - exactly the way i want it.

Just omit the background() instruction? Or move it to setup().
If you want to overlay the grid then hide it, it is harder, unless you memorize all the operations the user has done, or you take a snapshot of the current image (get()), forbid doing something will the grid is on, then restore the image to hide the grid.
Actually, you can still let drawing while the grid is on, but you have to draw both on visible display and in the hidden copy (which has no grid).

dear PhilLho,

thanks for giving a "starting point". it really helped me.
i tried to put your script together with my script and now get some trouble while running.

actually my script has to do:

- drawing rects (20x20) when pressing a.
- by clicking one sets the starting point.
- when pressing Return the same sort of rect - line has to appear right under the previous line of rects.
- when clicking to another point, it has to start again.
- when pressing TAB the grid (your grid appears.

my problem:

when pressing TAB the grid does not appear in the upper left corner (the inital defaut 0,0), but at the position of the mouse. which is  no surprise, as i set everything at the position of mouseX, mouseY. But I just want my rectangles to appear at the position of the mouse. the grid has to be always at (0,0) (to give the user a sort of orientation where to set the rectangles).

i hope you know a way to get out of this
thanks a lot.

static final int GRID_SIZE = 50;
boolean bShowingGrid;
PGraphics drawing;
int breite = 40;
int hoehe = 60;
int wertX, wertY;
int x1, x2 = 40;
int count;


void setup() {


 size (500,500);

 background (0);

 drawing = createGraphics (width, height, JAVA2D);
 drawing.beginDraw();
 drawing.background (0);
 drawing.endDraw();

}
void draw() {
}

void mouseClicked() {
wertX = 0;
 wertY = 0;
 count = 0;

}


void keyReleased() {

translate (mouseX,mouseY);

 x1 = breite/4;
 x2 = hoehe/4;
 wertX += x1;
 wertY -= x2/5;
 tastatur (wertX, wertY);

}

void DrawGrid()
{
 stroke(#005588);
 strokeWeight(1);
 for (int i = 0; i < width; i += GRID_SIZE)
 {
   line(i, 0, i, height);
   line(0, i, width, i);
 }
}

void tastatur (float typeX, float typeY) {
 switch (key) {

 case ENTER:
 count ++;
 
   wertX =  -count*x1/3;
   wertY = count*x2;
   break;

 case 'a':
   rect (typeX,typeY,20,20);
   drawing.beginDraw();
   drawing.rect (typeX, typeY, 20,20);
   drawing.endDraw();
   break;

 case TAB:
   {
     bShowingGrid = !bShowingGrid;
     if (bShowingGrid)
     {
       DrawGrid();
     }
     else
     {
       image(drawing, 0, 0);

     }
   }
   break;
 }
}