How slow is slow?

just draw lines all the way across / down rather than each individual rectangle

> I wonder if it'd make a differnce to work out the i limit once

this will be minimal compared with the computation done with working out which pixel to fill, and then filling that pixel for ever effected pixel.

There are indeed some problems in the code snippet you posted. The first obvious one would be the width and height you set for the rect. Currently you are setting it to i*16 and j*16, but it should be just 16. Also your for loops are a bit unconventional for drawing a 16x16 grid. Check out the following code example.

Code Example

1. int numGrid = 16;
2.  
3. void setup() {
4.   size(1000, 1000);
5. }
6.  
7. void draw() {
8.   numGrid = mouseX / 20; // remove line to keep it 16
9.   float w = (float) width/numGrid;
10.   float h = (float) height/numGrid;
11.   for (int i=0; i<numGrid; i++) {
12.     for (int j=0; j<numGrid; j++) {
13.       rect(i*w, j*h, w, h);
14.     }
15.   }
16.   frame.setTitle(int(frameRate) + " fps");
17. }

More in general, a great way to save resources on non-changing graphics (such as a grid), is to draw it to a PGraphics once in setup(), then in draw() just use the image. This will save all the calculations from the for loops, the rect calls etc.

@kooogy

I actually mentioned that lines are also slow.

@amnon.owed

The width and height of the rectangles was a mistake. I just accidently put the *i part in.

I dont think you really get what I want. It's maybe because of my explanation because I want a grid with 16X16 pixels cells. Is it really faster to draw it on a PGraphics and draw that image (can be up to 3000x3000 pixels) then to draw lines? 

 Edit

Lines seem to be slower so I'll try the PGraphics part.

Edit

Why does the window not show up when I use this:

1. PGraphics pg;
2. void setup() {
     size(1280, 720);
     pg = createGraphics(1280, 720,JAVA2D);
     pg.beginDraw();
     for (int i=0;i<width/16;i++) {
       for (int j=0;i<height/16;j++) {
         pg.rect(i*16, j*16, 16, 16);
       }
     }
     pg.endDraw();
   }
3. void draw() {
     image(pg, 10, 10);
   }

And why does it show nothing when I use this:

1. PGraphics pg;
2. void setup() {
     size(1280, 720);
     pg = createGraphics(1280, 720,JAVA2D);
   }
3. void draw() {
     pg.beginDraw();
     for (int i=0;i<width/16;i++) {
       for (int j=0;i<height/16;j++) {
         pg.rect(i*16, j*16, 16, 16);
       }
     }
     pg.endDraw();
     image(pg, 0, 0);
   }

Ah, a grid of 16x16 cells, got it.

The problem with both the codes you posted is an error in the line:

1.     for (int j=0;i<height/16;j++) {

The i should be a j.

1.     for (int j=0;j<height/16;j++) {