Problem coloring the Mandelbrot set on Android

Question

I have a problem with coloring the Mandelbrot set. This is my onDraw() procedure:

@Override
protected void onDraw(Canvas canvas) {
   g = Math.round(60+(iter_count*(2500/16)));
   iter_count++;
   for(int xx = 1; xx <= xMax; xx++) {
       for(int yy = 1; yy <= yMax; yy++) {
           schirmzupap(ar,br,bi,ai,xx,yy,xMax,yMax);
           n = 1;
           zr0 = zr;
           zi0 = zi;
           while ((n<g) && (zr*zr+zi*zi<4)) {
               zrh = zr;
               zr = (zr*zr)-zi*zi+zr0;
               zi = zrh*zi+zi*zrh+zi0;
               n++;
           }
           if (n==g) {                                                //[Coloring]
               paint.setARGB(255,0,0,0);  
           }
           if ((n/g) < (1/2)) {
               paint.setARGB(255,Math.round((n/g)*255),0,0);
           }
           if (((n/g) < 1) && ((n/g) > 1/2)) {
               paint.setARGB(255,255,Math.round((n/g)*255),Math.round((n/g)*255));  
           }
           canvas.drawPoint(xx, yy, paint);                           //[/Coloring]
       }              
    }
}

This is how it looks at the Java Android emulator: http://i55.tinypic.com/14ctqi8.png

This is how I want it to look: http://i54.tinypic.com/nh1aqe.png It's written in Delphi, but the coloring part is actually the same:

if n=g then image1.canvas.Pixels[xx,yy]:=RGB2TColor(0,0,0);
if (n/g)<(1/2) then image1.canvas.Pixels[xx,yy]:=RGB2TColor(Round((n/g)*255),0,0);
if ((n/g)<(1)) AND ((n/g)>(1/2)) then image1.canvas.Pixels[xx,yy]:=RGB2TColor(255,Round((n/g)*255),Round((n/g)*255));

Could someone help me please? Greetz,

Henry

Sanjay Manohar · Answer 1 · 2011-08-09T23:15:53.660

Don't do complex calculations in onDraw if it can be helped.
In general, use Math.floor() instead of round in these instances, as you don't really want things to be rounded upwards here.
Would be clearer with else if for the second and third conditions of your colouring algorithm. It seems that the third one is winning out over the second...?
(this is the real problem :-) I bet you declared n and g as integers! The divisions will be integer divisions unless you do this:

convert to double

(n/(double)g)

Remember an integer division e.g. 25000 / 25600 will == 0, and therefore all those pixels will get (255,0,0)

score 1 · Accepted Answer · answered Aug 10 '11 at 01:34

1

As Sanjay said your have a problem with division.

if ((n/g) < (1/2)) {...

if (((n/g) < 1) && ((n/g) > 1/2)) {....

You could fix in Sanjay-way. But take attention 1/2=0 1.0/2=0.5. Or less readeble but a bit faster

if((2*n)<g){...

and

if((n<g)&&(2*n>g)){...

answered Aug 10 '11 at 01:34

vsenik

518
5
12

Thank you very much! n and g were already declared as float. The reason was that I had to write 1.0/2 instead of 1/2. – Henry Aug 10 '11 at 10:30

Problem coloring the Mandelbrot set on Android

2 Answers2