How to solve the setColor() problem with multithreading?

Question

My fractal program uses a custom number of threads to draw the complete image. If I use just 1 thread, the image looks clean, if I use more threads some pixels of the image gets other colors. The problem ist the painting code:
g2d.setColor(col);
g2d.drawLine(x, y, x, y);as it is necessary to first set the color of the graphics object and then draw the line. I can solve this pixel color problem by synchronizing the Graphics2D object:
synchronized (g2d) {
  g2d.setColor(col);
  g2d.drawLine(x, y, x, y);
}but this slows the code down to times where just 1 thread is calculating the image. Unfortunately, I can't find a draw() method that has a color parameter.
I tried to divide the panel into multiple BufferedImages (1 for each thread) which solves the pixel problem but only the first BufferedImage get's drawn.

Answer 1

I solved my problem now. My solution is to use a separate BufferedImage for each Thread. I still had a bug where i didn't transformed the x/y coordinates of the complete image to the shifted x/y coordinates of the single BufferedImages, so that the threads draw the image out of the visible area which looked like all threads after the first only had a black image. After adjusting teh coordinates, the fractal now gets drawn correctly. And boy it's much faster with multiple threads - even on single core systems. On a dual core system, using 10-20 threads about doubles the performance as both cores have work almost all the time until the fractal is finished. :-)