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public void render(GameContainer gc, Graphics g)
{
int ambientLightLevel = 100; // 0 = black as the darkest night; 255 = as bright as day

// offscreen drawing

gOffscreenGame.clear();      
GL11.glEnable(GL11.GL_BLEND);
GL11.glBlendFunc(GL11.GL_ONE, GL11.GL_ONE_MINUS_SRC_ALPHA);
GL11.glColorMask(true, true, true, true);
// draw game elements
gOffscreenGame.flush()

gOffscreenLights.clear();
gOffscreenLights.setColor(new Color(255, 255, 255, ambientLightLevel));
gOffscreenLights.fillRect(0, 0, WIDTH, HEIGHT);
gOffscreenLights.setDrawMode(Graphics.MODE_ADD);
GL11.glColorMask(true, true, true, true);
// draw lights (white sprites on transparent background; tip: use sinosodial gradients;
// use slight white for places where a light color will be drawn above and strong white for lights that will be white in the end)
gOffscreenLights.flush();

gOffscreenLightColors.clear();
gOffscreenLightColors.setDrawMode(Graphics.MODE_NORMAL);
GL11.glColorMask(true, true, true, true);
// draw the light colors (colored sprites on transparent background; same size and shape as white lights)
// draw them exactly above slight white colored light sprites
gOffscreenLightColors.flush();

// on screen drawing

g.clear();

g.setDrawMode(Graphics.MODE_NORMAL);
g.drawImage(offscreenGame, 0, 0);
      
GL11.glBlendFunc(GL11.GL_ZERO, GL11.GL_SRC_ALPHA);
g.drawImage(offscreenLights, 0, 0); // here we draw the lights by using the light offscreen image as alpha map
      
g.setDrawMode(Graphics.MODE_ADD);
g.drawImage(offscreenLightColors, 0, 0); // here we add the light colors to the lights
   
g.setDrawMode(Graphics.MODE_NORMAL);
g.flush();
}

lightsImage = Image.createOffscreenImage(container.getWidth(), container.getHeight());