Efficiency

//Changing:
for(int y = 0; y < height; y++)
    img.setColorAt(x, y, c);
//to
img.setColor(c);
img.drawLine(x, 0, x, height);

/**
 * Gives the worlds background image a gradient between 2 colors.
 * Example call to this: createGradientBGImage(Color.RED, Color.BLUE);
 * This requires "import java.awt.Color;" and must be in a class that
 * extends greenfoot.World
 */
private int createGradientBGImage(Color left, Color right)
{
	long time = System.currentTimeMillis();
	int width = getWidth(),
		height = getHeight(),
		leftMult,
		rightMult,
		r, g, b;
	GreenfootImage img = new GreenfootImage(width, height);
	for(int x = 0; x < width; x++)
	{
		leftMult = 255 - (x*255/width);
		rightMult = 255 - leftMult;
		r = ((left.getRed()  *leftMult) + (right.getRed()  *rightMult))/255;
		g = ((left.getGreen()*leftMult) + (right.getGreen()*rightMult))/255;
		b = ((left.getBlue() *leftMult) + (right.getBlue() *rightMult))/255;
		Color c = new Color(r, g, b);
		//for(int y = 0; y < height; y++)
		//	img.setColorAt(x, y, c);
		img.setColor(c);
		img.drawLine(x, 0, x, height);
	}
	setBackground(img);
	return (int)(System.currentTimeMillis() - time);
}

public void test1()
{
	List<Integer> times = new ArrayList<Integer>();
	for(int i = 0; i < 10000; i++)
		times.add(createGradientBGImage(randomColor(), randomColor()));
	double time = 0.0;
	for(int t : times) time += t;
	time /= times.size();
	String exeTime = "" + time;
	if(exeTime.length() > 5) exeTime = exeTime.substring(0, 5);
	System.out.println("createGradientBGImage(...) execution time: " + exeTime + "ms");
}

static private Color randomColor()
{
	return new Color(Greenfoot.getRandomNumber(255), Greenfoot.getRandomNumber(255), Greenfoot.getRandomNumber(255));
}

img.setColor(new Color(r, g, b));
img.drawLine(x,  0,  x, height);

GreenfootImage bg = new GreenfootImage(getWidth(), getHeight());
Color leftColor = Color.blue;
Color rightColor = Color.green;
for (int x = 0; x < getWidth(); x++)
{
    int alphaLeft = x * 255 / 600;
    int alphaRight = (getWidth() - 1 - x) * 255 / 600;
    GreenfootImage left = new GreenfootImage(1, getHeight());
    left.setColor(leftColor);
    left.fill();
    left.setTransparency(alphaLeft);
    GreenfootImage right = new GreenfootImage(1, getHeight());
    right.setColor(rightColor);
    right.fill();
    right.setTransparency(alphaRight);
    bg.drawImage(left, x, 0);
    bg.drawImage(right, x, 0);
}
setBackground(bg);