Java Graphics.drawLine() inexact: drawn lines are too long

Question

I have a polygon with the vertices (0,0), (100,0), (100,100), and (0,100). I debugged the program and those are the exact lines that java is drawing. Instead of drawing an exact square, some of the lines are a pixel too long:
http://gyazo.com/7418546c51c9a10fc690b18afcc96360.png
(The green circle is just me testing the centroid). When I move the square out of the corner, you can see that the top left corner is the only corner that is exactly correct. Why is this happening? Right before drawLine, I wrote the lines to the console after they were converted to integer coordinates, and they were correct. So I can't see what could possibly be wrong except for the drawLine functions. drawLine:

        g.drawLine((int) line.getStart().getX(), (int) line.getStart().getY(), 
            (int) line.getEnd().getX(), (int) line.getEnd().getY());

The line has a start vector and an end vector. The vectors contain an x and y.

Even when doing this:

    g.drawLine(0, 0, 100, 0);
    g.drawLine(100, 0, 100, 100);
    g.drawLine(100, 100, 0, 100);
    g.drawLine(0, 100, 0, 0);

It still produces the same result. This works though:

    g.drawLine(0, 0, 100, 0);
    g.drawLine(101, 0, 101, 100);
    g.drawLine(100, 101, 0, 101);
    g.drawLine(0, 100, 0, 0);

Please post your code. It's probably an off by one thing, but it's hard to say without seeing some code. — jjm
Well we need to see how you called the method. Please provide a minimal working example... — Willem Van Onsem

Andie2302 Andie2302 · Accepted Answer · 2015-02-21T02:24:46

The example you provided is working:

    BufferedImage bi = new BufferedImage(200, 200, BufferedImage.TYPE_INT_ARGB);
    Graphics g = bi.getGraphics();
    g.fillRect(0, 0, bi.getWidth(), bi.getHeight());
    g.setColor(Color.red);
    g.translate(50, 50);
    g.drawLine(0, 0, 100, 0);
    g.drawLine(100, 0, 100, 100);
    g.drawLine(100, 100, 0, 100);
    g.drawLine(0, 100, 0, 0);
    JOptionPane.showMessageDialog(null, new ImageIcon(bi));

There is a bug https://bugs.openjdk.java.net/browse/JDK-8049901 that describes one possible issue.

Java Graphics.drawLine() inexact: drawn lines are too long

1 Answers