Change origin of canvas

Question

hey, i have a canvas that positions itself in the middle of the screen with two lines (x and y axis). the problem is that i want to be able to move the origin, which then draws the axis again.
thanks
Pedge
// GCanvas.java
import java.awt.*;
import java.awt.event.*;
import java.util.*;
* This class extends the canvas class to include the input and drawing of a polygon
* @author Geoff Hobson
* @version 1.1 (24 Nov 99)
class GCanvas extends Canvas
* the polygon object associated with this canvas.
private GPolygon polygon;
* indicates whether currently in process of colecting the vertices of a polygon.
private boolean building = false;
* the size of one pixel in the logical coordinates of the polygon - the scaling factor.
float pixelSize;
* the screen x coordinate of the logical origin - screen x position of y-axis.
int centerX;
* the screen y coordinate of the logical origin - screen y position of x-axis.
int centerY;
* the radius of the circle around the start point,
* a mouse press anywhere in this circle will be treated as selecting the centre point.
private int circleSize = 3;
* Constructs a GCanvas and associates it with the given Polygon poly.
* @param poly the polygon object which is to be drawn and manipulated
GCanvas(GPolygon poly)
polygon = poly;
addMouseListener
(new MouseAdapter()
{ public void mousePressed(MouseEvent evt)
pointSelected(evt.getX(), evt.getY());
); // end of new MouseAdapter
setBackground(Color.gray);
private int cX, cY;
* clears the canvas and sets it ready for the input of a new polygon.
public void newPoly()
building = true;
setCursor(Cursor.getPredefinedCursor(Cursor.CROSSHAIR_CURSOR));
repaint();
* The action to be performed when a point is selected by a mouse press.
* @param x the x coordinate of the selected point
* @param y the y coordinate of the selected point
private void pointSelected(int x, int y)
if (building)
addVertex(x, y);
* Add a new vertex to the polygon object
* @param x the x coordinate of the new vertex
* @param y the y coordinate of the new vertex
private void addVertex(int x, int y)
if (polygon.size() == 0)
polygon.addVertex(new Point2D(fX(x), fY(y))); // add first vertex to a polygon
else
Point2D v0 = polygon.getVertex(0);
int dx = x - iX(v0.x), dy = y - iY(v0.y);
if (dx * dx + dy * dy < circleSize * circleSize)
{    // mouse in circle around first vertex
polygon.closePolygon(); // the polygon has been closed
building = false;
setCursor(Cursor.getPredefinedCursor(Cursor.DEFAULT_CURSOR));
else
polygon.addVertex(new Point2D(fX(x), fY(y))); // add an other vertex to polygon
repaint();
* Converts logical to screen x coordinate
* @param x the logical x coordinate
* @return the screen x coordinate
int iX(float x)
return Math.round(centerX + x/pixelSize);
* Converts logical to screen y coordinate
* @param x the logical y coordinate
* @return the screen y coordinate
int iY(float y)
return Math.round(centerY - y/pixelSize);
* Converts screen to logical x coordinate
* @param x the screen x coordinate
* @return the logical x coordinate
float fX(int x)
return (x - centerX) * pixelSize;
* Converts screen to logical y coordinate
* @param x the screen y coordinate
* @return the logical y coordinate
float fY(int y)
return (centerY - y) * pixelSize;
* Paints the canvas with current state of polygon
* @param g the graphic context for the paint method
public void paint(Graphics g)
// initialise a blank canvas and draw axes etc
int maxX = getSize().width - 1; // get size of screen
int maxY = getSize().height - 1;
float rWidth = polygon.logicalWidth(); // get logical size of polygon
float rHeight = polygon.logicalHeight();
pixelSize = Math.max(rWidth/maxX, rHeight/maxY); // calculate scaling factor
centerX = maxX/2; // calculate position of axes
centerY = maxY/2;
int left = iX(-rWidth/2); // calculate edges of logical polygon
int right = iX(rWidth/2);
int bottom = iY(-rHeight/2);
int top = iY(rHeight/2);
// draw white rectangle for logical polygon
g.setColor(Color.white);
g.fillRect(left, top, right - left+1, bottom - top+1);
// draw frame around logical polygon rectangle
g.setColor(Color.darkGray);
g.drawRect(left, top, right - left, bottom - top);
g.setColor(Color.lightGray);
g.drawLine(left,centerY, right,centerY); // draw x axis
g.drawLine(centerX,top, centerX,bottom); // draw y-axis
// draw current state of the polygon
int n = polygon.size();
if (n == 0) return; // if there are no vertices
Point2D A, B;
A = (polygon.getVertex(0));
// Show tiny circle around first vertex:
g.setColor(Color.red);
if (!polygon.isClosed())
g.drawOval(iX(A.x)-circleSize, iY(A.y)-circleSize, circleSize*2, circleSize*2);
// draw the polygon
g.setColor(Color.blue);
for (int i=1; i<n; i++)
B = (polygon.getVertex(i));
g.drawLine(iX(A.x), iY(A.y), iX(B.x), iY(B.y));
A = B;
if (polygon.isClosed()) // draw side from last to first vertex
B = (polygon.getVertex(0));
g.drawLine(iX(A.x), iY(A.y), iX(B.x), iY(B.y));
} // end of paint
} // end of class GCanvas

Answer 1

How about declaring changing centerx and centery in the constructor?