Re: Questions about drawing on Canvas

Zerex71 <>
Tue, 29 Apr 2008 09:31:14 -0700 (PDT)
On Apr 29, 12:01 pm, Knute Johnson <>

Zerex71 wrote:


I am attempting for the first time to do some drawing on a canvas.
What I would like to do is draw a standard set of three orthogonal
axes and then superimpose a line (vector) on the canvas as well, to
aid in visualizing some rotations. The questions I have are as

1. If I want the vectors to have any thickness, it sounds like I am
going to have to call drawRect() instead of drawLine() - true or

See Stroke.

2. In my reading, I have seen code examples where they seem to extend
JPanel (I don't know why such a graphical object would subclass from a
panel, so I'm a bit puzzled by that) and override its paintComponent()
method. Why do you subclass from JPanel to draw rectangles, etc.?

Convention. You can use JComponent and some people recommend you do.
JComponent and JPanel which extends it are both Containers as well.
Sometimes you want to be able to add components to your GUI. Canvas is
not a Container. Be sure to read the docs for Canvas.

3. Must you do everything to each thing you want to draw that you
would do to other container components (e.g. my canvas) such as
frame.add(canvas) and canvas.setVisible(true), etc.?

You can add the Canvas to a Container (eg. Frame or Panel) before you
set the primary Frame visible. You should pack() or setSize() on the
container to get it to layout before you set it visible.

4. When you do a setColor(color), it seems like it's just telling the
Graphics object "you're about to do something and I want you to do it
with this color). Is that correct?


   Why can you get away with not

specifying the fg and bg colors each time (for example, if you want to
draw a red rectangle with a black border, I have seen in that example
where they make two calls to setColor())?

Because that's not the way it works. In your paint() method you set the
color you wish to draw with and then call the drawing method. If you
want to change colors, set a new color and draw again.

5. Is there a specific order to packing and adding things before
showing them? I'm a little confused on what the absolute last thing
is that you need to do when you want to show your combined UI. (In
other words, I have a frame which contains the canvas, the axes, and
the vector, and so far I can only get the canvas to be drawn.)

setVisible(true) is the last call on your GUI container. Are you
drawing in the paint() method? For AWT components (Frame, Panel, Canvas
etc.) all drawing must occur in the paint() method. For Swing
components it is the paintComponent() method.

I am hoping there are some simple answers here but unfortunately none
of the Java tutorials I have seen on Sun or in my manuals I printed
seem to provide any concrete answers. If you know of any good
websites that really explain the graphics paradigm well, I'm open to
hearing about them.

Post some simple code that demonstrates the problem you are having. It
must be compilable (unless you are asking about compile errors).


Knute Johnson
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*** BEGIN CODE ***

package transformation;

import javax.swing.*;
import java.awt.*;

 * @author mfeher
public class Visualizer
    // The purpose of this class is to provide a Swing-based
    // capability for our rotations to show the user a graphical
    // of what happens during the transformation process.

    // Default constructor (required but not used)
    public Visualizer() {}

    public Visualizer(final Vector start,
                      final Vector end) {
        // In this constructor, we should try to instantiate a canvas
        // (drawing window), a set of axes, and an initial vector.
        // Also, we should customize the view to show a "perspective
        // view" of the scene before performing the rotation.

        // Good resource for how to use the frame properly is located

        // Create the drawing canvas; customize its background and
        // foreground colors. Make the window appear right away.
        String title = "OneTESS Transformation Algorithm Visualizer";
        Color bg = new Color(17, 54, 171); // Background color (dark
        Color fg = Color.white; // Foreground color (white)
        Canvas canvas = new Canvas(); // The drawing canvas
        // Set the colors on the canvas, and force it to be a certain
        Dimension canvasSize = new Dimension(400, 400);
        // Set the max and min sizes to attempt to "lock" the
container to a fixed size
        // NOTE By themselves, these two calls do NOT limit the size.
        // They also make the initial size zero height.
        // TODO Find out what effect removing these LOC will have.
        // Setting the frame size seems to be just fine.
        // TODO Do we need to make the frame an explicit size or is
        // some layout manager handling that for us because we are
        // something contained by the frame?
        // Seems dumb and obvious but we better make sure it's visible

        JFrame frame = new JFrame(title, null); // Use default GC
        // Ensure the window closes with the app

        // Set the frame size - the canvas never changes size but it's
        // frame that we want to lock
// frame.setMaximumSize(canvasSize);
        // NOTE These three lines don't work in locking the frame,
        // so find another way...
        // NOTE Here's that way:

        // Add the canvas to the frame

        // Size the frame
// frame.setSize(400, 400); // Rough guess at size, had no

        // Create axes and draw them
// VisualAxes axes = new
VisualAxes(canvas.getForeground(), // Use the canvas fg
// new Vector(100.0, 100.0,
100), // May need to check to see if these coords will fit on the
100.0); // Set a reasonable length for each

        // NOTE This type of code can ONLY go into a paintComponent()
        // This is because g is of type Graphics which CANNOT be
        // and must be passed down by the painting infrastructure to
your overridden
        // paintComponent() method.
// g.setColor(canvas.getForeground());
// g.drawRect(10, 20, 30-1, 5-1);
// g.setColor(canvas.getForeground());
// g.fillRect(10+1, 20+1, 30-2, 5-2);

        // Create the vector to be rotated
        // DEBUG Try drawing just one axis and see what happens
        VisualVector vv = new VisualVector(new Vector(10.0, 0.0, 0.0),;
        // TODO Pass in a real vector at some point

        // Duh, set the vector to be visible too...sheesh, Java makes
        // you do everything like a baby
        // TODO Do we even need to do this?

        // TODO Did we even add the vector to the frame? Do we need
to do that?
        // If so, in what order?
        // QUESTION Do we add the vector to the frame or to the
canvas? Hmm...
        // NOTE We cannot add the vector to the canvas, so to the
frame it is...

        // TODO Ensure that when this app is ready to rock and roll,
        // that you can port it to the conference room and run it

        // Show the frame

        // Hmm, maybe we need to force a repaint of ALL components
        // TODO Do we need to set opacity on some object?

        // Draw the initial vector
        // TODO Figure out the proper way to force the object(s) to
        // paint themselves. I haven't been able to find a way to
        // do this but maybe there's hope in repaint()...
// vv.repaint();

        // Shall we try to animate the rotation to the final vector?

        // QUESTION Do we want to show the guntube as a vector, or
        // just show the effect of the initial muzzle vector rotating?

        // TODO If you want to get slick, see if you can allow the
        // user to see the rotation quaternion on there as the unit
        // vector plus twist angle.

        // TODO Somehow we have to define a "perspective view" such
        // that our coordinates (axes), when drawn, appear as "tilted"
        // (perspective) rather than side-on and thus losing one DOF.

        // TODO Figure out how to put arrowheads on the axes

        // TODO Investigate making the axes have some thickness
        // enough for the user to see
        // NOTE To answer this to-do, consider using drawRect()

    private void animateRotation() {
        // This method will perform the animation of the vector
        // NOTE We may need some form of slerp to do this...
        // We assume, as novice Java Swing programmers, that we will
        // have to on every cycle draw a vector, then erase it, and
        // draw a new vector based on the interpolated position over
        // the total length of the vector divided by dt, the time
        // of animation.

        // TODO When we animate the rotation, we should be using a
        // repaint scheme - but only the vector to be rotated should
        // be repainted. The axes should stay fixed.

    private final double dt = 0.0; // Parameter to govern speed of
    // Set to positive number for gradual motion
    // Set to zero for "snap-to" animation

    private Timer animationTimer; // Timer used to control animation

    // The amount of twist around the z-axis that we will rotate our
    // geometry for better viewability
    private final double perspectiveTwist = 0.0;
    // The amount of tilt away from vertical (pitch down) that we
    // will rotate our geometry for better viewability
    private final double perspectiveTilt = 0.0;

package transformation;

import javax.swing.*;
import java.awt.*;

 * @author mfeher
public class VisualAxes extends JPanel // TODO Figure out why we must
use JPanel
    // This class will provide a visual set of axes on the drawing
    // canvas so the user can see where their vectors are being

    // Default constructor
    public VisualAxes(Color color,
                      Vector O,
                      double length) {
        // Initialize the attributes of this class
        axesColor = color;
        origin = O;
        axisLength = length;

        // With this starting information, set up each axis.
        // Once this is done, we can try to draw them.
        // The x-axis will be (length, 0, 0) offset from the origin.
        // The y-axis will be (0, length, 0) offset from the origin.
        // The z-axis will be (0, 0, length) offset from the origin.

        xAxis = new VisualVector(origin.add(new Vector(axisLength,
0.0, 0.0)), axesColor); // Or something like this
        yAxis = new VisualVector(origin.add(new Vector(0.0,
axisLength, 0.0)), axesColor); // Or something like this
        zAxis = new VisualVector(origin.add(new Vector(0.0, 0.0,
axisLength)), axesColor); // Or something like this

        // TODO Ensure mathematically that the axes adhere to the
        // hand rule.

        // Theoretically at this point we have three orthogonal axes
        // starting at a common origin and having the same length, but
        // pointing in three different directions orthogonal to each
        // other.

        // TODO Consider adding an axis indicator character to the
        // screen for each axis such as "x", "y", and "z".
        // NOTE drawString() might do this for you

        // TODO May want to add some debug code to ensure the math
        // part is correct, by printing out the origin and axes

    public void show() {
        // Method to draw the axes

        // Draw the x-axis

        // Draw the y-axis

        // Draw the z-axis

        // TODO Do we need some kind of reference to the canvas
        // we're trying to draw on?

    public void hide() {
        // Method to hide the axes

        // Hide the x-axis

        // Hide the y-axis

        // Hide the z-axis

    // Required method to implement drawing of the axes
    public void paintComponent(Graphics g) {
        // First call the superclass to paint the background

        // Set the color
        // Draw each axis

    private Color axesColor; // Color of each axis
    private Vector origin; // The origin of the axes
    private double axisLength; // The length of each axis
    private VisualVector xAxis; // The x-axis
    private VisualVector yAxis; // The y-axis
    private VisualVector zAxis; // The z-axis

package transformation;

import javax.swing.*;
import java.awt.*;

 * @author mfeher
public class VisualVector extends JPanel
    // This class represents a vector which we can show to the user.
    // A visual vector is a graphical class which extends a normal
    // mathematical vector.

    public VisualVector(Vector v,
                         Color c) {
        // Default constructor

        // Set up the visual vector's characteristics
        theVector = v;
        vectorColor = c;
        // Now convert the math vector into an appropriate visual
        x = (int) Math.round(v.x);
        y = (int) Math.round(v.y);
        z = (int) Math.round(v.z);

        // DEBUG Print out the coordinates for grins
        System.out.println("Original vector to be drawn:");
        System.out.println("Original vector converted to graphics
        System.out.println("x = " + x + " y= " + y + " z= " + z);
        // NOTE Z-coordinates not used
        // NOTE I read something in the Java API documentation
        // something to the effect of how "user space" drawing got
        // to "system space" (?) coordinates. I believe this is
        // the 2D representation as the user/programmer would think of
        // an object as opposed to the "upper-left-corner-across-and-
        // graphics space, not the math conversion of 3D objects to 2D
        // objects. This latter process is probably what we will have
to do
        // explicitly.

        // TODO Figure out if we'll need this or even use it
        thickness = 0;

        // TODO Do we draw the vector now, or make an explicit call?

    public void show() {
        // Show (draw) the vector
        // TODO Do we need a canvas or graphics object to do this?

    public void hide() {
        // Hide the vector
        // TODO Do we need a canvas or graphics object to do this?

    // Required method to implement drawing of the axes
    public void paintComponent(Graphics g) {
        // First call the superclass to paint the background

        System.out.println("Attempting to paint a vector!");

        // Set the color
// g.setColor(vectorColor);
        // TODO Examples make it seem like you only call setColor()
        // but maybe you need to set the bg and the fg explicitly to
        // your graphics will show up
        setBackground(new Color(17, 54, 171)); // TODO Find a way to
pass in the bg color from the canvas

        // Draw the vector
        // For now, just draw a line
// g.drawLine(10, 20, 30, 40); // DEBUG
        g.drawLine(0, 0, x, y); // The actual line we're given
                                 // Somehow we need to pass the origin
into here
        // TODO This is not showing our line - do we need to use the
        // frame's origin instead?
        // TODO Do we need to set it to opaque?

    public Vector theVector; // The mathematical vector to be rotated
    private Color vectorColor; // The color of the vector
    private int x; // The integral value of the x-coordinate
    private int y; // The integral value of the y-coordinate
    private int z; // The integral value of the z-coordinate
    private int thickness; // Thickness of the vector to draw
                             // Assume zero means a line will be drawn

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