(This material can be found at http://graphics.lcs.mit.edu/6.837/f96/problem_sets/problem_sets.html.)

Some style notes: we use typewriter font to denote class names (for example Draw) and affix parentheses to denote methods (for example Draw.init()). Note that capitalization is significant, so that (for example) the class DrawLine is distinct from the method Graphics.drawLine.

• The drawing program currently only supports three colors. Add support for additional colors. First, add more addItem() calls in Draw.init() to add colors to the Choice component. Second, update the event handling code in Draw.action() to do the right thing when your new items are selected. Finally, add a setColor() call to DrawCircle.mouseDown() and DrawText.mouseDown() to make color selection work as it does in DrawLine.mouseDown().

• Add support to change the size, font ("Helvetica", "TimesRoman", "Courier") and style (Font.PLAIN, Font.BOLD, Font.ITALIC) of text. You can use a Choice or a TextEntry for string-valued input, and Checkboxes for selecting the style. Add code to Draw.init() which implements these UI components, and add code to Draw.action() to handle the corresponding events. Also, add fields to Options to hold the new state values in your applet. Finally, in DrawText.mouseDown(), you'll new a new Font(name,style,size) object, and call setFont() on the appropriate graphics object (of class Graphics). Note that this approach will not support changing fonts while in the middle of typing a string; for extra credit, implement such a capability.

• Add functionality to complete your applet's support for remote operation. (Note that since there is no true distributed object model in Java, the application must implement remote method invocation by sending data across the network.) Most of this is already implemented in the DrawObject and DrawConnection classes; however, you must fill in the following gaps in functionality. First, modify DrawObject.sendOptions() and DrawObject.readOptions() to handle all supported colors, and transmit some encoding of the fill flag options.fill. Second, modify the methods in DrawText to send events analogous to those sent by DrawLine and DrawCircle.

• Extend the applet to draw splines. Make a class DrawSpline (analogous to, e.g., DrawLine and DrawCircle) that subclasses DrawObject; add a button to Draw.init() which activates it; and add code to Draw.action() analogous to that of e.g. DrawLine. The simplest approach is to collect the positions of four mouse clicks, then draw the cubic Bezier spline defined by these four control points. You can use graphics.drawLine() to draw the line segments.

  For extra credit, make the current spline editable. In DrawSpline.mouseDown(), check whether the corresponding position is close to that of any control point. If so, let the user drag the control point around, while the spline is dynamically redrawn. Note: achieving smooth regeneration of the curve without disturbing the rest of the image will take some thought. The xor approach used by DrawCircle won't work well, since your spline-drawing routine is likely to draw some pixels twice (e.g., at shared endpoints of line segments, or where the spline self-intersects). One solution is to save the current image before any interactive spline drawing begins. Then, whenever you update the spline, first redraw the stored image, then draw the spline. Here is skeleton code to do this:

  class DrawSpline extends DrawObject {
    Image saveImg = null;
    public DrawSpline(...) {
      // Allocate image buffer of proper size
      saveImg = parent.createImage(parent.size().width, 
                                   parent.size().height);
      // Tricky part: tell Draw to update our 
      // image, rather than the screen image
      parent.update(saveImg.getGraphics());
    }
    void drawTheSpline() {
      // redraw what was there before
      graphics.drawImage(saveImg,0,0,null); 
      // render spline segments using a
      // series of calls to graphics.drawLine()
      // ...
    }
  }
  

• The drawRender class writes a rectangular block of pixels (some of which are transparent) into the graphics to realize a filled, multicolored circle. For extra credit, extend the drawRender class to do something more interesting.