Erase color behavioral improvements

src/main/java/com/defano/jmonet/tools/PencilTool.java

@@ -25,10 +25,11 @@ public PencilTool() {
     protected void startPath(Scratch scratch, Stroke stroke, Paint fillPaint, Point initialPoint) {
         Color pixel = new Color(getCanvas().getCanvasImage().getRGB(initialPoint.x, initialPoint.y), true);
 
-        if (getErasePaint() == null) {
+        if (getEraseColor() == null) {
             isErasing = pixel.getAlpha() >= 128;
         } else {
-            isErasing = (pixel.getRed() + pixel.getGreen() + pixel.getBlue()) / 3 <= 128;
+            Color eraseColor = getEraseColor();
+            isErasing = eraseColor.getRed() != pixel.getRed() || eraseColor.getBlue() != pixel.getBlue() || eraseColor.getGreen() != pixel.getGreen();
         }
 
         renderStroke(scratch, fillPaint, new Line2D.Float(initialPoint, initialPoint));

src/main/java/com/defano/jmonet/tools/builder/PaintTool.java

@@ -31,7 +31,7 @@ public abstract class PaintTool implements SurfaceInteractionObserver, CanvasCom
     private Observable<Stroke> strokeObservable = BehaviorSubject.createDefault(new BasicStroke(2));
     private Observable<Paint> strokePaintObservable = BehaviorSubject.createDefault(Color.BLACK);
     private Observable<Optional<Paint>> fillPaintObservable = BehaviorSubject.createDefault(Optional.empty());
-    private Observable<Optional<Paint>> erasePaintObservable = BehaviorSubject.createDefault(Optional.empty());
+    private Observable<Optional<Color>> eraseColorObservable = BehaviorSubject.createDefault(Optional.empty());
     private Observable<Integer> shapeSidesObservable = BehaviorSubject.createDefault(5);
     private Observable<Font> fontObservable = BehaviorSubject.createDefault(new Font("Courier", Font.PLAIN, 14));
     private Observable<Color> fontColorObservable = BehaviorSubject.createDefault(Color.BLACK);
@@ -145,8 +145,8 @@ void setFillPaintObservable(Observable<Optional<Paint>> fillPaintObservable) {
         this.fillPaintObservable = fillPaintObservable;
     }
 
-    void setErasePaintObservable(Observable<Optional<Paint>> erasePaintObservable) {
-        this.erasePaintObservable = erasePaintObservable;
+    void setEraseColorObservable(Observable<Optional<Color>> eraseColorObservable) {
+        this.eraseColorObservable = eraseColorObservable;
     }
 
     void setIntensityObservable(Observable<Double> intensityObservable) {
@@ -183,8 +183,8 @@ public Paint getStrokePaint() {
         return strokePaintObservable.blockingFirst();
     }
 
-    public Paint getErasePaint() {
-        return erasePaintObservable.blockingFirst().orElse(null);
+    public Color getEraseColor() {
+        return eraseColorObservable.blockingFirst().orElse(null);
     }
 
     public Color getFontColor() {
@@ -195,8 +195,8 @@ public Observable<Optional<Paint>> getFillPaintObservable() {
         return fillPaintObservable;
     }
 
-    public Observable<Optional<Paint>> getErasePaintObservable() {
-        return erasePaintObservable;
+    public Observable<Optional<Color>> getEraseColorObservable() {
+        return eraseColorObservable;
     }
 
     public Observable<Stroke> getStrokeObservable() {
@@ -297,7 +297,7 @@ public void applyRenderingHints(Graphics2D g2d) {
     }
 
     protected void erase(Scratch scratch, Shape shape, Stroke stroke) {
-        Paint erasePaint = getErasePaint();
+        Paint erasePaint = getEraseColor();
 
         Graphics2D g = erasePaint == null ?
                 scratch.getRemoveScratchGraphics(this, stroke, shape) :

src/main/java/com/defano/jmonet/tools/builder/PaintToolBuilder.java

@@ -22,7 +22,7 @@ public class PaintToolBuilder {
     private Observable<Stroke> strokeObservable;
     private Observable<Paint> strokePaintObservable;
     private Observable<Optional<Paint>> fillPaintObservable = BehaviorSubject.createDefault(Optional.empty());
-    private Observable<Optional<Paint>> erasePaintObservable = BehaviorSubject.createDefault(Optional.empty());
+    private Observable<Optional<Color>> erasePaintObservable = BehaviorSubject.createDefault(Optional.empty());
     private Observable<Integer> shapeSidesObservable;
     private Observable<Font> fontObservable;
     private Observable<Color> fontColorObservable;
@@ -217,26 +217,38 @@ public PaintToolBuilder withFillPaintObservable(Observable<Optional<Paint>> pain
     }
 
     /**
-     * Specifies the paint (color) that pixels are changed to when they're erased. Specify null for fully-transparent
-     * (default behavior).
+     * Specifies the color that pixels are changed to when they're erased (via the eraser or pencil tools). Specify null
+     * for fully-transparent (default behavior).
+     * <p>
+     * Note that this color does not affect the color of "void" pixels that are left when selecting a region and moving
+     * or deleting it. Further note that the default boundary behavior associated with
+     * {@link com.defano.jmonet.tools.FillTool} looks for fully transparent pixels, thus, when changing the erase color
+     * to a non-null value, erased pixels will not be filled by this tool (install a custom
+     * {@link com.defano.jmonet.algo.fill.BoundaryFunction} if such behavior is desired).
      *
      * @param paint The color that erased pixels should become; null means fully transparent.
      * @return The PaintToolBuilder
      */
-    public PaintToolBuilder withErasePaint(Paint paint) {
+    public PaintToolBuilder withEraseColor(Color paint) {
         this.erasePaintObservable = BehaviorSubject.createDefault(paint == null ? Optional.empty() : Optional.of(paint));
         return this;
     }
 
     /**
-     * Specifies an observable provider of the paint that pixels are changed to when they're erased. Specify null for
-     * fully-transparent (default behavior).
+     * Specifies an observable provider of the paint that pixels are changed to when they're erased (via the eraser or
+     * pencil tools). Specify {@link Optional#empty()} for fully-transparent (default behavior).
+     * <p>
+     * Note that this color does not affect the color of "void" pixels that are left when selecting a region and moving
+     * or deleting it. Further note that the default boundary behavior associated with
+     * {@link com.defano.jmonet.tools.FillTool} looks for fully transparent pixels, thus, when changing the erase color
+     * to a non-null value, erased pixels will not be filled by this tool (install a custom
+     * {@link com.defano.jmonet.algo.fill.BoundaryFunction} if such behavior is desired).
      *
      * @param erasePaintObservable Observable providing the color that erased pixels should become; null means fully
      *                             transparent.
      * @return The PaintToolBuilder
      */
-    public PaintToolBuilder withErasePaintObservable(Observable<Optional<Paint>> erasePaintObservable) {
+    public PaintToolBuilder withEraseColorObservable(Observable<Optional<Color>> erasePaintObservable) {
         this.erasePaintObservable = erasePaintObservable;
         return this;
     }
@@ -364,7 +376,7 @@ public PaintTool build() {
         }
 
         if (erasePaintObservable != null) {
-            selectedTool.setErasePaintObservable(erasePaintObservable);
+            selectedTool.setEraseColorObservable(erasePaintObservable);
         }
 
         if (shapeSidesObservable != null) {