Semi-transparent lines?

Question

Hi,
I’m trying to develop an extended cross hairs cursor. However the lines are rendered as semi transparent although the opacity is set to 1.0. Following code is for testing.
What’s wrong? Can somebody help me?
Thx!
@Override
public void start(Stage stage) throws Exception {
        StackPane root = new StackPane();
        root.setStyle("-fx-background-color:black;");
        Scene scene = new Scene(root, 300, 300);
        stage.setScene(scene);
        stage.setTitle("Cross Hairs Test");
        final Line hLine = new Line();
        hLine.setStroke(Color.WHITE);
        hLine.setStartX(0);
        hLine.setEndX(300);
        hLine.setOpacity(1.0);
        final Line vLine = new Line();
        vLine.setStroke(Color.WHITE);
        vLine.setStartY(0);
        vLine.setEndY(300);
        vLine.setOpacity(1.0);
        final double mouseX = 150;
        final double mouseY = 150;
        Rectangle rect = new Rectangle(300, 300);
        rect.setOpacity(0);
        rect.setCursor(Cursor.NONE);
        rect.setOnMouseMoved(new EventHandler<MouseEvent>() {
          @Override
          public void handle(MouseEvent me) {
               hLine.setTranslateY(me.getY() - mouseY);
               vLine.setTranslateX(me.getX() - mouseX);
        root.getChildren().addAll(hLine, vLine, rect);
        stage.show();
}

Answer 1

This is because the coordinates you specify are not referring to concrete pixels but can also be in between the pixels (in which case they will be anti-aliased). The center of the top left pixel is actually (0.5, 0.5). The top left of the top left pixel is (0, 0).
See this javadoc: http://docs.oracle.com/javafx/2.0/api/index.html?overview-summary.html and read the part of the interaction with the coordinate system.
Anyway, a quick fix is to add 0.5 to coordinates:
@Override
public void handle(MouseEvent me) {
  hLine.setTranslateY(me.getY() - mouseY + 0.5);
  vLine.setTranslateX(me.getX() - mouseX + 0.5);
}