Estou usando o Intellij IDEA para o desenvolvimento de javafx FXML. Eu usei o código a seguir para simplesmente desenhar um retângulo. No entanto, nunca apareceu.

Main.java

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        primaryStage.setTitle("Hello World");
        primaryStage.setScene(new Scene(root, 600, 400));
        primaryStage.show();
    }


    public static void main(String[] args) {
        launch(args);
    }
}

Controller.java

public class Controller implements Initializable {
public Mat image;
@FXML public Canvas img = new Canvas(300,300);
public GraphicsContext gc = img.getGraphicsContext2D();
@FXML private void drawCanvas(ActionEvent event) {
    gc.setFill(Color.AQUA);
    gc.fillRect(10,10,100,100);
}
@Override
public void initialize(URL location, ResourceBundle resources) {
        gc.setFill(Color.BLACK);
        System.out.println("color set to black");
        gc.fillRect(50, 50, 100, 100);
        System.out.println("draw rectangle");
    }
}

Eu usei setFill () e fillRect () nos métodos Button OnAction e de inicialização. Mas ainda não desenha os retângulos.

sample.fxml

<BorderPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Controller">
   <top>
      <ToolBar prefHeight="40.0" prefWidth="200.0" BorderPane.alignment="CENTER">
        <items>
          <Button mnemonicParsing="false" onAction="#drawCanvas" text="draw" />
        </items>
      </ToolBar>
   </top>
   <bottom>
      <AnchorPane prefHeight="41.0" prefWidth="600.0" BorderPane.alignment="CENTER">
         <children>
            <Label fx:id="co" layoutX="14.0" layoutY="9.0" text="co" />
            <Label fx:id="rgb" layoutX="144.0" layoutY="13.0" text="RGB" />
            <Label fx:id="zoom" layoutX="245.0" layoutY="9.0" text="zoom" />
         </children>
      </AnchorPane>
   </bottom>
   <center>
      <ScrollPane fx:id="img_pane" prefHeight="200.0" prefWidth="200.0" BorderPane.alignment="CENTER">
         <content>
            <Canvas fx:id="img" height="286.0" width="355.0" />
         </content>
      </ScrollPane>
   </center>
</BorderPane>