Override path with transparent color in javafx Canvas/GraphicsContext

Question

I'm wondering if there is any way in the GraphicsContext of a Canvas in JavaFX to override all pixels in a path in such a way that they become transparent again. Like the clearRect function but for any path.

I have looked at the different BlendModes and Effects but it seems like there is no way to actually override the alpha value which could be achieved by e.g

• rendering a transparent color with the Porter/Duff Src operator
• rendering a non transparent color with globalCompositeOperation destination-out in HTML5

I would be very glad if someone knew a way to achieve this without tessellating the path and using multiple clearRect calls. Thank you.

http://docs.oracle.com/javase/8/javafx/api/javafx/scene/canvas/GraphicsContext.html

---

Addition in response to the first comment: Instead of using a Path node I'm using a Canvas node to perform rendering using the GraphicsContext and thus, it is not possible to modify the color of a previously rendered path. I’m trying to do something like

Canvas canvas = …;
final GraphicsContext context = canvas.getGraphicsContext();
context.setFill(Color.BLUE);
context.fillRect(0,0,100,100);
…
…

and later on I want to reset the pixels in a path to transparent

context.setStroke(Color.TRANSPARENT);
context.beginPath();
context.moveTo(0, 0);
context.lineTo(10,10);
…
context.stroke();

However, as blending is applied this does not work. In other rendering libraries it is possible to use e.g. one of the above mentioned blending / compositing methods to achieve this but until now I was not able to identify such a functionality in javafx.

Answer 1

I haven't found a mechanism to clear a canvas depending on a Path. But here's how you could do it manually:

• Create a mask on which you draw the path
• Blend image and mask manually

Example with a blue rectangle that's been applied a mask with a path:

public class ManualBlendWithMask extends Application {

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

    @Override
    public void start(Stage primaryStage) {

        Group root = new Group();

        // create canvas
        Canvas canvas = new Canvas(200, 200);
        canvas.setTranslateX(100);
        canvas.setTranslateY(100);

        // create canvas with blue rectangle
        GraphicsContext canvasGC = canvas.getGraphicsContext2D();
        canvasGC.setFill(Color.BLUE);
        canvasGC.fillRect(0, 0, canvas.getWidth(), canvas.getHeight());

        // create mask
        Canvas mask = new Canvas( canvas.getWidth(), canvas.getHeight());
        GraphicsContext maskGC = mask.getGraphicsContext2D();
        maskGC.setFill(Color.WHITE);
        maskGC.fillRect(0, 0, mask.getWidth(), mask.getHeight());

        // draw path
        maskGC.setStroke(Color.BLACK);
        maskGC.setLineWidth(20);
        maskGC.beginPath();
        maskGC.moveTo(0, 0);
        maskGC.lineTo(400, 400);
        maskGC.stroke();

        // get image from canvas
        WritableImage srcImage = new WritableImage((int) canvas.getWidth(), (int) canvas.getHeight());
        srcImage = canvas.snapshot(null, srcImage);

        // get image from mask
        WritableImage srcMask = new WritableImage((int) mask.getWidth(), (int) mask.getHeight());
        srcMask = mask.snapshot(null, srcMask);

        PixelReader maskReader = srcMask.getPixelReader();
        PixelReader imageReader = srcImage.getPixelReader();

        int width = (int) srcMask.getWidth();
        int height = (int) srcMask.getHeight();

        // create dest image
        WritableImage dest = new WritableImage(width, height);
        PixelWriter writer = dest.getPixelWriter();

        // blend image and mask
        for (int x = 0; x < width; x++) {
            for (int y = 0; y < height; y++) {

                Color maskColor = maskReader.getColor(x, y);
                Color imageColor = imageReader.getColor(x, y);

                if (maskColor.equals(Color.WHITE)) {
                    writer.setColor(x, y, imageColor);
                }

            }
        }

        // clear canvas and fill it with the blended image
        canvasGC = canvas.getGraphicsContext2D();
        canvasGC.clearRect(0, 0, canvas.getWidth(), canvas.getHeight());
        canvasGC.drawImage(dest, 0, 0);

        // draw background with gradient
        Rectangle rect = new Rectangle(400, 400);
        rect.setFill(new LinearGradient(0, 0, 1, 1, true, CycleMethod.REFLECT, new Stop(0, Color.RED), new Stop(1, Color.YELLOW)));

        // show nodes
        root.getChildren().addAll(rect, canvas);

        primaryStage.setScene(new Scene(root, 400, 400));
        primaryStage.show();

    }

}

Of course it would be better if someone could come up with an example using BlendModes.