JavaFX Tip 2: Sharp Drawing with Canvas API

Dirk LemmermannApril 11th, 2014Last Updated: April 10th, 2014

2 84 1 minute read

When I initially started out working with the Canvas API I noticed that the results of my rendering code were somewhat blurry and even worse, inconsistent. Some lines were blurry, others sharp. Coming from Swing it took me some time to realize that this was caused by the coordinate system of JavaFX, which allows for double precision rendering.
To solve this problem all that is needed is to use coordinates “in the middle”. So in my code you now find a lot of methods called snapXZY() (similar methods can be found in the JavaFX code itself), which first casts the given coordinate to an integer and then adds .5 to it. The following screenshot shows the difference when using this approach.

The code below was used for this example:

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.control.Label;
import javafx.scene.layout.VBox;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

/**
 * Tip 2: How to render sharp lines in a canvas.
 */
public class Tip2DrawingSharpLinesInCanvas extends 
        Application {

	class MyCanvas extends Canvas {

		public MyCanvas(boolean drawSharpLines) {

			setWidth(150);
			setHeight(150);

			double w = getWidth();
			double h = getHeight();

			GraphicsContext gc = getGraphicsContext2D();
			gc.clearRect(0, 0, w, h);

			gc.setStroke(Color.GRAY);
			gc.strokeRect(0, 0, w, h);

			for (double y = 20; y <= h - 20; y += 10) {
				if (drawSharpLines) {
					// Snap the y coordinate 
					gc.strokeLine(10, 
                                   snap(y), 
                                   w - 10, 
                                   snap(y));
				} else {
					gc.strokeLine(10, y, w - 10, y);
				}
			}
		}

		private double snap(double y) {
			return ((int) y) + .5;
		}
	}

	@Override
	public void start(Stage stage) throws Exception {
		MyCanvas canvasBlurry = new MyCanvas(false);
		MyCanvas canvasSharp = new MyCanvas(true);

		Label labelBlurry = new Label("Blurry");
		Label labelSharp = new Label("Sharp");

		VBox.setMargin(canvasBlurry, new Insets(10));
		VBox.setMargin(canvasSharp, new Insets(10));

		VBox.setMargin(labelBlurry, 
                     new Insets(10, 10, 0, 10));
		VBox.setMargin(labelSharp, 
                     new Insets(10, 10, 0, 10));

		VBox box = new VBox();
		box.getChildren().add(labelBlurry);
		box.getChildren().add(canvasBlurry);
		box.getChildren().add(labelSharp);
		box.getChildren().add(canvasSharp);

		stage.setScene(new Scene(box));
		stage.setTitle("Tip 2: Sharp Lines in Canvas");
		stage.show();
	}

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

Reference:

JavaFX Tip 2: Sharp Drawing with Canvas API from our JCG partner Dirk Lemmermann at the Pixel Perfect blog.