import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.image.WritableImage;
import javafx.scene.image.WritablePixelFormat;
import javafx.stage.Stage;

public class CanvasErrors extends Application {
  public static void main(String[] args) throws Exception { launch(args); }
  public void start(final Stage stage) throws Exception {
    // create a simple surface wrapper around a canvas.
    final Canvas canvas = new Canvas(1800, 600);
    final Surface surface = new Surface(canvas);

    // layout the scene.
    Scene scene = new Scene(new Group(canvas));
    stage.setResizable(false);
    stage.setScene(scene);
    stage.show();

    // update the surface every pulse.
    AnimationTimer timer = new AnimationTimer() {
      int t = 0;
      @Override public void handle(long now) {
        t++;
        System.out.print((t % 60 == 0) ? "." : "");
        surface.clear(0x27, 0x2b, 0x38);
        surface.update();
      }
    };
    timer.start();
  }

  class Surface {
    private int width;
    private int height;
    private GraphicsContext context;
    private byte[] buffer;
    private WritablePixelFormat format = WritablePixelFormat.getByteBgraPreInstance();
    private WritableImage imageBuffer;

    Surface(Canvas canvas) {
      width  = (int) canvas.getWidth();
      height = (int) canvas.getHeight();
      context = canvas.getGraphicsContext2D();
      buffer = new byte[width * 4 * height];
      imageBuffer = new WritableImage(width, height);
    }

    void clear(int red, int green, int blue) {
      int idx = 0;
      for (int y = 0; y < height; y++) {
        for(int x = 0; x < width; x++) {
          buffer[idx++] = (byte) blue;
          buffer[idx++] = (byte) green;
          buffer[idx++] = (byte) red;
          buffer[idx++] = (byte) 0xFF;
        }
      }
    }

    void update() { // should work but you get java.lang.NullPointerException at com.sun.prism.impl.BaseGraphics.drawTextureVO(BaseGraphics.java:418) when using -server VM.
      context.getPixelWriter().setPixels(0, 0, width, height, format, buffer, 0, width * 4);  
    }

//    void update() {  // workaround - this works for some reason I don't really understand.
//    imageBuffer.getPixelWriter().setPixels(0, 0, width, height, format, buffer, 0, width * 4);
//    context.drawImage(imageBuffer, 0, 0);
//    }

  }
}