get rgb from a canvas?

Java 2D
#1

hi people, im doing a game in java, and i want to get a pixel value by determinate X,Y coordinates , from a canvas , i tried to convert the canvas to bufferedimage, but the result bufferedimage is all black :S


  //Getting the Pixel value
  int pixel=canvasToImage(this).getRGB(x,y);

//Method to convert the canvas to BufferedImage
 private BufferedImage canvasToImage(Canvas canvas) {
     int w = canvas.getWidth();
     int h = canvas.getHeight();
     int type = BufferedImage.TYPE_INT_RGB;
    
     BufferedImage image = new BufferedImage(w,h,type);
    
     Graphics2D g2 = image.createGraphics();
     canvas.paint(g2);
     g2.dispose(); 
      
     
     return image;
 }

what can i do?

#2

How are you painting the image on the canvas in the first place?

#3

i have a class that extends from Canvas, and i overrides the paint method to draw the images, the paint method used the Graphics from a BufferStrategy, i have an arraylist that store somes classes which content the BufferedImage and the X , Y information for their position,

sorry my english, spanish speaking , i hope that you can understand me :smiley:

#4

A Java Canvas is just a place on the screen to display graphics, but it never remembers those graphics, so for example, if you painted something onto it, and then drag another window over it, it would be wiped clean until the next time you paint onto it (either by its overridden paintComponent, or by the BufferStrategy). The canvas can never give you the graphics that are displayed on it.

I don’t think you are overriding the paint method as you say you are. that’s not how a bufferstrategy works.

Because you are using a BufferStrategy insted of overriding the canvas paintComponent() method, if you want to get what your canvas is displaying into an image, you have to do the same rendering to the image graphics as you do to the BufferStragegy graphics.

In this example, where a simple rendering loop is used:



private void renderLoop() {
	while(running) {
			do {
				do {
					Graphics2D g = (Graphics2D)bufferStrategy.getDrawGraphics();
					g.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
					g.setColor(Color.white);
					g.fillRect(0, 0, canvasWidth, canvasHeight); 
					g.drawImage(image1, 0, 0, null);
					g.drawImage(image2, 30, 50, null);
					g.dispose(); 
				} while (bufferStrategy.contentsRestored());
				bufferStrategy.show();
			} while(bufferStrategy.contentsLost());
	}
}

If you want to be able to render it onto an image whenever you like, you could replace it with:



private void renderLoop() {
	while(running) {
			do {
				do {
					Graphics2D g = (Graphics2D)bufferStrategy.getDrawGraphics();
					renderSingleFrame(g);
					g.dispose(); 
				} while (bufferStrategy.contentsRestored());
				bufferStrategy.show();
			} while(bufferStrategy.contentsLost());
	}
}

private void takeScreenShot(BufferedImage into) {
	Graphics2D g = (Graphics2D)into.getGraphics();
	renderSingleFrame(g);
	g.dispose();
}
private void renderSingleFrame(Graphics2D g) {
	g.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
	g.setColor(Color.white);
	g.fillRect(0, 0, canvasWidth, canvasHeight); 
	g.drawImage(image1, 0, 0, null);
	g.drawImage(image2, 30, 50, null);
}
#5

I’m not saying this is a good solution … But you can use the Robot class to create a screen capture of the canvas (by supplying its bounds). You’ll get a BufferedImage this way.