This code is handling pixels on a array with only BufferedImage.

ImageSample

package jp.yom;

import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;

public class ImageSample {
  
	static public void main( String[] args ) throws IOException {
		
		// PNG画像を読み込み
		BufferedImage	img1 = ImageIO.read( new File("ebi02.png") );
		
		int	w = img1.getWidth();
		int	h = img1.getHeight();
		
		// ピクセルを配列として取得
		int[]	px = img1.getRGB( 0,0, w, h, null, 0, w );
		
		// グレースケール化
		for( int i=0; i<px.length; i++ )
			px[i] = convGray( px[i] );
		
		// 新しいBufferedImageを作成し、ピクセルを配列でセット
		BufferedImage	result = new BufferedImage( w, h, BufferedImage.TYPE_INT_RGB );
		result.setRGB( 0,0, w, h, px, 0, w );
		
		// BufferedImageをPNGとして保存
		ImageIO.write( result, "png", new File("gray.png") );
	}
	
	static private int convGray( int argb ) {
		
		int	a = (argb>>24) & 0xff;
		int	r = (argb>>16) & 0xff;
		int	g = (argb>>8 ) & 0xff;
		int	b = (argb    ) & 0xff;
		
		int	avr = ( r + g + b ) / 3;
		int	gray = avr & 0xff;
		
		return (a<<24) | (gray<<16) | (gray<<8) | gray;
	}
	
}