合并BufferedImages会留下黑色边框

Question

对于一个类项目，我们需要采用一个实现Sobel算法的程序，并在多个线程中运行它，以便熟悉线程。我有程序正确地运行程序（据我所知），但每当我使用多个线程实现它时，我看到一个黑色边框沿着分隔区域运行，缓冲区似乎不是写入目的地BufferedImage。

以下是负责线程化和编写文件的方法的一些代码：

public void processImage(int threads)
{
    try{
        File imgFile = new File(bmpFile);
        image = ImageIO.read(imgFile);

        w = image.getWidth();
        wThr = (image.getWidth()/threads);
        h = image.getHeight();
        inData = new int[wThr*h];
        BufferedImage[] pieces = new BufferedImage[threads];

        //instantiate array used for storing pieces of image
        for (int i=0; i<threads; i++){
            pieces[i] = new BufferedImage(wThr, h, BufferedImage.TYPE_BYTE_GRAY);
        }

        wThr = pieces[0].getWidth();
        h = pieces[0].getHeight();

        //instantiate target image
        combined = new BufferedImage(w, h, BufferedImage.TYPE_BYTE_GRAY);

        //split into threads, each one taking a division of the image and running algorithm
        Thread[] threadList = new Thread[threads];
        for (int i = 0; i < threadList.length; i++) {
            image.getRaster().getPixels((i*wThr), 0, wThr, h, inData);  
            threadList[i] = new Pr1();
            threadList[i].start();
            try{
                threadList[i].join();
            }catch (InterruptedException ie) {}

            //Write images to pieces and draw pieces individually onto target image
            pieces[i].getRaster().setPixels(0, 0, wThr, h, outData);

            Graphics2D g = combined.createGraphics();
            g.drawImage(pieces[i], i*(wThr), 0, null);
            g.dispose();

        }
            outFile = new File("1.bmp");
            ImageIO.write(combined, "BMP", outFile);
    }
    catch(IOException e)
        {
        // Handle the exception here!
        }
    }

这是使用2个线程处理时的图像：http://i.imgur.com/n5gasAW.png?1

任何人都对如何修复此问题有任何见解？我已经改变了BufferedImages和数组的各种参数，但我仍然没有运气。