Sobel边缘检测后，图像有白色边框

我试图找到图像的边缘 . 我的主要问题是，为什么我的边缘检测 has white border ？

这里的代码：

public static BufferedImage executeSobelEdgeDetection(BufferedImage inputImage, int threshold) 
{
    int width = inputImage.getWidth();
    int height = inputImage.getHeight();
    double red = 0.0;
    double green = 0.0;
    double blue = 0.0;
    BufferedImage borderedImage = new BufferedImage(width+2, height+2, BufferedImage.TYPE_INT_RGB);

    for(int x = 0 ; x < width; x++)
    {
        for(int y = 0 ; y < height; y++)
        {
            Color color = new Color(inputImage.getRGB(x ,y));
            red = color.getRed();
            green = color.getGreen();
            blue = color.getBlue();
            borderedImage.setRGB(x+1, y+1, new Color((int) red, (int) green, (int) blue).getRGB());
        }
    }

    BufferedImage outputImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);


    for (int x = 1; x <= width; x++) 
    {
        for (int y = 1; y <= height; y++) 
        {

            double gx = 0;
            double gy = 0;


            //Left Column Kernel
            Color color = new Color(borderedImage.getRGB(x-1 , y-1));
            red = color.getRed();
            green = color.getGreen();
            blue = color.getBlue();
            double intensity = red + green + blue;
            gx = gx + (intensity) *-1;
            gy = gy + (intensity) *-1;

            color = new Color(borderedImage.getRGB(x-1 , y));
            red = color.getRed();
            green = color.getGreen();
            blue = color.getBlue();
            intensity = red + green + blue;
            gx = gx + (intensity) *-2;
            gy = gy + (intensity) * 0;

            color = new Color(borderedImage.getRGB(x-1 , y+1));
            red = color.getRed();
            green = color.getGreen();
            blue = color.getBlue();
            intensity = red + green + blue;
            gx = gx + (intensity) *-1;
            gy = gy + (intensity) * 1;

            //Middle Column Kernel
            color = new Color(borderedImage.getRGB(x , y-1));
            red = color.getRed();
            green = color.getGreen();
            blue = color.getBlue();
            intensity = red + green + blue;
            gx = gx + (intensity) * 0;
            gy = gy + (intensity) * -2;

            color = new Color(borderedImage.getRGB(x , y));
            red = color.getRed();
            green = color.getGreen();
            blue = color.getBlue();
            intensity = red + green + blue;
            gx = gx + (intensity) * 0;
            gy = gy + (intensity) * 0;

            color = new Color(borderedImage.getRGB(x , y+1));
            red = color.getRed();
            green = color.getGreen();
            blue = color.getBlue();
            intensity = red + green + blue;
            gx = gx + (intensity) * 0;
            gy = gy + (intensity) * 2;

            //Right Column Kernel
            color = new Color(borderedImage.getRGB(x+1 , y-1));
            red = color.getRed();
            green = color.getGreen();
            blue = color.getBlue();
            intensity = red + green + blue;
            gx = gx + (intensity) * 1;
            gy = gy + (intensity) * -1;

            color = new Color(borderedImage.getRGB(x+1 , y));
            red = color.getRed();
            green = color.getGreen();
            blue = color.getBlue();
            intensity = red + green + blue;
            gx = gx + (intensity) * 2;
            gy = gy + (intensity) * 0;

            color = new Color(borderedImage.getRGB(x+1 , y+1));
            red = color.getRed();
            green = color.getGreen();
            blue = color.getBlue();
            intensity = red + green + blue;
            gx = gx + (intensity) * 1;
            gy = gy + (intensity) * 1;


            double length = Math.sqrt((Math.pow(gx, 2) + Math.pow(gy, 2)));

            length = length/4328 * 255;


            if(length <= threshold)
            {
                length = 0;
            }
            else
            {
                length = 255;
            }

            outputImage.setRGB(x-1, y-1, new Color((int) length, (int) length, (int) length).getRGB());
        }
    }

    return outputImage;
}

这是输入图像作为示例：

输出Sobel边缘检测：

如果看不到边框，可以保存图像并使用IrfanView或其他具有黑/深背景的东西打开（因为边框有白色）