递归尝试在两种颜色之间创建垂直线性渐变

I am trying to implement a method that involves setting a startColor and endColor for the first and last row of pixels and then settings the middle row to a colour which is the average of the RGB values of startColor and endColor. I am using recursion to continue doing this until all the rows of pixels have been coloured. This algorithm seems to work when I test it out on paper, however, when I run it, it generates an image in which some rows of pixels have been skipped throughout the image.

The image that is generated is:

起始和结束颜色分别是绿色和蓝色。

我的代码：

public MainWindow()
    {
        InitializeComponent();

        linearGradientImage(Color.Green, Color.Blue);

    }

    public static void linearGradientImage(Color startColor, Color endColor)
    {
        Bitmap gradientBitmap = new Bitmap(720, 1280);
        createGradient(gradientBitmap, 0, gradientBitmap.Height - 1, startColor, endColor);
        gradientBitmap.Save("gradient.png");
    }

    public static void createGradient(Bitmap bm, int startY, int endY, Color startColor, Color endColor)
    {
        if (endY > 1 + startY)
        {
            for (int x = 0; x < bm.Width; x++)
            {
                bm.SetPixel(x, startY, startColor);
                bm.SetPixel(x, endY, endColor);
            }

            Color midColor = Color.FromArgb
                (
                    (startColor.R + endColor.R) / 2,
                    (startColor.G + endColor.G) / 2,
                    (startColor.B + endColor.B) / 2
                );

            int midY = (endY + startY) / 2;

            if ((endY + startY) % 2 != 0)
            {
                createGradient(bm, startY, (int)(midY - 0.5), startColor, midColor);
                createGradient(bm, (int)(midY + 0.5), endY, midColor, endColor);
            }
            else
            {
                createGradient(bm, startY, midY, startColor, midColor);
                createGradient(bm, midY, endY, midColor, endColor);
            }
        }
        else
        {
            return;
        }
    }

也许，重新审视我的代码可以发现我似乎缺少的东西。任何建议表示赞赏。谢谢！