放大的位图图像之间的间隙

Question

我试图通过在每个角上使用2张图像来绘制标题，然后重复另一张图像以填充两者之间的区域。这是未升级时的样子（正确）：

但是当我尝试放大图像时，底部和右侧会变得透明。我在相同的坐标上绘制了一些彩色矩形，以表明它们是正确的：

当我关闭插值时，间隙变得完全透明：

我不知道为什么会这样，特别是因为它在绘制矩形时效果很好。

源代码：

protected override void OnPaint(PaintEventArgs a_EventArgs)
{
    var gfx = a_EventArgs.Graphics;

    var left_size = new Rectangle();
    var right_size = new Rectangle();
    var middle_size = new Rectangle();

    left_size.Height = Height;
    left_size.Width = Math.Max(1, (int)Math.Floor((left_size.Height / ((double)HeaderLeftBitmap.Height)) * Resources.ItemsHeaderWhiteLeft.Width));
    left_size.X = 0;
    left_size.Y = 0;

    right_size.Height = Height;
    right_size.Width = Math.Max(1, (int)Math.Floor((right_size.Height / ((double)HeaderRightBitmap.Height)) * Resources.ItemsHeaderWhiteRight.Width));
    right_size.X = Width - left_size.Width;
    right_size.Y = 0;

    middle_size.Height = Height;
    middle_size.Width = Math.Max(1, (int)Math.Floor((middle_size.Height / ((double)HeaderMiddleBitmap.Height)) * Resources.ItemsHeaderWhiteMiddle.Width));
    middle_size.X = left_size.Width;
    middle_size.Y = 0;

    //gfx.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.NearestNeighbor;
    //gfx.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.None;

    bool toggle = false;

    for (; middle_size.X < right_size.X; middle_size.X += middle_size.Width)
    {
        gfx.FillRectangle(toggle ? Brushes.Red : Brushes.LightGreen, middle_size);
        toggle = !toggle;

        gfx.DrawImage(HeaderMiddleBitmap, middle_size);
    }

    gfx.FillRectangle(Brushes.Blue, left_size);
    gfx.FillRectangle(Brushes.Blue, right_size);

    gfx.DrawImage(HeaderLeftBitmap, left_size);
    gfx.DrawImage(HeaderRightBitmap, right_size);
}

图像（通过资源文件加载）：