Fast DrawLine() in Silverlight

public static void DrawLineFast(this WriteableBitmap bmp, int x1, int y1, int x2, int y2, int color)
{
    // Use refs for faster access (really important!) speeds up a lot! 

    int w = bmp.PixelWidth;
    int[] pixels = bmp.Pixels;

    // Distance start and end point

    int dx = x2 - x1;
    int dy = y2 - y1;

    const int PRECISION_SHIFT = 8;
    const int PRECISION_VALUE = 1 << PRECISION_SHIFT;

    // Determine slope (absoulte value)

    int lenX, lenY;
    int incy1;
    if (dy >= 0)
    {
        incy1 = PRECISION_VALUE;
        lenY = dy;
    }
    else
    {
        incy1 = -PRECISION_VALUE;
        lenY = -dy;
    }

    int incx1;
    if (dx >= 0)
    {
        incx1 = 1;
        lenX = dx;
    }
    else
    {
        incx1 = -1;
        lenX = -dx;
    }

    if (lenX > lenY)
    { // x increases by +/- 1

        // Init steps and start

        int incy = (dy << PRECISION_SHIFT) / lenX;
        int y = y1 << PRECISION_SHIFT;

        // Walk the line!

        for (int i = 0; i < lenX; i++)
        {
            pixels[(y >> PRECISION_SHIFT) * w + x1] = color;
            x1 += incx1;
            y += incy;
        }
    }
    else
    { // since y increases by +/-1, we can safely add (*h) before the for() loop, since there is no fractional value for y
        // Prevent divison by zero

        if (lenY == 0)
        {
            return;
        }

        // Init steps and start

        int incx = (dx << PRECISION_SHIFT) / lenY;
        int index = (x1 + y1 * w) << PRECISION_SHIFT;

        // Walk the line!

        int inc = incy1 * w + incx;
        for (int i = 0; i < lenY; i++)
        {
            pixels[index >> PRECISION_SHIFT] = color;
            index += inc;
        }
    }
}