def alpha_blend(rgb, intensity, alpha=0.7):
    """
    Combines an RGB image with an intensity map using "alpha" transparent blending.
    https://en.wikipedia.org/wiki/Alpha_compositing

    Parameters
    ----------
    rgb : ndarray
        An MxNx3 RGB array of floats ranging from 0 to 1 (color image).
    intensity : ndarray
        An MxNx1 array of floats ranging from 0 to 1 (grayscale image).
    alpha : float
        This controls the transparency of the rgb image. 1.0 is fully opaque
        while 0.0 is fully transparent.

    Returns
    -------
    rgb : ndarray
        An MxNx3 RGB array representing the combined images.
    """
    return alpha * rgb + (1 - alpha) * intensity
