blend_modes.py 文件源码

def _compose_alpha(img_in, img_layer, opacity):
    """
    Calculate alpha composition ratio between two images.
    """

    comp_alpha = np.minimum(img_in[:, :, 3], img_layer[:, :, 3])*opacity
    new_alpha = img_in[:, :, 3] + (1.0 - img_in[:, :, 3])*comp_alpha
    np.seterr(divide='ignore', invalid='ignore')
    ratio = comp_alpha/new_alpha
    ratio[ratio == np.NAN] = 0.0
    return ratio