python生成指定尺寸的缩略图，先对图片进行缩小，然后进行裁剪，直到生成规定的尺寸

def MakeThumb(path, sizes=(75, 32, 16)):
    """
    缩略图生成程序 by Neil Chen
    sizes 参数传递要生成的尺寸，可以生成多种尺寸
    """
    base, ext = os.path.splitext(path)
    try:
        im = Image.open(path)
    except IOError:
        return
    mode = im.mode
    if mode not in ('L', 'RGB'):
        if mode == 'RGBA':
            # 透明图片需要加白色底
            alpha = im.split()[3]
            bgmask = alpha.point(lambda x: 255-x)
            im = im.convert('RGB')
            # paste(color, box, mask)
            im.paste((255,255,255), None, bgmask)
        else:
            im = im.convert('RGB')
 
    width, height = im.size
    if width == height:
        region = im
    else:
        if width > height:
            delta = (width - height)/2
            box = (delta, 0, delta+height, height)
        else:
            delta = (height - width)/2
            box = (0, delta, width, delta+width)
        region = im.crop(box)
 
    for size in sizes:
        filename = base + "_" + "%sx%s" % (str(size), str(size)) + ".jpg"
        thumb = region.resize((size,size), Image.ANTIALIAS)
        thumb.save(filename, quality=100) # 默认 JPEG 保存质量是 75, 不太清楚。可选值(0~100)