python图像大小调整 保持长宽比 增加背景色

from PIL import Image

def resize(image_pil, width, height):
    '''
    Resize PIL image keeping ratio and using white background.
    '''
    ratio_w = width / image_pil.width
    ratio_h = height / image_pil.height
    if ratio_w < ratio_h:
        # It must be fixed by width
        resize_width = width
        resize_height = round(ratio_w * image_pil.height)
    else:
        # Fixed by height
        resize_width = round(ratio_h * image_pil.width)
        resize_height = height
    image_resize = image_pil.resize((resize_width, resize_height), Image.ANTIALIAS)
    background = Image.new('RGBA', (width, height), (255, 255, 255, 255))
    offset = (round((width - resize_width) / 2), round((height - resize_height) / 2))
    background.paste(image_resize, offset)
    return background.convert('RGB')

https://stackoverflow.com/questions/44370469/python-image-resizing-keep-proportion-add-white-background

https://stackoverflow.com/questions/273946/how-do-i-resize-an-image-using-pil-and-maintain-its-aspect-ratio

https://www.cnblogs.com/RChen/archive/2007/03/31/pil_thumb.html