goodcharacters 动图替换背景
背景目标
goodcharacters 这个网站提供了文字书写顺序的动图，但动图上有些水印标识，处于文字下方的背景图，看着总是怪别扭的，所以尝试着把背景给换了。
简单展示
原图是这个样子的：
处理之后是这个样子的：
原理分析
gif 图片是由一帧一帧的图片组成的动画，所以我们需要对 gif 进行一帧一帧的拆解，把每一帧的背景图去掉之后再加上新的背景图，然后再重新组装成一张 gif 图片。
零、安装依赖
整个处理的过程，我们需要用到 Python 的图片处理库：Pillow，使用 pip 直接安装最新版 7.0.0。
pip install Pillow
一、获取背景图
比较幸运的是 gif 的第一帧就提供了一张完整的背景图，这样就不需要手动去拼凑一张背景图，直接从 gif 中提取出第一帧就可以了。
二、拆解 gif
按照 Pillow 的官方文档将 gif 图片先拆成 png 看一下
input_image = Image.open(input_name)
for index in range(0, input_image.n_frames):
input_image.seek(index)
frame = input_image.convert("RGBA")
frame.save("./imgs/frame%d.png" % index)
WTF! 咋回事？笔画的颜色不对，还多了很多方块；Pillow 对 gif 的读取存在 bug，需要先打个补丁。
from PIL import Image, ImageFile
from PIL.GifImagePlugin import GifImageFile, _accept, _save, _save_all
# workaround initialization code
class AnimatedGifImageFile(GifImageFile):
def load_end(self):
ImageFile.ImageFile.load_end(self)
Image.register_open(AnimatedGifImageFile.format, AnimatedGifImageFile, _accept)
Image.register_save(AnimatedGifImageFile.format, _save)
Image.register_save_all(AnimatedGifImageFile.format, _save_all)
Image.register_extension(AnimatedGifImageFile.format, ".gif")
Image.register_mime(AnimatedGifImageFile.format, "image/gif")
# end of workaround initialization code
prev_frame = None
for index in range(0, input_image.n_frames):
input_image.seek(index)
frame = input_image.convert("RGBA")
if prev_frame and input_image.disposal_method == 1:
updated = frame.crop(input_image.dispose_extent)
prev_frame.paste(updated, input_image.dispose_extent, updated)
frame = prev_frame
else:
prev_frame = frame.copy()
frame.save("./imgs/frame%d.png" % index)
这个时候再看一下，png 图片都正确了。
三、抹去背景图
经过第二步的处理，我们现在已经有了每一帧的图片，以及它的背景图片，接下来就是要删除每一帧图片中的背景图片。
+
得到
对于颜色叠加的算法没有了解过的可以详细的看下这篇文章，iconfinder 图标去水印；这里拿已经得到的公式做一下推导。
$$\alpha_t = 1 - (1 - \alpha_o)(1 - \alpha_w)$$ $$\alpha_w = \frac{\alpha_t - \alpha_o}{1 - \alpha_o}$$
$$R_t\alpha_t = R_o\alpha_o(1 - \alpha_w) + R_w\alpha_w$$
问题点：
- 如果背景底色是完全不透明的，是没有办法求出字的不透明度的（字有可能是不透明的颜色覆盖到背景上去的，也有可能是半透明的颜色和背景的颜色叠加起来的）
基于问题1，公式中就出现了2个变量，现在不透明度是求不出来了，是不是可以根据公式2来求出字的不透明度，好在这个例子中字的颜色是固定的，拿 Photoshop 给文字取色
$$\alpha_w = \frac{R_o\alpha_o - R_t\alpha_t}{R_o\alpha_o - R_w}$$
这样对图上的像素点进行遍历，然后求出没有背景图的字图
def calc_pixel(input_rgba, background_rgba):
if background_rgba == input_rgba:
return 0, 0, 0, 0
background_red, background_green, background_blue, background_alpha = background_rgba
input_red, input_green, input_blue, input_alpha = input_rgba
output_alpha = None
if background_alpha < 255:
output_alpha = 255 * (input_alpha - background_alpha) / (255 - background_alpha)
if output_alpha == 0:
return 0, 0, 0, 0
if output_alpha is None:
output_red = 4
output_green = 2
output_blue = 4
output_alpha = 255 * (background_red * background_alpha - input_red * input_alpha) / (background_red * background_alpha - output_red * 255)
return output_red, output_green, output_blue, round(output_alpha)
output_red = (input_red * input_alpha - 255 * background_red * background_alpha) / output_alpha + background_red * background_alpha
output_green = (input_green * input_alpha - 255 * background_green * background_alpha) / output_alpha + background_green * background_alpha
output_blue = (input_blue * input_alpha - 255 * background_blue * background_alpha) / output_alpha + background_blue * background_alpha
return round(output_red), round(output_green), round(output_blue), round(output_alpha)
四、添加新的背景图
在添加新的背景图之前，我们用Photoshop 做两个新的背景图出来，作为演示
用 Pillow 可以很方便实现背景图和文字图的合并
new_background_image = Image.open(new_background_name)
new_frame = new_background_image.copy()
new_frame = Image.composite(output_frame, new_frame, output_frame)
五、重新组合成 gif
output_image = output_frames[0]
output_image.save(output_name, save_all=True, optimize=True, append_images=output_frames[1:], duration=frames_duration, loop=0)
最终代码
from PIL import Image, ImageFile
from PIL.GifImagePlugin import GifImageFile, _accept, _save, _save_all
import argparse
import os
# workaround initialization code
class AnimatedGifImageFile(GifImageFile):
def load_end(self):
ImageFile.ImageFile.load_end(self)
Image.register_open(AnimatedGifImageFile.format, AnimatedGifImageFile, _accept)
Image.register_save(AnimatedGifImageFile.format, _save)
Image.register_save_all(AnimatedGifImageFile.format, _save_all)
Image.register_extension(AnimatedGifImageFile.format, ".gif")
Image.register_mime(AnimatedGifImageFile.format, "image/gif")
# end of workaround initialization code
def calc_pixel(input_rgba, background_rgba):
if background_rgba == input_rgba:
return 0, 0, 0, 0
background_red, background_green, background_blue, background_alpha = background_rgba
input_red, input_green, input_blue, input_alpha = input_rgba
output_alpha = None
if background_alpha < 255:
output_alpha = 255 * (input_alpha - background_alpha) / (255 - background_alpha)
if output_alpha == 0:
return 0, 0, 0, 0
if output_alpha is None:
output_red = 4
output_green = 2
output_blue = 4
output_alpha = 255 * (background_red * background_alpha - input_red * input_alpha) / (background_red * background_alpha - output_red * 255)
return output_red, output_green, output_blue, round(output_alpha)
output_red = (input_red * input_alpha - 255 * background_red * background_alpha) / output_alpha + background_red * background_alpha
output_green = (input_green * input_alpha - 255 * background_green * background_alpha) / output_alpha + background_green * background_alpha
output_blue = (input_blue * input_alpha - 255 * background_blue * background_alpha) / output_alpha + background_blue * background_alpha
return round(output_red), round(output_green), round(output_blue), round(output_alpha)
def attach_background(output_frame, new_background_image):
new_frame = new_background_image.copy()
new_frame = Image.composite(output_frame, new_frame, output_frame)
new_frame = new_frame.convert('P')
return new_frame
def replace_background(input_name, background_name, new_background_name, output_name):
if not os.path.exists(input_name):
print("图片不存在")
return
input_image = Image.open(input_name)
background_image = Image.open(background_name)
new_background_image = Image.open(new_background_name)
prev_frame = None
output_frames = []
frames_duration = []
for index in range(0, input_image.n_frames):
input_image.seek(index)
frame_info = input_image.info
frame = input_image.convert("RGBA")
if prev_frame and input_image.disposal_method == 1:
updated = frame.crop(input_image.dispose_extent)
prev_frame.paste(updated, input_image.dispose_extent, updated)
frame = prev_frame
else:
prev_frame = frame.copy()
output_frame = Image.new(frame.mode, frame.size)
w, h = frame.size
for x in range(w):
for y in range(h):
background_rgba = background_image.getpixel((x, y))
input_rgba = frame.getpixel((x, y))
output_rgba = calc_pixel(input_rgba, background_rgba)
output_frame.putpixel((x, y), output_rgba)
new_frame = attach_background(output_frame, new_background_image)
frames_duration.append(frame_info["duration"])
output_frames.append(new_frame)
output_image = output_frames[0]
output_image.save(output_name, save_all=True, optimize=True, append_images=output_frames[1:], duration=frames_duration, loop=0)
def output_path_or_default(input_path, output_path):
if output_path:
return output_path
root, ext = os.path.splitext(input_path)
return root + ".clean" + ext
if __name__ == "__main__":
parser = argparse.ArgumentParser(description = "watermark remover for iconfiner")
parser.add_argument("--output", dest = "output", help = "output image")
parser.add_argument("input", help = "input image")
args = parser.parse_args()
output = output_path_or_default(args.input, args.output)
replace_background(args.input, "background.png", "background_red.png", output)
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