你写过什么有趣的程序？

写一点程序来折腾折腾图片总是有趣的事情，至少可以做完了以后发给朋友炫耀一番。不过大多数时候得到的回应都是，你用什么P的？ 呵呵，我是写程序写的！你才P图，你们全家都P图！#莫名其妙的优越感

最有趣的是当时在读一篇讲 image blending 的论文。说白了就是先把两个图片的 Guassian Pyramid 算出来，然后再出来 Laplacian Pyraimd算出来，然后在每一层把他们 alpha blend 在一起，再全部拼起来。我知道这么说没人听得懂，废话别多说，show me the code

Reduce

采样图片（4个像素点取1个）

convolve the image with given kernel 
small = convolveResult[::2, ::2] 
return small

Expand

扩展图片（1个像素点扩4个）

initialize big as double size on both dimension. 
big[::2, ::2] = image
convolveResult = convolve big with given kernel
return convolveResult *4

Gaussian Pyramid:

创建gaussian Pyramid，其实就是一层层采样

set current to input image 
loop until depth reaches
    reduce the current
    append reduce result to pyramid 
    set current to reduce result
return pyramid

Laplacian Pyramid:

创建Laplacian Pyramid，其实就是记录每一层采样中流逝的信号，以后还能加回来

for every level of input pyramid p
    compute p[currentLevel] - expand(p[nextLevel])
    append result to pyramid
return pyramid

blend

把每一层通过alpha blend，合起来，组成新的pyramid

for every level
    get black, white and mask of current level
    computer white*mask + black * (1-mask) 
    set the result as the level of return pyramid
return pyramid

collapse

把整个pyramid还原成图片，大功告成

current = last level image
loop from the last level to the second level
    expand(current)
    current = expand result + upper level image 
return current

来看看成品吧

左边右边两张图，加上一张黑白的mask