image processing - How to remove the known background from captcha in C# -

for example, there 2 pictures.

captcha:

background:

how remove background captcha in c#?

one simple method is: can iterate image's pixels. every pixel, need subtract background pixel value original corresponding pixel value. however, simple method is, not detect edges , not separate foreground background. therefore, see dots in resulted image, result of subtraction black dots of background.

image

in order achieve c#, need image processing library. if don't have one, can here details.

if not want use method, can edge detection algorithms, these quite difficult. in addition, not guaranteed work these captcha images.