From the Visual Cryptography by Shamir and Noar.
Let us take the pixel expansion as 2. Therefore, each pixel will be represented by 2 pixels (also called as subpixels) in a share. Since this is a (2,2) scheme, we will have 2 shares. In order to create share1 for a white pixel, a fair coin is tossed with 2 possibilities of subpixels - Black&White or White&Black - either one of these 2 choices is chosen to represent a white pixel. It is clear that Share1 is random.
In order to create share2 for a white pixel, is a coin tossed again? If yes, then how is reconstruction of a white pixel assured? If no, then how is share2 random? Assuming that in the reconstructed image, the white pixel will be 50% black and 50% white.