Does the black color in the alpha channel images represent alpha = 0 or alpha = 1?

What algorithm is used to compute the alpha values of some object in the image, such as this koala?

As a follow up question, how do we determine the optimal alpha value for the object in the image?

Does the black color in the alpha channel images represent alpha = 0 or alpha = 1?

What algorithm is used to compute the alpha values of some object in the image, such as this koala?

As a follow up question, how do we determine the optimal alpha value for the object in the image?