About the contextual. Yes and no. A photo with a dog with a toy, as exampe, ok.
Digital editing: "before sharpening" "after sharpening".
Toy review: "picture of the toy in usage".
Dog's coat: "my cute doggy's coat"
Dog meme: "adorable dog playing with a toy"
All these are valid alt-texts. Things like describing the coat pattern belongs in the text or image caption, not in the alt-text.
Opinions about this will differ. But one should not hide the information in the alt-text. And the context shifts with purpose, but it does not mean you should transcribe different parts of the picture. You should not transcribe it to begin with. How would that even read? Oh, look, here my dog looks sharper as in the previous image? That is not alt-text, that is caption.