Those are always edge cases. I would be hesitant to provide a simple rule on how to deal with these kinds of answers, although one thing should be clear: **"You cannot do it" *can* be a valid answer.**


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However in this particular instance it's clearly this:

> It doesn't solve the real problem. If OP asks "Is <XYZ> possible", he really means "How do I do <XYZ>".

In general, it doesn't make a lot of sense to ask whether something is possible. Asking *how to achieve it* is always preferred, and if the answer is, "it's not possible", then so be it. Clearly, in this case, the OP needs to do something very specific – a workflow in GIMP – and just telling them you can't do it is

- not particularly helpful
- maybe not even true (who knows, maybe it's possible but the answerer hasn't found a way to)


So, this question here can't be compared with others where you can actually *prove* that something cannot be done. Unless you can technically argue that it's not possible – and will never be – then that's a full answer to the question. I could come up with plenty of examples where this would be acceptable: Imagine asking how you can write data to a CD-ROM, or how you can make a low quality compressed movie high quality. It's just not feasible.