Is this the correct Forum to ask OpenCV questions in relation to a Raspberry Pi Based Robotics project? I asked one previously but a moderator moved it to Stack Overflow as they did not believe it was Raspberry Pi related?
I'll discuss this issue in the abstract a bit before focussing on openCV questions in particular. Keep in mind that closing and migrating questions is normal and not a bad thing. No one needs to feel guilty about it, either for asking a question or for voting it off-topic.
I've addressed that here before by saying in my opinion (and practice as a moderator for the past 4 years), this means the community can deal with gray area questions at their discretion, either in terms of providing an answer or voting to close. Moderators, who are elected to help maintain the interests of the community, can close questions without having to collect four more votes. We're also the only ones that can directly migrate questions to another site, which is obviously significant WRT the gray area questions. I rarely migrate questions but often close them as off-topic and recommend somewhere more appropriate.
Because this is a discretionary activity, there is bound to be some inconsistency in the result. In my opinion, that is not a bad thing either, as it simply reflects the real and inevitably inconsistent forces at play (= who does what when).
To me "the interests of the community" (including that at large, i.e., the Stack Exchange network as a whole) include two potentially contending considerations:
Where is the OP ("original poster" of a question) going to be best served? I think a lot of people believe that the point of closing a question is to save us the bother, etc., even if this means foisting it off on the OP and/or some other site. That's not completely false, but more importantly, it is about indicating that you will be better served elsewhere. Being smaller, we can't provide the coverage that Stack Overflow in particular provides. Questions that might sit here unanswered for days can get a collection of answers there in minutes (a bit of an idealized case, but also one that could easily be real).
Giving people here the opportunity to provide answers that will enhance the site generally. That last clause is important, because what this does not mean is giving people the opportunity to provide poor answers that would be quickly shot down on a larger site. We absolutely do have some great regulars here who have written many outstanding answers that probably would not have been written elsewhere, simply because they represent an exceptional effort in which luck I think plays an important role -- as in, this question was lucky to end up in the right place at the right time (= who does what when). Unfortunately though, you cannot count on such luck to win out statistically against ignored or badly dealt with questions simply because the demand for good answers is greater than the potential supply.
That said, I hope it is clear why this is a policy and why it is important to enforce. It might be one of our most important policies. I do see this site as a place many people are introduced to the Stack Exchange network, and migrating questions or closing them and recommending another site is part of the dynamic nature of that network, something I do not think has any close parallel in the world.
So, openCV is obviously not specific to the Raspberry Pi. Stack Overflow has almost twice as many questions tagged 'opencv' as we have in total. I (alas) still have not gotten around to using openCV, so it is not crystal clear to me the extent to which it is possible to have a question which really is pi specific (meaning I am pretty sure it is, I am just not sure about the specifics), but here are a couple of things to think about:
If you can remove all the references to the brand of hardware and specific OS distro without changing the meaning of the question much, you are probably better off asking on Stack Overflow.
If you ask on Stack Overflow first and get information back that doesn't jibe with what's happening for you, perhaps it is a Pi specific issue.
If you ask on either place and don't get any feedback within a day or so, feel free to delete it and post it elsewhere, but please don't cross post simultaneously.
WRT #1, it is possible to be uncertain about whether the hardware and even more so the OS (especially for newcomers to linux) is that relevant. When in doubt, feel free to ask, but make this issue explicit, i.e., say you are not sure about the relevance, and don't take offense when instead of an answer you get someone explaining why the Pi is not a critical element of the question and where you will likely be better served.