Simply tuning the model to generate a diverse range of people when a) the prompt already implies the inclusion of a person with a discernible race/ethnicity and b) there aren't historical or other contingencies in the prompt which make race/ethnicity not interchangeable, would not feel overbearing or degrading to performance. E.g. doctors/lawyers/whatever else might need some care to prevent the base model from reinforcing stereotypes. Shoehorning in race or ethnicity by rewording the user's prompt irrespective of context just feels, as I said, hamfisted.
Rome had a north african emperor. we don't have pictures of him, and 'race' is a modern invention. Ancient people didn't care about that. to be worried that a model does not reproduce white roman emperors is to be worried about its replication of popular images of roman emperors over the last 100 years or so. In this case it is not accurately replicating popular images of roman emperors, and if that's good or bad is up to you. but to say it is not accurately replicating roman emperors themselves? well, its not doing any worse.
We have pretty good clarity that Septimius Severus wasn't racially African. His parents were of Italian and Carthaginian descent. To portray a Roman emperor -- with no further specification -- as black is to intentionally misrepresent the historical record. I use the term "race or ethnicity" because this was the language Bard used when referring to its rewording of my prompt. That other cultural portrayals of emperors have likewise been inaccurate doesn't mean I should be satisfied with the same from Imagen, especially when there are competing image models which will dutifully synthesize an image of much higher correspondence to my request.
I didn't say ”imperial concept”, its an “age of imperialism” concept (though, in retrospect, “age of exploration” is when it started, it just really gained salience in the age of imperialism; though whether it was ~1300 or ~1600 years too late to apply to him isn’t a big difference.)
At least in the late Roman Republic, there was absolutely a concept of race that unified e.g. the various Gallic tribes, or differentiated the peoples of the Roman East. It's always been a sociopolitcal concept. But the Romans were aware of e.g. North Africans versus dark-skinned Africans.
