An earlier post commented on the repetition that using backdrops can produce.
Another suggested backdrop splines which could produce curved corners. and I fully agree that these would look better - especially on a virtual model railway.
However, the segments of splines are by their very nature repetitions, and quite short ones would be needed to produce a curved corner.
I have been giving this some thought, and have come to the conclusion that the only answer for producing "curved" corners would be a scenery item consisting of a number of short sections each showing part of a long image, such as a forest scene, and of course with matching between adjacent sections. The sections would need to be angled to move progressively from a short section to one at right angles to the first. The number of sections needed would have to be the subject of experimentation, and the angle between each one and the next would have to be determined mathematically.
I hope this makes sense! - it does to me.
I've also realised how helpful the length of a side of a Trainz baseboard is to anyone creating a backdrop with matching sections - 720 is a factor of every number from 1 to 20 inclusive except for 7,11,13 and 14 so backdrops could be made for any of these fractions of a baseboard.
Ray