Hi Mathias !
Orthophotos under overhangs are pretty common problem in aerial photogrammetry.
As you said, to be able to get the most accurate orthophotos of the masked floor, you will firstly need the classify the ground points to be able to get a DTM (digital terrain model) instead of the DEM (digital elevation model).
Then, you will have to compute the orthomosaic. But, as PhotoScan create it using pieces of images which have been taken from the best place (i.e. the most vertically above a point), it will probably project the roofs on the floor...
Two solutions to correct that:
- As you said, previously the orthomosaic generation, by masking on pictures that have been mostly taken at the vertical above the roof. It may take a time and it may be difficult to copy a mask from a picture to another one (as the UAV moved) and also difficult to use the magic wand we can use to mask the sky on terrestrial imagery (as the roof isn't a plain color)...
- Subsequently the orthomosaic generation, by editing the produced orthophoto using polygon shapes and assigned images.
Hope it helps !
Regards