In creating a model from JPeg photos, I went through the trouble of editing out all of the parts of the photos that were not part of the model (leaving Alpha=0 "transparent") and stored the edited photos to PNG files. In my mind this should have told PhotoScan to treat the transparent parts of the photos as a sort of mask to make the solver ignore those areas and consider them to not contribute to the solve of the model. But the solve created a lot of snow-like looking white stuff where there should have been no geometry. My photos had no white yet a lot of "white stuff" ended up in my model. I can only presume that the solver was ignoring the Alpha in the PNG files and was placing a default white background behind the photos and then interpreted it as part of the substance of the model. This should not have happened.