Why does the app use Direct2D?

Initially, the app used wxDC, wxImage, and wxBitmap for all 2D drawing operations; I replaced these with Direct2D:

for full alpha channel support
as a learning experience (honestly, I expected to revert my changes without committing)
with the expectation that there would not be a significant performance difference

That expectation was wrong: it removed many multi-second freezes when handling large amount of image data. The primary reason seems to be:

wxBitmap supports native pixel formats, but supports very few operations.
wxImage supports common operations (e.g. scaling), but uses RGB, with the alpha channel stored separately.
so, every conversion between the two required iterating over every pixel, on the CPU.

Moving to Direct2D: