I got interested in Blitz3D because one of my favorite games, Stevie G's "Z-Virus" (a port of an Acorn Archimedes game called "Zarch") comes with .BB source code. I only just discovered in late 2021 that I could download Blitz3D for Windows, and have been having a lot of fun with it, including recompiling from scratch. (A big piece of my motivation is that there's a "v2" of Z-Virus, but the precompiled binary has always set off my antivirus software, so I couldn't run it until after recompilation from source. That generated a smaller (!) binary that does not set off antivirus, by the way.)
All of that was on 32-bit Windows XP on a 2008-vintage HP Pavilion laptop. Worked great. Gameplay has you flying a little ship over a gently-rolling ground, textured with colors indicating degree of viral infection, over which the author, through clever programming, managed to overlay black shadows of your own and enemy ships. On 64-bit WIndows 10 on a completely-different, newer, HP Laptop, the game runs, and can be played -- but the "ground," instead of being textured, is rendered in solid black except for a thin border of the expected texture colors, around its perimeter.
My gut feeling is that whatever tricks the author is playing to get realistic shadows in Blitz3D, they simply don't produce the same visual effects in the new environment as they "always did before." I have the newest graphics drivers (apparently much newer than can even be had from HP's own website), and Windows did prompt me to install DirectPlay -- and I did it -- when I first ran the game.
So: (1) something's different, and (2) anybody have any ideas of what/why, and any ideas for fixing it?