My speculation is that sprites can be generated rather painlessly from good 3D Blender models either to 2D or 3D. I am skeptical to sprites generated other ways because I like the ability to do changes later and the indifference between 2D and 3D, now changing a massive amount of PNG files by hand does not sound very smart (or better sounds very time-consuming in comparison to Blender-generated sprites). You can find here a Sonic in Blender-generated animation, now a 3D-game-assimilator needs to manage the camera (adjust positions, make it for shoots, etc).
Now I warmed up this q with the discussion here with helper questions below, to reveal my amateurship around this area.
1. is there some Dimension requirements for sprites? 2. alpha-channeled? 3. is there any specification for sprites? 4. what kind of borders should sprites have? Any margin between pics? 5. Which colors to avoid in sprites? 6. Is there some open-source tools tailored for sprites-generation? 7. am I good to go with brute-force with Gimp/InkScape/Blender/Imagemagick -combo?
So how are sprites really generated? With Blender in combinations with some *ix tools? By which tools if not with them? How do you maintain/process the sprites over time?