Rockstar Games producer Rich Rosado gave a big interview to The Gamer before the release Grand Theft Auto: The Trilogy — The Definitive Edition. He dived deep into the development process of the remasters.
"So when it comes to redoing them [games], you hit a crossroads.
You can either try and rebuild them and improve them, and in
doing so that means rescripting everything and adding new
physics. Or you acknowledge that these games had a very secret
sauce that people fell in love with, and it would be a fool's
errand to try and replicate every single element—the good bits
and the frustrating bits. So one of the first choices we made for
these remasters was to leave them intact."
GTA producer also gave an explanation on new characters' look, somewhat different from the original design.
"When you're doing these geometry updates, some are easier than
others. You can look at an arched bridge and see the low geometry
and fix it. But when you come across a character like, say,
Claude, who was built in the PS2 era and doesn't have a lot of
detail in the face, it's tricky. You're adding three times the
amount of triangles to the character, trying to put detail where
there was no detail before. Now you have a problem, because you
have to add definition to these characters that people have
gotten to know over the years and have their own mental image
of."
Developers also worked on characters with Rockstar North, including some of the original artists. Rosado compared NPCs and characters from the originals with Michelin Man. He said their bodies were separated into chunks. Updated games work on two different engines running parallel with each other. The visuals is covered by Unreal Engine 4. But the scripts and everything else is still based on old RenderWare from the original titles.
The most difficult game to update was GTA 3. The assets in these game are too old to reuse. Some of the texture were reworked by hand, but mostly the materials were updated with machine learning. There were also some big troubles with optimization. The team was working hard to make the games work properly on Nintendo Switch.