New vehicles:
GTA3 Refresh use HD cars made by OnePiece but also some new cars made by Jemoeder51 converted from other GTA Games. Some new cars: Infernus from GTA IV and the Cloverbach GT (a mix of the Hellenbach GT from GTA LCS and Clover from GTA SA). The crappy dodo plane with cut of wings is now changed to a nice looking learjet.
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Textures:
GTA3 Refresh has much better quality textures.
It used textures made by Jemoeder51 and textures from other GTA Games.
Like the new road textures are from GTA SA and the new building textures are from GTA IV
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Misc:
GTA3 Refresh has more, better and more realistic miscellaneous than the original game like realisic car reflections
GTA IV style water, real effects fixed collisions and much more.
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Buildings:
As say earlier, the game uses new high quality building textures from GTA SA and GTA IV.
But the are also brand new buildings added like the Claude mansion: a big mansion with a jacuzzi in the backyard with people in it.
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Graphics:
The mod uses a brand new timecycle to make the graphics much better and more realisic. It uses also ENB Series for bloom effect and even better car reflection.
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Sound:
GTA3 Refresh uses also new sound for guns, vehicles, background noise, water sound, walking sounds.
Most of the gun sound are from GTA IV and GTA Vice City.
The background noise is from GTA IV
The walking sound is also from GTA IV
The water sound is recorded in real life by Jemoeder51
This is a very well done mod. It makes the game much more realistic looking and its not so damn dark. Kudos on the people who worked on this. I can tell a lot of effort went into it.