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Okay, to make awesome normal maps for your custom models you will need to have 2 3d meshes. it doesn't need a texture, it can just be a plain model.
1. Create your low polygon game model. then save it like mymodel_lowpoly.obj (it has to be obj for the tool to read the model)
2. Sculpt your model until its mid to high polygon. then save it as something like mymodel_Highpoly.obj
3. Download the tool Nvidia Melody. Its free, so don't worry. If you read the homepage you'll see why you need it to create you normal map. For people that don't like to read the official page:
use the tool to create your normal map. You model looks like it's high poly but will render extremely fast because it's low poly. enjoyNVIDIA Melody creates high quality normal maps that make a low-poly model look like a high-poly model. Simply load your low-poly working model, then load your high-poly reference model, click the "Generate Normal Map" button and watch Melody go to town.
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Edited: Forgot to add some fundamental information , its now there
edit again: Once you have your normal map you can use it on an animated model of another format. you just have to create another material in your 3d modeling software and apply the map. to your diffuse mapped (that's fancy lingo for textures) and animated model. then it's ready for your game
Nuther edit: Formatted the post for easier reading and added a recommended content creation pipeline that more simply states what was said at the top of the post..
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The reason I posted this:
looking at a lot of peoples projects and being an artist myself I have come to the conclusion that most of the projects have "programmer art" and having an easy way to make some decent normal maps would greatly improve the quality of ALL our games. SO I have brought to your attention the free easy tool Nividia Melody. If you can make a halway decent model in both low and high polygon, you can crate a good normal map with this and make your games better
NOW GO MAKE HALF-LIFE 3
respond and say how much this helped you.
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This is a recommended content creation pipeline for your games when using Melody for model making:
Make your low poly models
Use your low poly models to make high poly models.
Import your models into melody to create your normal maps. The models must be in either 3ds format or obj format for melody to read your models, but the 3ds format's polygons are limited for some reason, so use .obj
Texture and animate your models.
Put them into your game.
Done