Making of Varga by Paul Tosca
Below is a comparison between the two methods , obviously the combination overlay/multiply is the way to go.
If the vectors stored in the normal map are normalized then the red and green channel will suffice because the normal component can be computed from the tangent , bitangent and the calculation should be done when displaying , if this is the case then simple overlay will work fine because the info in the blue channel won't be used but most apps will use the blue channel also so you should go for overlay/multiply combination.
Of course for the example above you could just make a selection and overlay them with normal mode but most of the time you will have details in both maps that need to be overlaid.
Now that you know how to combine normal maps why would you want to do that?Well , i will give you some examples.
After you will finish the diffuse textures you can create a bump map with fine detail ( pores in skin and other granularities like leather , cloth ... etc ), then use nvidia filters to create a normal map that will be overlaid on top of the normal map computed from geometry ( this way the fine detail from diffuse and normal will register correctly )
It can help you in other situations as well:
When i sculpted the torso for Varga i forgot to sculpt the scar of the missing breast and later on when i realized that i had some weird problems with the scenes and could not load them so i just sculpted a scar on a new plane and overlaid it on top
Also when i was sculpting Umbra i have gone too far with the number of polygons and later on i realized i was too timid with the musculature ... but it was too late ... i have already broken the model into pieces ...
To remedy the situation i started sculpting the muscles on a new plane using as a guide a temporary normal map baked from an earlier version , then computed a normal map between this geo and a quad plane and overlaid it on top of the old normal map to reinforce the muscles.
A final image with Umbra
