Alpha channel on normal map isn't working

[CaptainExtremis]

Hey. I've been trying to make a model look metallic for the past day but I've just kept hitting a wall. As I understand it,the normal map's alpha channel is used to determine what areas of the model have a metallic sheen/phong, so I've tried baking the normals in Blender but the alpha channel always comes out white. I'm fairly certain this is because I'm baking a procedural texture instead of a hi-poly to lo-poly mesh. How would I add an alpha channel post-bake?

[The Renderer]

I don't know where you got the idea that the alpha channel of a normal determines the metallicness, maybe that's the case for certain engines or games, but it is not the norm, and definitely not how it works in Blender.

A normal map determines, well, the normals, i.e. the shape of the surface. They usually don't even have an alpha channel since it serves no purpose (hence it's all "white" if you force Blender to create one).

You can try to bake the metallicness to a separate black and white texture and then add this as an alpha channel to the normal texture using any 2D software (like Gimp), if that's what you need for the game/engine.

[CaptainExtremis]

Ah, I should have specified that. I'm working on a model for the Source engine, and Source uses the normal map's alpha channel to determine shininess/metallic/phong (usually). I assumed I'd have to add the alpha in post, but I am not smart enough to figure out how without cocking it up.

[The Renderer]

As I said, just bake the texture in Blender like any other and attach it to the normal map later, it's not difficult. Just make sure you use the same UV map to generate them, so they match.

[CaptainExtremis]

Will do. Thanks for the insight!