I want to make a low-poly building with a reflectance to the windows only. When I put a reflection cubemap on it it goes on the whole thing, the concrete and everything. How could I make this without making separate polys for the windows with a separate texture?
Use built in reflections shaders, they take reflection mask from alpha channel of the diffuse texture.
How exactly do I make an alpha channel? Transparency?
Use an image editor, like GIMP to alter the Alpha channel seperately. this way, you can use it as an independent piece of data. I usually save these images as RGBA-png’s myself.