Ripping an AO Out of a Normal Map

So, let's say you want to convert some legacy material to PBR.  You got a normal map, but you got nothing that looks like shadows you could exploit to make an AO map.  Well, fret no more, you can generate a quick and dirty AO from a normal map.  It's not going to be perfect, but it's going to be good enough for most purposes.

I won't get into the full nuts and bolts of how a normal map works.  But what I can tell you is the blue channel of a normal map describes how directly a pixel is looking at the the viewer, we can exploit this because pixels angled away from the camera got steep slopes which don't catch direct light as well, such as ambient light.

So open your normal map in gimp.  Click Colors->Components->Decompose

Now either hide your red and green layers so you can see your blue layer.  I prefer to right click each layer and select delete though, since red and green are effectively junk.

Once you have that done you are almost there.  The one catch with a a blue apha-channel is it cannot go lower then mid-tone grey.  So we need to use the curves tool to fix that.

Click Colors->Curves.

A floater will pop up for curves.  Now this window lets you distort the way things go from light to dark.  We are going to change the definition of what black is so that anything that is mid-tone gray or darker becomes black.

See that diagonal line accross the grid?  Click that lower-left dot and drag it to the right so it looks like this:

Now click ok.  This should get you close enough for use as an AO map for your metal-rough map.  If you are really fussy you might notice the very bottom of valleys and pits will be white in the places where the pixels are facing straight up where in a proper AO map these deep down crevices would be dark.  Using a paintbrush or pencil tool to pain in darkness manually can be done to touch this stuff up.  But if you ignore it and just roll with what you got most people won't notice.

Either way MISSION ACCOMPLISHED!