Inverting a Normal Map

[arabellajones]

I did a quick coin-thing today, a prim with normal maps. Nothing all that fancy, but one of the normal maps was showing the high parts of the surface as low. Can you invert a normal map?

Yes you can. I use the GIMP

Go to the Colours menu and pick Curves. It shows this graph, a line from bottom left to top right. Leave the Blue channel alone, but change Red and Green, line from top left to bottom right. You just have to drag the end markers.

OK, maybe some normal maps are wrong for SL in other ways. In this case I got this result.

There are various ways to get from a height map to a normal map, plug-ins or websites. These two images may help you figure out if the generated Normal map is right for SL. You could just use whatever height option your creation tool has. But that's not an choice if you download a freebie.

If you use Photoshop there's almost certainly a way to doing the same. There may be another way of doing it in the GIMP. But this makes it pretty clear what you need to do.

[ChinRey]

Nice.

I made a FilterForge Filter that does the job a few years ago btw, so if you have FF, you can download it here.

It's usually only the y "swizzle coordinates" (the green channel) that needs to be inverted. More info about it here.

 

[Quarrel Kukulcan]

5 hours ago, arabellajones said:

OK, maybe some normal maps are wrong for SL in other ways. In this case I got this result.

Shouldn't you have reversed the green only? Normal maps in SL should look like they're lit with green from the top and red from the right.

[arabellajones]

I'll give the green-only a try... DirectX and OpenGL, eh. What a surprise.

[Quarrel Kukulcan]

You definitely want "up" slopes (away from the surface and toward the camera) to be green on top and red on the right. Look at this experiment. The sun is directly overhead.

The top right sphere (relative to the ^ arrow) looks properly raised. The bottom left is properly sunk. The other two have the highlight 90° off from a realistic direction.