Bump normals flipped on certain faces

[evanerichards]

Ok. So I've got a really head scratcher of a problem here. This isn't a vray problem I don't think, but I was hoping someone could help me identify the problem.

I'm on maya 2016 using vray 3.4

I have a building model. It has diffuse maps. I used those to generate normal maps. So all the normal maps should be accurate and correct.
The model itself has all of its surface normals pointing out. They are all correct as far as I can see. When I look at the normals pass they all look correct. When I look at a AO render there is nothing black (another indication that the normals are not flipped).

However, when I render out the building, certain faces have the bump normals flipped. Where brick morter is usually indented, now it is popping out. How is this possible? It isn't the normal maps I think since they were all generated from the diffuse textures and surface normal direction shouldn't even enter into it.

None of the UV shells are flipped either. They are all blue in mayas UV editor however when I flip them so that they are backwards, it seems to fix the problem. However, this isn't really a viable solution since when I flip a non-symetrically shaped UV shell it doesn't match the textures anymore.

Any thoughts on what this is or how to fix this?

[yolov]

Can you share an example?

[evanerichards]

Give me a couple minutes and I'll post some screenshots.

[Morbid Angel]

This might happen if the geometry has negative scale. See if that is the case, or post problematic piece.

[evanerichards]

Well, if the geometry had a negative scale wouldn't that effect the whole model? And not just certain faces?

[Morbid Angel]

This is correct, but you never know with maya

I could also suggest freeze all transforms on it and unlock/set normal angle

[evanerichards]

model.zip

Attach the image into the shader bump/normals slot.

What's even happening here?

[Morbid Angel]

switch the color space on normal map file loader to raw.

[yolov]

This might be the possible reason, yes. With 2016 and later maya versions we support the file node color management but the file node itself in no way makes a difference if there is a normal map plugged in or a diffuse texture. I think there is a way to improve that though.

[Morbid Angel]

I just thought to plug in a solid color with same values as blank normal map, 240,0.5,1 and got proper result. This suggested that the way the map was being read was wrong. I think normal map does not need to be gamma corrected in any way, and setting color profile to sRgb offset that value.

[evanerichards]

Wow. This is amazing. Thanks Morbid and yolov.

I'm still not sure why this works, seems kind of like a magic trick. I feel like when I apply a solid color neutral normal map, I understand that the sRGB color space throws it off, but it seems like it should throw it off equally for every face since every face is getting exactly the same color. But I still see some specific faces looking like the bump normals are flipped. Does that make sense?

I was experiencing this problem on the models someone else made, but not on the models that I made. I feel like it should be consistent. Either all normal maps are off without using RAW, or none of them are.

So, does that mean that all normal maps should be applied RAW and not sRGB?

[Morbid Angel]

For normal maps, no gamma correction or any kind of correction should be applied.

For difference in normals shading on your mesh, it happens in areas where there are more polygons, my guess is - all of the normals are wrong no matter where they are, some exhibit more noticeable shading issues then others, but imho they are all incorrect. I was surprised to find that your object was rendering for so long, being almost a simple plane. This meant that the "bad" normals caused it to sample/shade incorrectly and made the render time what it was.