Shader to identify filpped normals?

[evanerichards]

I'm working with a model that was not very skillfully built. There are a lot of faces with normals that are flipped. It's very difficult to fix. What would help is some sort of a shader that would allow you to render the scene and see all of the faces with normal facing in the opposite direction. I've been using an AO shader for this because anything with a flipped normal appears black, but it can also give false positives when two objects are really close together and the black is actually occlusion.

So my question is, would it be easy to build something like this? I feel like using the hypershade you could build a shader that would do exactly that. Show all surfaces flat white except faces with flipped normals which would appear bright red. Or something like that. I'm just not advanced enough to build something like this myself. Is there anything like this built in? And if not, how would one go about creating such a shader?

Thanks!

[vlado]

You could add a Normals render element and see if you get faces with their back to the camera.

Best regards,
Vlado

[pechart]

If I understand you right you just need to apply a VRayMtl2Sided to your object with 2 different shaders to front/back and the translucency set to 0. Or isn't that what you want?
By the way, I always enable Backface culling in the Shading Menu of the viewport. That way you can see "holes" where the normals are flipped.

[bigbossfr]

- Create a blendColor utility
- create a samplerInfo utility
- connect the flipped normal output of the samplerInfo into the blend on the blendColor
- connect your blendColor into a VrayLightMtl

Render.

[evanerichards]

@ vlado - I was doing that, as well as an AO pass. But it can be a little difficult to pick out the flipped normals as the colors all sort of blend together and can look pretty similar sometimes. It can be difficult to distinguish between the teal and the green colors etc.

@ pechart - Backface culling!!! Brilliant! I never knew what it did before now. Very useful. Thanks! I'll try the VRayMtl2Sided trick too and let you know what I find.

@ bigbossfr - That is EXACTLY the trick I was looking for. It worked perfectly. Now, the question is, and this is more of a maya question than a vray question but I know you are a coder/programmer, if maya can identify the flipped normals is there some way to select all those flipped normal faces at the same time and flip them back without having to click and select each one individually?

[bigbossfr]

@ bigbossfr - That is EXACTLY the trick I was looking for. It worked perfectly. Now, the question is, and this is more of a maya question than a vray question but I know you are a coder/programmer, if maya can identify the flipped normals is there some way to select all those flipped normal faces at the same time and flip them back without having to click and select each one individually?

Ahah, i am not a programmer, i do code for fun.

So,
1) Go to select ---> select using constraint
2) Set properties "location" to "OnBorder"
2) Select all vertices of your object
3) Set "constrain" to "all and next"
4) Now, go to selection ----> convert selection ----> to contained faces
4) Go to normals ---> reverse

[Morbid Angel]

Also you may try the following:
in the viewport go to lighting and turn off two sided lighting. This will render the reverse normal polygons as black. So visually you can see it in maya viewport in real time. You can also do normals/conform, which will try to set all the normals to the majority of the normals facing direction, so if most of your normals are facing outwards, it will reverse the other ones too. Though for this to work I believe all the faces must be attached.

[nbreslow]

Also you may try the following:
in the viewport go to lighting and turn off two sided lighting. This will render the reverse normal polygons as black. So visually you can see it in maya viewport in real time. You can also do normals/conform, which will try to set all the normals to the majority of the normals facing direction, so if most of your normals are facing outwards, it will reverse the other ones too. Though for this to work I believe all the faces must be attached.

What he said - works like a charm and much simpler.

-Nick

[evanerichards]

@ bigbossfr - That did flip some normals, but not the ones that were facing the wrong way. I have never used those particular selection commands before and it has opened my eyes to a lot of possibilities. Thanks for the help.

@ Morbid Angel - The two sided lighting is a good trick. I'm going to be using that. The "normals...conform" didn't seem to work. Like you said, there may be certain criteria that must be met before it works correctly. Thanks for the tips!