Hi guys I had a look into previous threads on this subject , but I didnt find what I was looking for.
I am using a imported model from maya with normals facing in and out. So I really need to reorganise the normals. What would be the correct way of doing this. Would it be best to attach a point SOP and delete the normals , then apply another Point SOP to create new normals ?. I did notice reading the help if you enter a minus into each channel under the normals tab ie -$NX it will reverse the normals , pretty handy in some cases.
I do have a feeling that deleting the normals attribute would help and then using a point SOP set to add normals would be the way to go. would this be the correct way to go ? and if so how would I use the attribute SOP to delete a normal attribute ?

R

The Facet SOP is your one stop solution for solving problems like this.

The main assumption I am making is that you really don't care to keep the Maya normals around. They are probably incorrect since they were originally vertex normals and they won't be correct in Houdini. Your shading will be off.

So, append a Facet SOP and look at the parms. It's like a small “fix-my-polys” factory. The parms are in the order that they are executed.

In your case, simply turn on “Orient Polygons” as I suspect you have some reversed primitives in your geometry. Then you can turn on Post-Compute Normals. This will rebuild the point normals for correct shading.
You can append a Point SOP as you eluded to and simply blow away the point normals. In your specific case, you did mention that some normals point inward, not a good thing as I suspect that primitives are reversed in your geometry. Used to see this a lot in many of the free models you would get off the net.

Jeff thanks for the answer , I wish I could say that I have had success … but using the facet SOP has not changed my normals from pointing still into the head.
The polys look ok apart from the normals , in other applications you can tell that some polys are flipped easily, are there any other indicators I can use ?. I must say Im finding this excercise quite fustrating.Is there no easy way to flip faces

R

Display the primitive normals and turn off the point normals. Adjust the scale if needed with the the display options > Misc folder.

Now check to see if you have some primitives that have normals pointing inwward. They should be all pointing outward.

If there are some pointing inward, then try the Facet SOP and the “Orient Polygons” option. That only works if the points are all consolidated.

If that doesn't work, then you will need to find those inward facing polys and reverse them manually with a Reverse SOP.

I would blow away the point normals unless you really needed them. You can always easily recreate them later on.

Jeff …. thanks for the extra help .Like a noob I had point normals showing not , primitive normals. Using the reverse SOP changes their colour …nice interactive feature.
The facet SOP didnt have any overall effect that I could see , but I have left it in there.After you reverse the polys the model looks black , due o a lack of material /colour So I just ammended a point SOP to add colour and blew away the point normals. job done !!

Thanks for all your help


R

The facet Sop computes normals based on the prim normals, so once the prims are reversed the “compute normals” function in the facet sop that follows should fix the point notmals as well.

Cheers