win 10 / x64 / 2019.20140
Hi, not sure if this effect is by design or not, so please ignore all of this if it is supposed to be this way for other reasons!
Simple occurrence to reproduce, create a rectangle SOP, and turn on the pink surface normals.
There are 3 plane orientations that can be created, the default of XY faces the node viewers “camera” and thus works as expected in terms of shading and other things that use the normal.
The same can be said for the YZ plane. normal faces to the right.
In both of those configs, the normal faces in the positive side of that axis.
However, the third ZX configuration where the plane sits on the “floor” the surface normal actually faces down.
This obviously isn’t tough to fix with a transform sop or other methods, but it can create a lot of confusion for why normals may look incorrect when rendering onto a flat rectangle used as a floor geometry, or otherwise.
Also, a box scaled down very small on Y produces a different result, due to the normal facing up.
I’ve run into this in the past with out realizing what it was, and never quite figured out why normal maps on some geometry seemed flipped, or odd. I know the shaders have some internal code to flip the normals in the instance of backface vs front face, but I am also finding some odd behavior there, and will post a separate thread on that.