How Do I Get Faces With Normals Facing Away?

new.gif MAXScript Frequently Asked Questions

A user asked:

Can anyone tell me how to get faces that have normals pointing outside of the current viewport?


You can get the face normal in view space, then you just have to check the sign of the Z component. If it is negative, it is facing away from the camera!


theObj = Teapot segs:10 --create a teapot

convertToMesh theObj --convert to EMesh

theMeshCount = theObj.numfaces --get the number of faces in the object

selArray = #() --init. an array to collect faces

for f = 1 to theMeshCount do --loop through all faces


in coordsys (inverse (viewport.GetTM())) theFN = getFaceNormal theObj f

if theFN.z < 0 then append selArray f --if Z is negative, add to array


setFaceSelection theObj selArray --set the face selection in the EMesh

max modify mode --go to modify mode

select theObj --select the mesh

subObjectLevel = 3 --go to Face SO level

modPanel.addModToSelection (deleteMesh()) --add a delete mesh, preserving the SO selection



Result: All triangles facing away from the camera will be deleted!

Teapot_Backfaces_Deleted_CameraView.gif Teapot_Backfaces_Deleted_SideView.gif

The image on the left shows the teapot with deleted backfaces as seen from a camera viewport. The image on the right shows the teapot and the camera as seen from the side. Note that for the screenshot, the backface culling was disabled (you can see the backfaces on the right image), but the left image looks just like it would with backface culling enabled! This is because the script deletes the same faces the viewport backface culling would skip!

See also

Frequently Asked Questions