VRay Decal cannot take effect on the back of a cube

[123456_]

I have uploaded the problem video and engineering files

[zhanet.georgieva]

Hi 123456_,

When applying a V-Ray Decal to a 3D object, it’s normal for the decal not to appear on the back side of the object. There might be some confusion because there’s a "Project on Back" option enabled in your scene. This option actually allows the V-Ray Decal to be projected onto the back face of the surface (like the reverse side of a single polygon), not the back side of the entire 3D object. So, it won't make the V-Ray Decal appear on the back of the whole object, just the reverse side of individual surfaces.

[123456_]

Hi 123456_,

When applying a V-Ray Decal to a 3D object, it’s normal for the decal not to appear on the back side of the object. There might be some confusion because there’s a "Project on Back" option enabled in your scene. This option actually allows the V-Ray Decal to be projected onto the back face of the surface (like the reverse side of a single polygon), not the back side of the entire 3D object. So, it won't make the V-Ray Decal appear on the back of the whole object, just the reverse side of individual surfaces.

According to what you said, why did the back of the cylinder in my picture turn red due to the influence of V-Ray Decal? It should be unaffected like a cube

[fixeighted]

You have the normal angle set to 180 (default, at least in Max, is 90), which makes it wrap the cylinder.
If you actually did want it to wrap the box too then you need to increase the bend parameter until it does so

[zhanet.georgieva]

V-Ray Decal can't detect normals that are exactly at 180 degrees. If an object has a completely flat surface, its normals are exactly at 180 degrees.