How to check if face in mesh is culled/invisible to camera?

Development Programming & Scripting C++
1

Hi everyone,

If I have a static or skeletal mesh, is there a way to get the IDs of all the faces in that mesh that are invisible to the camera (culled)?

Obviously the Unreal Engine knows it should not draw them, but I couldn’t find a way to access that info in the reference.

Perhaps unnecessary, but I have attached two images to illustrate.

Thank you for any ideas or feedback!

2

That’s normally handled at the shader level, not the application level. For example, you can use the SV_IsFrontFace Semantic in HLSL to find out if a triangle is front facing or not - but that data isn’t saved off each frame or send back to the CPU.

3

Hi Matt, thank you for this information. I managed to script it inside 3D Studio Max, but I didn’t know it would be so hard to in UE4.

Indeed I would need the data since I want to do complex vertex animation which is different for each vertex.

As I’m new to UE4, am I correct in stating that I would have to study writing a new shading model with my own custom geometry shaders embedded in the shader section of the engine?

Like in this article: Unreal Engine 4 Rendering Part 6: Adding a new Shading Model | by Matt Hoffman | Medium

If that is the case then I have a long study period ahead of me…

4

You could get the normal of each polygon and do a dot product with the camera direction vector. Any face that is less than 90 degrees from the camera direction is facing away from the camera.

EDIT: This won’t find polygons that are facing the camera but hidden by other surfaces between the polygon and the camera.

5

Hi GBX, that is exactly what I did in 3DSMax, I also added a raycasting part to find front facing faces that were occluded by other faces.

I was just hoping that info on faces (like being culled/visible or not) would be available inside UE4 giving way to a much faster and realtime version.

But I’m really disappointed now that things like access to faces, vertices et cetera is way more difficult than in 3DSMax. ;-(

6

That is too slow to be done on CPU… this is why you need “shader code” to calculate that (on GPU)

7

Thanks Bruno, I looked into it and I had no idea that HLSL can be used for ray casting, but I saw a lot of examples online.

Would this be possible from a Custom expression or do I need to embed that code in the engine?

8

I have no idea… In regard to HLSL code I’m as smart as a rock :slight_smile:
There’s some graphics programmers around AnswerHub from time to time, they might help…

@RyanB

9

Awesome, thanks!