How to understand and dtermine the vertex order?

1

Hi all,

The book Rendering with Radiance (1998, P29), introduces that “The vertex order is very important, especially for polygons with holes. Normally, vertices are listed in counterclockwise order as seen from the front (the room interior in this case).”

I am wondering that when I use genbox command to generate a room, how to determine the vertex order, should I see from the front or room interior?

Thank you in advance for your help and consideration.

Cheers,
Hao

2

Hey Hao,

I am not the most experienced user but I’ll try to help.

Indeed, what you said about the definition of polygons is true. However, the genbox command does not take vertices as inputs. Instead, one corner is at the origin, while the x,y, z values that you pass describe the coordinates of the opposite corner.

Have I misunderstood something?
Alex

2 Likes
3

Hi Hao,

adding to that - genbox creates 6 surfaces of the box with surface normals pointing outward by default. This means the order of the vertices is accordingly. With -i you can generate a box with surface normals pointing inward. If you try both, you will see that the order of the vertices of the single surfaces is reversed.

Best,
David

2 Likes
4

Also, the surface normal orientation is unimportant for some materials, and important for others. The Radiance reference manual tries to be clear about when it matters. An opaque material will be visible from the inside or outside, unless the -bv flag is used during rendering.

Best,
-Greg

1 Like
5

Hi Alex,

Thank you for your help. Your reply is right about the way how to use genbox to “define” a box.

It is my fault for giving the misleading information. There are some points that are unclear in my question. What I was confused about was when I used the genbox command to generate a box, for the generated box how to understand the vertex order of each surface of the box.

With the reply of @David_Geisler-Morod1, I think I understand the vertice order of each surface. The default of genbox for the generated surfaces is outward. With the Right-Hand Rule on P48 and the normal direction of the surface (outward), I am able to understand the vertex order of each surface.

As @Greg_Ward said, the surface normal orientation is unimportant for some materials and important for others e.g. single-sided materials. Besider, as shown in his book, for polygons with holes, the vertex order is also very important.

Thank you very much for your guys helping and I truly appreciate it.

Cheers,
Hao

6

Hi Greg,

Thank you for your explanation. By the way, with the introduction of Glass in Radiance reference manual and my understanding, for Glass, the surface orientation doesn’t matter, because it will transmit and reflect light. Am I right?

Cheers,
Hao

7

Hi Hao,

the common glass “material” is an optimized model for transmission and reflection by thin glass panes. It assumes symmetry by default, so orientation does not matter.

Please note though that “real” fenestration may include coated glass panes - and here outside and inside reflection are different. So if you model a coated glass e.g. using Optics, make sure that the orientation matches the modelling assumptions.

Best, Lars.

2 Likes
8

Hi Lars,

Thank you for your reply. It helps me to have more confidence in the common glass “material”. I will be careful when dealing with fenestration with coated glass panes. Thanks for the reminder and I appreciate your explanation.

Cheers,
Hao