Face normal + Object rotation..?

Coding Python Support
1

Hi all.
I am newly into python in blender and I am working on a rewrite of a section script.
It is goinf well and I can now get a section from a mesh, but I have a small problem…

I get a the normal of a polygon, but the normal is relative to the local space of the face… I need the normal in world space… So:
1: I get the local normal and rotate it by the objects rotation. Problem: how do I rotate a normal?
or…
2: I get the normal relative to world space diractly. Can that be done?

Thanks. Hope to get the script done soon so I can share it…

2
  • removed incorrect suggestion *

Sorry about that, normalized matrix are used for quaternion multiplication but not for normals.
I didn’t use this stuff in a while and got it confused.

3

Hi ejnaren,

ideasman’s code just needs a slight tweak if your object is non-uniformly scaled (as that can change normal direction also) if that’s the case for you, you can use:

matrix = obj.matrix_world
matrix_new = matrix.to_3x3().inverted().transposed()
no_world = matrix_new * no
no_world.normalize()

The last line assumes that you want to maintain unit normal length (i.e. length of 1)

Hope that helps.

Cheers,
Truman

4

Thanks ideasman and trumanblending!
You just cant buy support like this.

5

As said I am very new to blender scripting and while this helped me it is acually unclear to me why.
I am porting a 2.49 scropt so I need not know to much about scripting to get results.
In the 2.49 script one applies a matrix to a vector by just multiplying it, but in the 2.5 I need to use matrix.transposed() ??
How come. Has the matrices in 2.5+ been flipped?

Anyway Thanks again. When I post the script/addon/tool maybe someone would like to look it over. There might be much easier ways to do this now or I might have unseen bugs crawling around the code. Even though I can know cut things by a plane in blender again :slight_smile:

6

Ok, so this is a first for me, but I would like to share what I have done so far, but I am not sure how to proceed about this.
The script is still not ported 100% but it works and can cut an object, creating a section. The FILL option in hardcoded TRUE in the code because I havent yet figured out how to interact with the user through python.
Also I am not really into matrix calc so I was kind of fumbling in blind, but with the above help got it to work.
If anyone would like to look at it go for it as I have some other things I need to do before continuing with this :slight_smile:

Link to newly ported script (2.65): CrossSection.py

Link to original script (2.49): CrossSection

Thanks.

Ejnaren

PS: If anyone have good ideas as to what is the best way to get into blender I would be happy to hear about it.

7

Yes, @TrumanBlending changed this since 2.4x.
http://wiki.blender.org/index.php/User:TrumanBlending/Matrix_Indexing

8

Hey ejnaren, I like your port. It works fine for me except an error on line 252, changed that to

bpy.context.tool_settings.mesh_select_mode=(False,True,False)

For the next steps:
You could make an Operator out of it, so its callable by a button in the GUI.
There you could also have a Panel with a boolean Property for the fill option.
In the internal Text Editor there some samples for UI Panels and Operators.

Feel free to ask if you get stuck.

9

Thanks Afeitadora!
I am in the middle of launching my portfolio site and deliver my exams for this semester. After this I think I will finish the script as you mentioned.
I have also thought of improving it with an option for multiple face sections so it can be ued for more intersections.
For that I might call on help :smiley:

Roger over