topic Re: Selecting only visible (not obscured) 3D faces in AutoCAD Forum

Selecting only visible (not obscured) 3D faces

jbokic2F9MM — Fri, 01 Mar 2024 19:18:33 GMT

I work in mining, where we can get 3D objects representing overburden / clay / ore bodies / etc. Most of the time these are represented as 3D faces that represent a watertight solid (for lack of a better description).

We would like to represent these objects in Civil 3D as top and bottom surfaces but the best workflow I can currently think of is to use a visual style that shades the surfaces and then manually select them. Not an efficient process.

Is there a workflow or a tool that will allow the selection of only the top/visible 3D faces? I've posted a screenshot below and provided an example drawing.

Re: Selecting only visible (not obscured) 3D faces

paullimapa — Fri, 01 Mar 2024 21:46:39 GMT

May want post here

https://forums.autodesk.com/t5/civil-3d-forum/bd-p/66

Re: Selecting only visible (not obscured) 3D faces

jbokic2F9MM — Sun, 10 Mar 2024 18:45:04 GMT

Thanks paullimapa, I will if nobody here knows of a solution but the separation of surfaces seemed more of an AutoCAD than Civil 3D issue.

Re: Selecting only visible (not obscured) 3D faces

leeminardi — Sat, 02 Mar 2024 21:52:35 GMT

The following program changes the layer of selected 3dfaces that are facing you. I've done minimal testing on the file you posted.

;Changes the layer of seleted 3dfaces that ;are facig towards you. ;L. Minardi 3/2/2024 (defun c:FacingMe (/) (setq lay (getstring "\nEnter a layer name: ") ss (ssget '((0 . "3dface"))) i 0 ) (command "_ucs" "V") (while (setq en (ssname ss i)) (setq ed (entget en) pt1 (cdr (assoc 10 ed));get points of 3DFACE from entity data pt2 (cdr (assoc 11 ed)) pt3 (cdr (assoc 12 ed)) pt4 (cdr (assoc 13 ed)) v12 (mapcar '- pt2 pt1) v13 (mapcar '- pt3 pt1) FaceNormal (cross v13 v12) ) (setq viewNormal (trans '(0 0 1) 1 0)) (setq vis (dot FaceNormal viewNormal)) (if (> vis 0.) (progn ;(princ "\nVisble") (if (not (tblsearch "layer" lay)) (command "_.-layer" "_n" lay "") (command "_chprop" en "" "_LA" lay "") ) ) ) (setq i (+ i 1)) ) ; end while (command "ucs" "p") (princ) ) ;cross product (defun cross (a b / crs) (setq crs (list (- (* (nth 1 a) (nth 2 b)) (* (nth 1 b) (nth 2 a)) ) (- (* (nth 0 b) (nth 2 a)) (* (nth 0 a) (nth 2 b)) ) (- (* (nth 0 a) (nth 1 b)) (* (nth 0 b) (nth 1 a)) ) ) ;end list ) ;end setq c ) ;end cross ;Dot Product (defun dot (a b / dd) (setq dd (mapcar '* a b)) (setq dd (+ (nth 0 dd) (nth 1 dd) (nth 2 dd))) ) ;end of dot

Re: Selecting only visible (not obscured) 3D faces

jbokic2F9MM — Sun, 10 Mar 2024 22:51:35 GMT

@leeminardi thank you that works brilliantly! Does this work as the 3D faces are based on the right hand rule or is it for a different reason?

Re: Selecting only visible (not obscured) 3D faces

vladimir_michl — Mon, 11 Mar 2024 06:58:15 GMT

See the Normals utility on Autodesk Apps:

https://apps.autodesk.com/ACD/en/Detail/Index?id=6799700742884340036&appLang=en&os=Win32_64

Vladimir Michl, www.arkance-systems.cz - www.cadforum.cz

Re: Selecting only visible (not obscured) 3D faces

leeminardi — Mon, 11 Mar 2024 11:26:08 GMT

@jbokic2F9MM yes, using the right-hand-rule the face normals in your model consistently pointed inward. Looking at the sign of the dot product of the normal with a vector perpendicular to the screen is used to determine if the face is visible. I'm glad that you found the program helpful.

Re: Selecting only visible (not obscured) 3D faces

jbokic2F9MM — Mon, 11 Mar 2024 15:52:54 GMT

That helped tremendously, would never have thought of that until seeing your code.

Re: Selecting only visible (not obscured) 3D faces

jbokic2F9MM — Mon, 11 Mar 2024 15:53:48 GMT

Thanks @vladimir_michl , this also looks like a good solution.

Re: Selecting only visible (not obscured) 3D faces

robert06 — Fri, 21 Jun 2024 09:10:52 GMT

A very useful lisp, thanks.
Only thing is, that it filters the faces of the opposite side from the view direction (the bottom ones, if the view is from the top).

Re: Selecting only visible (not obscured) 3D faces

robert06 — Fri, 21 Jun 2024 09:34:56 GMT

Correction provided by ChatGPT:

Summary of Changes:

Fixed the cross product calculation to ensure the correct face normal direction.
Modified the dot product calculation to correctly sum the products of the components.
Adjusted the order of vector subtraction to ensure proper calculation of face normals.

(defun c:FacingMe (/ lay ss i en ed pt1 pt2 pt3 pt4 v12 v13 FaceNormal viewNormal vis)
(setq lay (getstring "\nEnter a layer name: ")
ss (ssget '((0 . "3DFACE")))
i 0
)
(command "_ucs" "V")
(while (setq en (ssname ss i))
(setq ed (entget en)
pt1 (cdr (assoc 10 ed)) ;get points of 3DFACE from entity data
pt2 (cdr (assoc 11 ed))
pt3 (cdr (assoc 12 ed))
pt4 (cdr (assoc 13 ed))
v12 (mapcar '- pt2 pt1)
v13 (mapcar '- pt3 pt1)
FaceNormal (cross v12 v13) ; calculate face normal
)
(setq viewNormal (trans '(0 0 1) 1 0)) ; view normal in WCS
(setq vis (dot FaceNormal viewNormal))
(if (> vis 0.)
(progn
;(princ "\nVisible")
(if (not (tblsearch "layer" lay))
(command "_.-layer" "_n" lay "")
(command "_chprop" en "" "_LA" lay "")
)
)
)
(setq i (+ i 1))
) ; end while
(command "ucs" "p")
(princ)
)

;cross product
(defun cross (a b / crs)
(setq crs (list
(- (* (nth 1 a) (nth 2 b)) (* (nth 1 b) (nth 2 a)))
(- (* (nth 2 a) (nth 0 b)) (* (nth 2 b) (nth 0 a)))
(- (* (nth 0 a) (nth 1 b)) (* (nth 0 b) (nth 1 a)))
) ;end list
) ;end setq crs
) ;end cross

;Dot Product
(defun dot (a b / dd)
(setq dd (mapcar '* a b))
(apply '+ dd)
) ;end of dot