topic Re: offsetting a hole in a PRISM, suggestions? in Libraries & objects

offsetting a hole in a PRISM, suggestions?

Anonymous — Fri, 22 Feb 2008 15:23:34 GMT

I have a triangle shaped object created with a PRISM. I need to cut a triangle shaped hole inside it that is equidistant (or basically offset) on all 3 sides. The width of my trim(determined by user) determines the offset.

Any suggestions on how to get the coordinates for the interior triangle?

The angles of the interior triangle are all the same as the overall triangles object.

This may be more of a math question than an
ArchiCAD question unless someone can tell me some secret offset command for inside a PRISM.

Re: offsetting a hole in a PRISM, suggestions?

Anonymous — Fri, 22 Feb 2008 17:54:29 GMT

You could create a copy of the prism and use the "Resize" command to reduce the object the required amount. Be sure to select the center of the prism as the base point for the resize command.

Then use SEO with the newly created object to create the hole. Finally, place the new object on a hidden layer.

Re: offsetting a hole in a PRISM, suggestions?

Anonymous — Fri, 22 Feb 2008 17:58:01 GMT

Can you do all that with scripting or do I have to give up the ability to select multiple trim sizes?

Re: offsetting a hole in a PRISM, suggestions?

TomWaltz — Fri, 22 Feb 2008 18:09:54 GMT

How good are you with Trig and linear algebra? You could probably script it out, but it won't be pretty.

I think there are few objects like this in the Archicad library that you could look at for examples.

Re: offsetting a hole in a PRISM, suggestions?

Anonymous — Fri, 22 Feb 2008 18:14:52 GMT

At the moment all I'm doing is spinning my wheels about different ways to use Sin, cos and tan to get the necessary angles. The initial object works correctly to change its shape based off user input roof pitches including when the roof pitches are not the same. The inner holes I've got working about 80% correctly. THey just aren't working for all cases. I'm trying to decided if I need an if-else loop to get the various cases to work right.

I don't think I do since the outer most layer works correctly. But yah, it's long and ugly.............

Re: offsetting a hole in a PRISM, suggestions?

Anonymous — Fri, 22 Feb 2008 18:41:29 GMT

Taking a cue from Dominick's suggestion,
would it be possible to make a cutpolya with
the same parameters as the prism then
mul the cutpolya to make it proportionally
smaller and use it to cut the hole in the prism ?
The local origin of the prism would have to be
the geometric center of the shape.
Peter Devlin

Re: offsetting a hole in a PRISM, suggestions?

Anonymous — Fri, 22 Feb 2008 21:28:11 GMT

I was able to make my object work by using an if-then statement for when the two pitches were not equal and a whole lot of math. I'm very curious as to how much shorter the code would be if I used the cutpolya command or attempted to script an SEO action. Thanks for all of the ideas.

Re: offsetting a hole in a PRISM, suggestions?

Anonymous — Fri, 22 Feb 2008 21:47:35 GMT

I don't know what you are trying to make but it occurs to me
that the easy way to make this shape with the offset the same
regardless of the size and shape of the triangle is to make
the shape using the TUBE command.
Peter Devlin

Re: offsetting a hole in a PRISM, suggestions?

LiHigh — Sat, 23 Feb 2008 04:20:01 GMT

Peter wrote:
I don't know what you are trying to make but it occurs to me
that the easy way to make this shape with the offset the same
regardless of the size and shape of the triangle is to make
the shape using the TUBE command.
Peter Devlin

Spot on! TUBE command is the way to go.

Re: offsetting a hole in a PRISM, suggestions?

Anonymous — Sat, 23 Feb 2008 04:49:26 GMT

tube is the go, but I personally would love to see an answer to the question I have had many instances where offsetting a prism or a path is needed, but the trig is a headache. There must be an easy "plug in" bit of code using the cosine rule and co-ordinate geometry..
Any maths genius's or physics Nerds up for the challenge?

Re: offsetting a hole in a PRISM, suggestions?

LiHigh — Sat, 23 Feb 2008 07:10:44 GMT

Getting the offset coords is no problem. But, determine the direction of offset requires complex math. I'll see if I've the spare time....

Re: offsetting a hole in a PRISM, suggestions?

Anonymous — Sat, 23 Feb 2008 07:25:22 GMT

Footing 1 11 - slab accessory 2d Script has a good example of offsetting a poly, its not commented enough but you will get the gist.

!-------------------------
! Start the polygon offset
!-------------------------

For i=crd_begend[1] To crd_begend[2] ! Precalculate the data
pen 2 ! Draw the polygon beFore the offset
If i<crd_begend[2] Then
data[3]=ac_coords[i+1][2]-ac_coords[2] ! Direction vector
data[4]=ac_coords[i+1][3]-ac_coords[3] ! Direction vector
Else
data[3]=ac_coords[crd_begend[1]+1][2]-ac_coords[2] ! Direction vector
data[4]=ac_coords[crd_begend[1]+1][3]-ac_coords[3] ! Direction vector
EndIf

If abs(data[3])<epszilon Then data[3]=0
If abs(data[4])<epszilon Then data[4]=0

length=sqr(data[3]^2+data[4]^2) ! Make the unit vector
vect_x=offset*dir_x*data[4]/length
vect_y=offset*dir_y*data[3]/length

data[1]=ac_coords[2]+vect_x
data[2]=ac_coords[3]+vect_y
next i