FPRISM_

[Anonymous]

Am I right in thinking that FPRISM_ (and therefore HPRISM_) have a limit to the angle of the hill?
I note that this parameter for the angle of the hill must be equal to 0 or less than 90, but when I use an angle above approx. 80 degrees the hill 'slope' does not become greater.
Thus, for example, if the FPRISM_ has a base 0.2M square; thickness 6.3M; hill height 5.4M;and the hill angle is set to 85 the resulting object is wrong!!
Here's the GDL script of the 3D object, where the angle is 88.

RESOL 16
FPRISM_ mat, mat, mat ,mat,
4, 6.3, 88,5.4,
0,0, 15,
0,.2,15,
.2,.2,15,
.2,0,15

Any insight would be appreciated.

I know I can achieve the result I want using either the RULED object or 4 CUTPLANEs, but I am puzzled by the behaviour of the FPRISM_ in the circumstances I have described.

[david nicholson-cole]

Graphisoft develpment team seem to have a problem with the number 80 and beyond. I have been doing LIGHTs recently, and there is an absolute limit to the splay of a light to 80 degrees. A fraction over this and its an error. Undocumented, I believe, but true.

[David Maudlin]

Mike:

From the GDL Reference Guide for FPRISM_

Restriction of parameters:
n >= 3, hill_height < thickness

So the issue may be the thickness, not the angle.

David:

From the GDL Reference Guide for LIGHT:

Parameter restriction:
alpha <= beta <= 80°

Hope this helps.

David

[Anonymous]

I am pretty sure that my example stes the hill height as being less than the thickness!

[David Maudlin]

Mike:

I created a new library part using your script, and added an angle parameter for the FPRISM_ angle so I could easily change this parameter. I then placed this object in the 2D plan, created a section/elevation view from the side, and moved the origin to the bottom of the slope to measure the angle to the top. I am getting 85 degrees when the parameter is set to 85 degrees, and correct readings at other angular settings. I cannot reproduce the problem you are describing. How are you measuring the angle?

David

[Anonymous]

David

You are correct. The angle does work, but the specific case I want is for use in a street light. The column base is small compared to the height, and the taper I want to effect is small, but important. The actual angle I need to use for a street light of 7.2M total height is about 89.4 degrees which should result in what is actually, I suppose, a frustrum - i.e the FPRISM_ does not end such that all sides meet at a single point.

My original assertion that the FPRISM fails at about 80 degrees is probably wrong.

Using the example script I gave previously, I have found that it fails to increase the angle of the hill at 88 degrees. This is presumably something to do with the circular geometry (TAN?) internal routine.

I would be interested to learn whether you can reproduce this.

Mike

[David Maudlin]

Mike:

Yes, I am getting the same limitation: any angle above 88 degrees results in a 88 degree slope. Besides the coders at Graphisoft, I don't know who can explain this behavior. At this point I would just use the Ruled statement (as you mentioned) to generate this form, to me this would be easier than fooling around with 4 cutplanes.

Good Luck.

David

[Anonymous]

David

Thanks for confirming my findings.

I resorted to using a RULED object, but the behaviour is strange - perhaps Graphisoft need to limit the parameter to a maximum of 88 degrees?

Mike

[Greg Kmethy]

This is a technical limit of the modeler. The theoretical limit is 90 degrees, however, because of the segmentation of the body, it will be less, depending on the size of the body and the resolution.