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jim78b

i use isometric drawing in autocad , and i draw a line length 20mm autocad draw a line length 20 but if i use a lisp

 

isoblocks.lsp

 

https://autocadtips1.com/2012/03/21/autolisp-easily-create-isometric-blocks/

 

the lengt hof the lines change? why?is another isometric projection?

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CyberAngel

An isometric drawing is by definition a projection, but it's orthographic. In other words everything is projected, but it's all projected at the same ratio. In a perspective view, the projection angle varies based on distance from the camera.

 

From what I can tell, this LISP routine automatically projects horizontal lines to 30 degrees. If you want a 20mm line in the block, you'll need to draw it at some length that becomes 20 at 30 degrees. It's a trigonometry problem, but I forgot everything I learned about trig in the 80s.

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jim78b
1 hour ago, CyberAngel said:

An isometric drawing is by definition a projection, but it's orthographic. In other words everything is projected, but it's all projected at the same ratio. In a perspective view, the projection angle varies based on distance from the camera.

 

From what I can tell, this LISP routine automatically projects horizontal lines to 30 degrees. If you want a 20mm line in the block, you'll need to draw it at some length that becomes 20 at 30 degrees. It's a trigonometry problem, but I forgot everything I learned about trig in the 80s.

Yes .i can not edit this lisp ...i think was projected on the same ratio .if you draw a linee in .autocad in isomode the length is the same you enter the Value in ortho mode .

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jim78b

Can you edit the lisp please ?

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SLW210

I have merged your threads, please post the same question only once in the most appropriate forum.

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jim78b

Thanks

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lrm

In the precise definition of an isometric drawing lines (object edges) that are parallel to one of the principal axes (x, y, or z) are drawn true length at an angle of 8 or - 30° or vertically.  Thus a 20mm long edge in the x direction of the 3D model would be drawn at an angle of 30°.  An isometric projection on the other hand is the result of making a parallel projection (as opposed to a perspective projection) of the 3D shape to a plane.  The plane for an isometric projection is perpendicular to any of the diagonals of a cube.  Each of the principal axes of the 3D object form an angle of about 35.2644° (asin(1/sqrt(3)) with the projection plane. 

An object rotated to an isometric orientation yields geometry that is scaled by about 82%  (0.81649…).  This is equal to the cosine of 35.2644°.  Thus the 20mm line would be about 16.33mm long in the projection (20 * cosine(35.2644) ) .

 

Bottom line?  Forget the math and scale the isometric projection to yield an isometric drawing by scaling it up by  this factor or 1.224744… (the inverse of 0.81649).  

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