Re: LOGO-L> Local and global 3D geometry (was:RE: 3D Logo source ?)

[Tommaso Russo <trusso@tin.it>]

George Mills wrote:

> I think I had considered your option #1 and have not ruled out
> adding it. I think your basically saying after each ROLL or PITCH
> (even perhaps SETXYZ) that it define a new 2D origin in which, where
> ever the turtle is currently pointing after one of these commands
> defines HEADING = 0.

Yes, this was what I had in my mind, but as I read the concept rephrased
by you I realizad why it is NOT a good idea: it would give problems to a
2d procedure using setpos and CALLING 3d procs (imagine E.G. a 2D city
map, probably painted using absolute coordinates, and the project of
inserting on it 3D models of main churchs and monuments: each call to 3D
procedures would redefine the 2D origin. Good-bye map.)

> One thing that is very powerful about Brians engine is you
> can implement your suggestion #2.
> 
> to myperspective
> perspective
> make "redefp "true
> erase "pos
> erase "setpos
> copydef "pos "posxyz
> copydef "setpos "setposxyz
> end

Yes, this would make working my procs rememberpos and restorepos, but is
anyway unsatisfactory. If somebody then plots a function using SETPOS
(LIST :X FUNCTION :X) the redefined setpos will complain it needs a 3
item list.

3D is a complex matter and logo programmer have to deal with it
intelligently. I think the only solution now is leave MSWlogo 6.3 as it
stands and warn them that a 2D procedures will work well in 3D only if
it use relative commands plus, at most, absolute setheading.

>>Only now I've realized the meaning of roll, pitch and heading in
>>MSWlogo: they output the angles a turtle at home must rightroll, uppitch
>>ad rigth IN THIS ORDER to gain current orientation. It was not clear, is
>>not documented and can be understood only reading the library procedure
>>setorientation.
>I forgot to answer this comment.
>See online help "Understand your Orientation in 3D" which covers this
>subject in detail. Of which there are numerous links to, including
>the online help on SETORIENTATION.

I've read carefully that chapter, and the deeper information it gives is
"SETORIENTATION knows ... it has to do things in the correct order." The
right order can be seen only unburying and examining setorientation
itself. The triplet choice of towardsxyz, that is hard-coded, can be
only guessed testing it. Hope you'll document these internal of MSWlogo,
because logo programmers need these details.

Thank you very much for the attention and for the powerful tool you gave
to the logo community.
  
-------------
Tommaso RUSSO
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