Mini-Challenge #2

jeff hammond

@box said:

You can use the width and the corner angles to work out the other sides of the two triangles. Technically you only need to solve one other side. The maths involved is beyond my whiskey sozzled brain.

sober up a bit and try to explain a little better (I kid, I kid)
I'm not sure which width and corner angle you're talking about?

Box

Actually the whiskey is from years past.

I think, to use your 2x4 analogy, you have a fixed width 2" you have two known angles, the two corners you want to fit between. There must be a formula to work out the lengths of the other two sides of the two triangles formed at the ends given the fixed length and known angles.

jeff hammond

you could get all those lengths by dividing the triangle into 2 right triangles then using trig.

once the maths get involved to that degree, it'd probably be better to solve with grasshopper..
was just wondering if anyone can see some sort of geometric relationship in which things like mid snaps and mirroring and ??? can be used to solve.
?

pilou

First speedy idea
Make your form a surface then
Seems you can use the "magenta" pinky inference ? (I don't remember the color)
Infinite solutions or maybe no solution for some dimensions?

Imperial unities are chiness for me!
And problem seems more funny when your quadrangle is not plane and find the biggest or longer rectangle!

Seems some Fredo tools can help!

pilou

Yes the are an infinity of quadrangles!
If you make the inverse, Draw the rectangle on view top
You can draw any number of little triangle at the end of the rectangle
A anywhere on the left for have possibility to draw a triangle

Then extend this little triangle and draw 2 big sides
Take a longer smaller than the end of rectangle so a constraint
Take a another longer smaller than the end of rectangle on the other side so a constraint again

slbaumgartner

Yes Frenchy, in general. But I took Jeff's challenge to mean that all sides and angles of the quadrilateral were fixed and known. I think you can get from here to there using the cosine rule and some algebra, but banging my head against the possibility of a purely geometric, non-trig solution first.

jeff hammond

@slbaumgartner said:

but banging my head against the possibility of a purely geometric, non-trig solution first.

don't bang tooo hard
fwiw, I saw this question at a rhino forum:
http://discourse.mcneel.com/t/geometry-riddle/14342

at first, I thought it was going to be do-able via geometry but since then, a few very smart people have commented in the thread and/or posted solutions-- all of which seem iterative by nature (in some sense-- trial&error)

nobody has proven that a more simple (straight-forward) geometric approach isn't possible so that's why there's still a shred of hope but again-- don't bang your head too hard.

@frenchy-- yeah, doing it backwards (start with the 'board' and draw the 'room' around it) has infinite solutions.. (it also shows that an infinite number of gons can work. (4,5,6,etc) sided shapes can be drawn around the rectangle.
the question is the other way though.. start with a known shape and bisect it with a rectangle.

(also-- units aren't important.. can be any unit and doesn't need to be imperial)

slbaumgartner

For shapes with an odd number of corners more info is necessary, as there is no innate definition of "opposite corner".

jeff hammond

@slbaumgartner said:

For shapes with an odd number of corners more info is necessary, as there is no innate definition of "opposite corner".

right- once you've come up with a method, more info would be needed in the case of odd or multi numbered corners.. though i think the initial problem boils down to 'fit a rectangle between two angles".. the number of sides is irrelevant..

slbaumgartner

@jeff hammond said:

@slbaumgartner said:

For shapes with an odd number of corners more info is necessary, as there is no innate definition of "opposite corner".

right- once you've come up with a method, more info would be needed in the case of odd or multi numbered corners.. though i think the initial problem boils down to 'fit a rectangle between two angles".. the number of sides is irrelevant..

[attachment=0:qfxswsu7]Untitled.jpg[/attachment:qfxswsu7]

Right, but when you eliminate the full shape you have to provide more info to pin down the problem. You can move and rotate either corner without changing its angle!

jeff hammond

ok

TIG

You need to know several things...
The distance between the two points where the angles are located [the line forming one diagonal of the quad']
You also need to know the four angles subtended to that line by each of the four lines forming the two angles.
From that you can do some calculations to give the range of x/y sizes for the potential rectangles that will touch all four lines [aka the sides of the quad'].
And i assume that no one quad' internal angle can be >180 deg ?
Does a rectangle that touches a side at the vertex itself 'in the limit', also 'count' ?

jeff hammond

@tig said:

Does a rectangle that touches a side at the vertex itself 'count' ?

i suppose there are a few cases where a corner of the rectangle would coincide with a vertex of the quad (it would be the point at which if the angles were offset any further, that particular quad wouldn't have a solution at all.. not all quadrangles will be solvable)

for all intents and purposes, the .skp in the original post has an unsolved shape in which all the things which need to be known are known.. so, using that one as an example, can you find a solution for it?

Box

@jeff hammond said:

has an unsolved shape in which all the things which need to be known are known.. so,

There could be hidden unknowns or maybe known unknowns, perhaps even unknown unknowns that we either do or don't, may or may not, have known about whether we know about not knowing about them or not.

jeff hammond

slbaumgartner

@box said:

@jeff hammond said:

has an unsolved shape in which all the things which need to be known are known.. so,

There could be hidden unknowns or maybe known unknowns, perhaps even unknown unknowns that we either do or don't, may or may not, have known about whether we know about not knowing about them or not.

I don't know about that...

TIG

Who knows ?
...
Someone does...

pgarmyn

From TIG's drawing
There are 6 constraints A1, A2, B1, B2, distance A-B and "thickness" of the inscribed rectangle
Thus we need, twice, a 7*7 matrix to solve each of the two corners (A-B). Who can define them?

The solution must give the distance (radius of the tangent circle) between the axe of the inscribed rectangle and each point (A and B).

sdmitch

http://youtu.be/_0TY6HHL25Y
A graphical solution.

jeff hammond

@sdmitch said:

A graphical solution.

hey sdmitch..
that's only doing one side of it though.. the difficulty comes when trying to fit the rectangle between two corners.
do you have a solution for that?