Rotate and Intersect a plane

Garry K

I'm probably just too tired to see it - oh well.

I first create a face which is parallel to the Y axis and offset 1/2"
I then create a line and intersect the plane of the face.

The line
10.000000",0.000000",5.000000"
10.000000",5.000000",5.000000"

The plane
-0.0
1.0
-0.0
-0.5
the result of the intersect
(11", 0.5", 5.5")

I was expecting ( 10", 0.5", 5" )
It appears that the the plane may have its point at the origin.

Here is the code

y = 0.5.inch
face = grp.entities.add_face( [[0,y,-10],[0,y,200],[200,y,200],[200,y,-10]] )

p1 = [ 10.inch, 0.inch, 5.inch ]
p2 = [ 10.inch, 5.inch, 5.inch ]
line = [ p1, p2 ]
p3 = Geom.intersect_line_plane( line, face.plane )

Garry K

I should mention that I am planning on rotating the face. If there is a way to simply rotate a plane then that would even be better.

TIG

A plane is defined by:
plane = [a_point, a_normal_vector]
So you can rotate a vector using a rotation-transformation:
tr = Geom::Transformation.rotation(a_point, an_axis_vector, an_angle)
Remember that:
angle**.degrees**
allows you to specify it in degrees - no need to mess on with radians/PI etc.
Also there are built-in constants for:
ORIGIN [0,0,0], X_AXIS [1,0,0], Y_AXIS [0,1,0], Z_AXIS [0,0,1]
You can reverse any vector with:
vector.reverse
To transform a vector use:
vector.transform!(tr)
which rotates the vector from where it is currently.

You can use:
entities.transform_entities(transformation, objects)
'objects' can be vertices, edges, faces, group, component_instance etc
These can move, scale or rotate.
Note that Points must be transformed with:
point.transform(transformation)

Garry K

Thanks TIG.

Ah - I see this could be handy if you have 2 lines and want to see where they would meet even if you have to extend one or both lines.

Essentially if I want to determine if the point returned from intersect_line_line is on both lines - which answers the problem that I need to solve - then I have to perform an additional check to see if the resulting point is not nil and lies on the line.

So I can rotate 2 points around the origin ( which is what I need to do ) and then convert the rotated edge to a vector.

cos_theta = Math.cos( angle )
sin_theta = Math.sin( angle )

x3 = cos_theta * x1 - sin_theta * y1
y3 = sin_theta * x1 + cos_theta * y1

x4 = cos_theta * x2 - sin_theta * y2
y4 = sin_theta * x2 + cos_theta * y2

And since I want to rotate a vertical plane z stays as 0 and I simply pop in dx and dy of the edge for the vector and use the first point as is. Then I can use intersect_line_line

TIG

You are muddling 'edge' and 'line'.
In SketchUp an edge has a line.
edge.line >> [point, vector]
To see if two lines intersect
geom.intersect_line_line(line1, line2)
returns a point if they do, or nil if they don't.
If you have a point it has 4 possible vertex matches: if it falls on the start or end of an edge, and there are 2 edges...
point==edge1.start.position point==edge2.start.position point==edge1.end.position point==edge2.end.position
returns true or false
If the point falls on an edge you test it thus:
point.vector_to(edge1.end.position)==edge1.line[1]
and
point.vector_to(edge2.end.position)==edge2.line[1]
If true the intersect's point falls on the edge [between the start/end] and not on a vertex.

Garry K

Sorry - I am not making myself clear. From a CAD point of view. What I have drawn are 2 lines that do not intersect.

I initially assumed ( at 2:00 in the morning ) that the intersect_line_line method took 2 CAD lines as arguments.

I now realize that the method wants 2 mathematical lines expressed as a ray and in this case will return a point because mathematically these CAD lines are simply 2 points on an infinite line and these 2 infinite lines do indeed intersect.

All I was trying to do was find a method that would give me a point if the 2 lines that I drew cross each other.

In CAD these are 2 lines

TIG

Please stop calling them 'lines' - I know that's the CAD name but in SketchUp Ruby code they are best called 'edges' - I know some SketchUp tools refer to drawing 'lines' and there's even an API method add_line() that adds an edge BUT when we are talking code - especially if we are taking about edge.line - it's a recipe for confusion to muddle up the two terms...

I tried to explain the difference between an edge and a line [edge.line]...

The two perpendicular edges you drew will only return a point with the intersect_line_line() IF they intersect - i.e. they are 'coplanar'... looking down 'in plan' does not mean they do intersect if their 'z' values not equal...
Assuming they do return a point, then my previous post explains how you can then tell if that point falls 'in space', or either of the edges' vertices, or on an edge in between its start/end...
It'd be quite possible to slide your example edges around so that they then return a point 'in space' that is on neither edge...