True projection (not cut)

[chennes]

I'm working on finishing up the OpenSCAD workbench's CSG import, and one of the final functions that OpenSCAD supports is projection. We have an implementation for a "cut" projection, which is straightforward, but now I'd like to implement true projection, e.g. generating basically the "shadow" of an object on the XY plane. I thought that by creating a Part::Plane object and then using plane.Shape.project(stuff_to_project) would do it, but either that's not what project does, or I'm using it wrong.

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>>> sphere = Part.makeSphere(10)
>>> plane = Part.makePlane(100,100)
>>> projection = plane.project([sphere])
>>> projection.Wires
[]

Suggestions, or alternate ways of going about it?

[Roy_043]

maybe look at:

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projection.Edges

[chennes]

So interestingly, there are two edges, but their bounding boxes appear to be zero (ish):

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>>> projection.Edges
[<Edge object at 000001AAECF828A0>, <Edge object at 000001AAECF84A20>]
>>> projection.Edges[0].BoundBox
BoundBox (0, 0, 0, 3.06162e-15, 4.49928e-31, 0)
>>> projection.Edges[1].BoundBox
BoundBox (0, 0, 0, 3.06162e-15, 4.49928e-31, 0)

[Roy_043]

I am not sure what is going on, but this seems to work OK:

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box = Part.makeBox(10,5,2)
plane = Part.makePlane(100,100)
projection = plane.project([box])
projection.Edges[2].BoundBox

=> BoundBox (10, 0, 0, 10, 5, 0)

[jmcornil]

Hello

From the tries I made, I think that the projection() method projects the edges of the shape which is to be projected.

Please run each of the following exampes in a new (fresh) FreeCAD file !

Example 1 : a box rotated

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box = App.ActiveDocument.addObject("Part::Box","Box")
box.Placement = App.Placement(App.Vector(0,0,20),App.Rotation(App.Vector(0.29582,-0.714173,-0.634388),66))

plane = App.ActiveDocument.addObject("Part::Plane","Plane")
plane.Length=60.00
plane.Width=60.00
plane.Placement=App.Placement(App.Vector(-30.00,-30.00,0.00),App.Rotation(App.Vector(0.00,0.00,1.00),0.00))
plane.ViewObject.Transparency=80

App.ActiveDocument.recompute()

proj = plane.Shape.project([box.Shape])
Part.show(proj)

App.ActiveDocument.recompute()

Example 2 : a sphere just translated the project of which seems to be a point.

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sph = App.ActiveDocument.addObject("Part::Sphere","Sphere")
sph.Placement =  App.Placement(App.Vector(0,0,20),App.Rotation(App.Vector(0,0,1),0))

plane = App.ActiveDocument.addObject("Part::Plane","Plane")
plane.Length=60.00
plane.Width=60.00
plane.Placement=App.Placement(App.Vector(-30.00,-30.00,0.00),App.Rotation(App.Vector(0.00,0.00,1.00),0.00))
plane.ViewObject.Transparency=80

App.ActiveDocument.recompute()

proj = plane.Shape.project([sph.Shape])
Part.show(proj)

App.ActiveDocument.recompute()

Example 3 : the same sphere rotated so that its "boundary half circle" is parallel to the plane ;
it is this half circle which is projected

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sph = App.ActiveDocument.addObject("Part::Sphere","Sphere")
sph.Placement =  App.Placement(App.Vector(0,0,20),App.Rotation(App.Vector(1,0,0),90))

plane = App.ActiveDocument.addObject("Part::Plane","Plane")
plane.Length=60.00
plane.Width=60.00
plane.Placement=App.Placement(App.Vector(-30.00,-30.00,0.00),App.Rotation(App.Vector(0.00,0.00,1.00),0.00))
plane.ViewObject.Transparency=80

App.ActiveDocument.recompute()

proj = plane.Shape.project([sph.Shape])
Part.show(proj)

App.ActiveDocument.recompute()

By the way, I had a look to the Edges of a sphere and I found something I don't understand :

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e0,e1,e2 = sph.Shape.Edges

e0.BoundBox
# BoundBox (3.06162e-16, -7.4988e-32, 25, 3.06162e-16, -7.4988e-32, 25)

e1.BoundBox
# BoundBox (3.06162e-16, -1.22465e-15, 15, 5, -7.4988e-32, 25)

e2.BoundBox
# BoundBox (3.06162e-16, -7.4988e-32, 15, 3.06162e-16, -7.4988e-32, 15)

If e1 seems to be correct and I can see its Curve (the half circle)

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e1.Curve
#Circle (Radius : 5, Position : (0, 0, 20), Direction : (-2.44929e-16, -2.22045e-16, -1))

I don't understand e0 and e2, the Curve of which gvinig error

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e0.Curve
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: undefined curve type

I also have a look at the definition f the project() method : line 1983 of the file
https://github.com/FreeCAD/FreeCAD/blob ... ePyImp.cpp
but I must admit that I am not up to par on this sort of thing. If anyone could help it would be welcome (may be @Chris_G).

Unfortunately the only information I could find in the automatic documentation of python modules is:
"project (...)
Project a list of shapes on this shape "

Sorry to have made such a long post but I would like to understand and progress ...

[onekk]

from https://github.com/FreeCAD/FreeCAD/blob ... hapePy.xml

Seem to be also this costruct:

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    <Methode Name="makeParallelProjection" Const="true">
      <Documentation>
        <UserDocu>Parallel projection of an edge or wire on this shape
makeParallelProjection(shape, dir) -> Shape
        </UserDocu>

Hope it helps

Sources, mention:

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# include <BRepProj_Projection.hxx>

and 

# include <BRepAlgo_NormalProjection.hxx>

I suppose that these are OCCT things, maybe inspecting the OCCT sources or documentation will make some light on the matter, or maybe ask to @wmayer?

Regards

Carlo D.

[Chris_G]

I don't understand e0 and e2, the Curve of which gvinig error

An edge is a topological wrapper around one, or more, geometry curves.
These curves can be both a 3D curve, and / or 2D curves defined on a 3D surface (called p-curves).
Let's explore more deeply the 3 edges of a sphere.
Copy / paste the following scripts in the python console.
The first edge :

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sphere = Part.makeSphere(10)

# The 3D curve is undefined
sphere.Edge1.Curve

# Its length is 0
sphere.Edge1.Length

# Does it have a p-curve ?
curve2d, surface, placement, first_param, last_param = sphere.Edge1.curveOnSurface(0)
print("{} on {}".format(curve2d, surface))

# It is a 2D line on a Spherical surface
# What are the endpoints of this line ?
curve2d.value(first_param)
curve2d.value(last_param)

This is the 2D representation of the face of a OCCT Sphere :

2D_sphere.png (10.16 KiB) Viewed 1587 times

The above edge corresponds to the upper horizontal edge of the square.
It is the north pole of the sphere : a line in the 2D space of the sphere that vanishes to a 3D point in the 3D space

Let's have a look the second edge :

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sphere.Edge2.Curve
sphere.Edge2.Length
# This edge has a valid 3D curve

# Does it have also p-curves ?
curve2d, surface, placement, first_param, last_param = sphere.Edge2.curveOnSurface(0)
print("{} on {}".format(curve2d, surface))
curve2d.value(first_param)
curve2d.value(last_param)
# this edge corresponds to the right vertical edge of the square

# Does it have another p-curve ?
curve2d, surface, placement, first_param, last_param = sphere.Edge2.curveOnSurface(1)
print("{} on {}".format(curve2d, surface))
curve2d.value(first_param)
curve2d.value(last_param)
# So this edge is also the representation of the left vertical edge of the square

So, this "Greenwich meridian" edge is a wrapper around 3 different geometric entities :
- a 3D curve (a circle)
- the 2 lines2D in the parametric space of the sphere that are sewed together to form the seam of the sphere

The third edge is pretty similar to the first one, but represents the south pole of the sphere, and the lower horizontal edge of the square :

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sphere.Edge3.Curve
sphere.Edge3.Length
curve2d, surface, placement, first_param, last_param = sphere.Edge3.curveOnSurface(0)
print("{} on {}".format(curve2d, surface))
curve2d.value(first_param)
curve2d.value(last_param)

[chennes]

So it seems that the "project()" function is not going to give me quite what I need here. Obviously the TechDraw WB is able to get the 2D projection, so there's code someplace that does this, but I can't figure out where. Can any of our TD or BRep experts point me in the right direction?

Ping

Ping

Ping

[jmcornil]

Thank you very much @Chris_G for this good explanation !

But for the parameters of a sphere wouldn't it be rather
0 <= U <= 3.14 and -1.57 <= V <= 1.57
as we find with :

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sphere.Faces[0] .ParameterRange 

[Chris_G]

This is not what I get :

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sphere = Part.makeSphere(10)
sphere.Faces[0] .ParameterRange 
# ---> (0.0, 6.283185307179586, -1.5707963267948966, 1.5707963267948966)