Which command to flatten object to Cplane?

Front view , I wish to flatten the object I am viewing in FRONT view to the Cplane behind it .
I expect to see a thin yellow line, a planar item only visible in front view.

FlattenSrf when viewed in top view doesnt give me a thin yellow line, but a shape.

Smash, squish, steamroller, whatever it might be called !

Object is 3D surface.


Hi Steve - ProjectToCPlane should do it.


Steve, if you only want the projected silhouette if the object, use Silhouette and then ProjectToCPlane to project the resulting curve(s) to the CPlane.

Pascal,…I tried ProjectToCplane and the object still remained ‘thick’.

Can one flatten it ?

Hi David, may run with just the outline if so, thanks, that’ll give me something.


What do you mean by “thick”? The result of ProjectToCPlane should be a zero thickness curve/surface/polysurface… However depending on the shape of the original object the curve/surface/polysurface may fold back on itself.

If you project the silhouette you can create a flat surface using PlanarSrf with the projected silhouette. You may need to join the parts of the projected silhouette first.

Hi David, Steve - Rhino will try not to project objects that will create degenerate surfaces - like a box being crunched so its sides become lines. I must say it does some unexpected things to skewed boxes though…

Still looking at this…


Not always true. See the attached file with an ellipsoid projected using ProjectToCPlane. A folded surface was created.EllipsoidPTCP.3dm (62.7 KB)

Looks like ProjectToCPlane won’t project an object containing a surface which is normal to the CPlane. The attached file has two boxes added. One with surfaces normal to the CPlane and it won’t project. The second box was rotated so no surfaces are normal to the CPlane and it does project as a folded polysurface. EllipsoidPTCP2.3dm (96.9 KB)

by thick, I simple meant it still had 3D shape, hadnt become zero/atom/pixel thickness !

I have just drawn as a test a sphere, given it projectToCplane, and its still spherical.

doesnt get flattened.

Silhouette solves the task I needed this for. Still wonder though how any object can be ‘flattened’.


I just tried a sphere. ProjectToCPlane does not work if the seam is perpendicular to the CPlane. “Unable to project 1 object” appears on the command line.

If the sphere is rotated so the seam is not perpendicular to the CPlane then ProjectToCPlane works.
EllipsoidPTCP3.3dm (127.8 KB)

attached the object, will it flatten ?

seen in FRONT view, I need to flatten it against the ‘back wall’ so to speak !

I need it a thin line in TOP view !

and how is it done ?

flatten test.3dm (620.5 KB)


ProjectToCPlane won’t flatten the polysurface in the Front view, but does flatten the perimeter curve which was also in the file.
flatten test DC.3dm (708.0 KB)

I select the object, run the ProjectToCPlane command in front view and it says unable to project one object.
How did you get the perimeter to do so, or was that silhouette in use ?

Like to know though if the object can be flattened or if not why not. Pascal ???


The perimeter curves were in the file you uploaded. I selected the curves and the polysurface. The curves flattened; the polysurface did not flatten.

I see,…ok… I used your silhouette method to do so.

So that just leaves us with… how can one flatten the object itself along with surface ?


What do you want to achieve by “flattening” the surface?

For single surfaces you can also use SetPt. To flatten on the XZ plane start SetPt, select the object, check only the “Set Y” box, and enter 0 as the value to set to.

Plan was I was needing a quick 2D view of the object with surfaces still there., I didnt want to be trying to resurface it.


So is this a bug? With V6, when I use ProjectToCPlane, I get the message ‘Unable to project 1 object’. I’d expect it to work as David has described Silhouette followed by ProjectToCPlane works. If it doesn’t work this way, shouldn’t it?

Isn’t projecting a 2D silhouette of an object on a CPlane the intended use? If not, what is?

Hi Mitch - the goal of ProjectToCPlane is to flatten the actual geometry to the plane. This is not the same as flattening the peerimeter curve. Some geometry cannot be flattened - that is what you are running into, I imagine.