There are many ways to trim a curve in a Grasshopper definition, and you will need to use a different strategy for every circumstance. In this video I try to introduce a few simple methods that can be used to start thinking about how this works parametrically.

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On instagram: @parametric_phil
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Essential learning resources for GH and Rhino:
   • Resources for Learning Rhino and Gras...  

RHINO:

Rhino Tutorials from McNeel
https://www.rhino3d.com/tutorials

Rhino Command List
http://docs.mcneel.com/rhino/5/help/e...
To find specific command catagories:
click "Open topic with navigation" at bottom of any command list webpage

Rhinoceros User Forums
https://discourse.mcneel.com

Simply Rhino - Rhino3d Video Tutorials
   / @simplyrhino3d  

GRASSHOPPER:

Grasshopper Primer
http://grasshopperprimer.com/en/

Grasshopper Tutorials from McNeel
https://www.rhino3d.com/tutorials

Grasshopper Component Reference
https://rhino.github.io/

Rhinoceros/Grasshopper User Forums
https://discourse.mcneel.com/c/grassh...

Original Grasshopper Forum (Legacy Forums)
https://www.grasshopper3d.com/forum

Food4Rhino (tons of add-ons for GH and Plug-ins for Rhino)
https://www.food4rhino.com/

Nick Senske on Youtube (Great GH videos from many years ago. I can't speak to his non-grasshopper content, but it's probably awesome.)
   / nsenske