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intersect-thumb

New users of SketchUp are often confused when trying to intersect geometry. The intersection edges "seem" to be created however their faces are not separated by them and they cannot delete excess geometry. In a little bit complex model it is not only frustrating but also confusing as many of the other entities can hide the main "culprit"; in which editing context we are applying the Intersect tool.

In this SketchUp tutorial, we shall examine the different reasons and how the editing context can affect how intersection is created and behaves.

In the scene below, there are two groups we wish to intersect with each other. Select both and Intersect Faces either With Model or With Selected (in this case, as there is no other geometry around, it is indifferent).

Basic intersection in SketchUp

Notice however that neither group has been affected by the intersection. If we move them apart, we can see both intact and we can also see the intersection lines created - but they are only hovering in the thin air. Unless you wanted specifically this (why not, there can be cases), most of the times this is not the desired effect.

The reason simply is: intersection always works in the editing context you are in (in this case, at top - model - level) so the lines are also created in this context and since we have the geometry we want to affect inside the groups, they remain unaffected (note that groups and components are the way to separate geometry and keep it unaffected by other geometry so this is actually the intended behaviour).

Basic intersection in SketchUp

All right, let's see what happens if we enter the editing context (aka "edit") of one of the groups; double click on it then select all (with a selection box or triple clicking or Ctrl+A etc. - actually, if you triple click the group when opening it (instead of double clicking), SU will also auto-select all its inner geometry). Right click again and Intersect (now we also have "Intersect With Context" - more about the different kind of intersection in our next tutorial).

Basic intersection in SketchUp

As we can see (after hiding the other group), the intersection lines have now been created in the group's editing context and they even affect geometry dividing the faces. Now our problem is however (if it is a problem and we did not intend it this way) that these edges only appeared in this group (the other, now hidden group is still unaffected).

Basic intersection in SketchUp

If we want both groups to be affected (and maybe eventually create one set of geometry of them), we need to get their geometry into the same editing context - either a level higher (in our case that's the top, model level) or my moving the geometry of one into the other. Below we do the first variation: select bot, right click and "explode". 

Basic intersection in SketchUp

Now the groups vanished, all the geometry is at top level and can be intersected affecting all. This may often not be what we want however (after all, we made the groups to separate geometry from other geometry and now we are back where we probably started).

Basic intersection in SketchUp

A more convenient method (to keep geometry separated inside a group) is to edit one, select all, cut...

Basic intersection in SketchUp

...or explode one (now everything is auto-selected) and cut...

Basic intersection in SketchUp

...edit the other group and Edit menu > Paste in place (one of my favourites - best is to set up a shortcut key under Preferences > Shortcuts - I have Shift+Ctrl+V).

Basic intersection in SketchUp

Now you can perform the intersection operation affecting the geometry sets of both (previous) groups while you are still separated from other parts of the model inside a single group.

Basic intersection in SketchUp

Just a glimpse into the next tutorial: above I used "Intersect With Model" while the below screen-shot shows ("Intersect With Context". The difference will be that anything outside the group (but intruding into the space it occupies) will affect the intersect operation above but only what's inside will be affected below.

Basic intersection in SketchUp

In our case (as there's nothing else in the model) it's indifferent but see the intersection edges nicely created below.

Basic intersection in SketchUp

Registered members are welcome to discuss this tutorial on the forums.