- [Instructor] In the previous movie,
we deleted polygons based upon a volume selection.
And that's great for non-destructive workflows.
I want to detour for a moment
into another way to select and delete polygons,
and I'm going to do it this way
to illustrate the issues that may arise
from a principle known as topology dependence.
Topology is the structure of a model,
how all of its sub-objects are connected to one another.
And dependence, means that certain effects
are going to depend upon the topology.
And if you're not careful,
and you don't quite understand how this works,
you will run into trouble
when you change the level of detail on a parametric model.
Let's take a look.
I've got my monitor stand here, I'll select it,
and in the Modify panel,
once again, we've got a box that's a parametric primitive,
it's being tapered and then bent.
I want to delete the polygons at the bottom here.
And to do that, I can use an Explicit Selection modifier.
In other words, I can actually click on individual polygons
and select them.
From the modifier list,
at the top, we have selection modifiers,
let's choose Poly Select.
And now we have a bunch of sub-object types
we haven't seen before.
This is for a polygon mesh,
we have a different set of sub-object types,
and we can open up the sub-object types up here too,
and see their names.
We've got vertex, which is point,
edge, which is a line between two points,
border, which is an open edge,
it's an edge that only has a polygon face on one side.
And we got polygons of faces which are flat surfaces
that are actually renderable,
and finally, elements which are collections of faces
or collections of polygons.
In this case, we want to be in polygon or face selection mode,
so choose polygon sub-object mode.
And if we click on a polygon,
it'll be selected and highlighted in red.
I could go around here holding down control,
and clicking on each one of these one by one
but that's kind of tedious.
I can make this a lot easier by using paint selection mode,
and I can just drag my mouse
across the polygons I want to select.
So I'll go up to the selection region types
on the main toolbar.
The default is Rectangular Selection Region.
Hold the mouse button down, you'll get a fly-out
and choose the bottom icon which looks like an aerosol can,
and that is called Paint Selection Region.
And when you hover your mouse over some surface,
and then click and drag,
you'll be able to select whatever's underneath the cursor.
But notice when I did that,
if we orbit with Alt + Middle mouse button,
I selected these faces on the top and the front.
I didn't want that, I only wanted to select the polygons
that were under the cursor,
and additionally facing towards me,
and not behind some other polygon, okay?
So to illustrate,
we've got some optional culling ability here.
So this means, if one of these switches is on,
then polygons that are either pointed away from me,
or pointed away and hidden behind other polygons
will not be selected.
I choose back-face,
and then click off of the object to select nothing,
we'll do our paint selection again here, click and drag.
Notice that I'm not selecting anything on the top,
because those faces are pointed away or back-facing,
but I did select these ones here,
because they are pointed towards my current view.
Even though they're behind something else,
they're still are being selected.
The mode I want to be in actually is occluded,
so turn that on.
Once again, deselect everything by clicking off the object,
and then click and drag to select some polygons.
Okay, we can remove polygons
from the current selection using the Alt key.
So hold down Alt
and just click on some polygons or faces to deselect them.
Now I've got the selection I want
and I can add my Delete Mesh modifier from the list,
and the modifier list choose, delete mesh,
and then those polygons have been removed.
Now so far, this is very similar
to the method we saw in the previous movie
using Volume Select.
Because actually, we could go down
into the polygon sub-object mode in the Poly Select,
and we could actually change our selection.
And that would pass up the stack
to the Delete Mesh modifier.
So just illustrate, if I pull down control
and select some other polygon here,
getting closer with Ctrl + Alt + Middle mouse.
If I turn show end result on,
we can see that that polygon got deleted, okay?
So this is non-destructive to a certain extent,
in as much as we can change the selection,
and that will be rippled up the stack.
I'll unselect that polygon with Alt and click.
Okay, but we could run into trouble.
If for example, we wanted to change
the overall level of detail
by adjusting the number of segments on the box.
So let's take a look at this, okay?
I can go down to the bottom of the stack
and click on the box base object,
and I get this warning that pops up.
A modifier exists in the stack that depends on topology.
Changing parameters may have undesirable effects.
The dependent modifier is Poly Select,
are you sure you want to continue?
Well, you know what?
When most people see this dialogue box
they have no idea what's going on
or what's meant by this language.
So, what it's trying to tell us,
is that if we change the topology,
it's going to break our model.
Do we really want to change topology?
Well, this time I'm going to say yes,
because I want to show you the issue.
So I'll click yes,
and we are down at the bottom of the stack
in our box base object.
And we can see here,
which parts of the object have been deleted
by our Delete Mesh modifier.
Let's now try changing up the level of detail
by adjusting the segments.
I can reduce the number of segments,
and we can see if I have fewer length segments.
Then more of my model is being deleted,
why is this happening?
Well, it's happening because,
selection using this Poly Select Modifier
operates on some invisible ID numbers.
So not to go too deeply into the weeds once again,
each vertex or each point on the model
has a unique numeric ID number.
So every single point on this object has a number.
We don't normally see that,
but it does have an effect in cases such as this.
When we change the level of detail
by adjusting the number of segments,
we are shifting those vertex numbers around on the model.
And that in turn means, different vertices, different faces,
are going to be selected by our Poly Select Modifier,
and then in turn, different faces or different polygons
are going to be deleted.
Well, technically it's the same polygons
that are being deleted,
but they're in different locations on the model.
Okay, so that's what is meant by topology dependence.
And we can get ourselves into a lot of trouble here.
This is a fairly innocuous situation,
where it's actually relatively easy
for me to get back to where I was,
I can just set my length segments back to a hundred,
and there's no real problem here.
But there are lots of situations
where topology dependence will get you into trouble,
so you need to understand how it works.
Okay, so I've just restored it back the way I had it.
So, if you need to select certain areas of the model,
and preserve a non-destructive parametric workflow,
then I recommend that you use the Volume Select modifier
instead of an Explicit Selection modifier
such as Poly Select.
That is how topology dependence affects parametric modeling.