NEED HELP with "Extrude" basics


(Cousin Hepcat) #1

Help, I’m reading through the following Blender documentation and get totally lost on the very first page of the section about “extrude” (in Chapter 6): I’ve understood everything else up to chapter 6, including the definitions of vertices, edges, and 3- and 4-point faces:

http://download.blender.org/documentation/htmlI/ch06s04.html

When I tried to do the “gus” gingerbread man extrusion example in the documentation, he came out pretty messed up looking, and I’m sure it’s because I don’t understand this section. When I finished with Gus, he looked ok from a distance, but in a zoom-in, had strange places where he seemed to “turn inside out”, parts that were “twisted”, multiple faces where there should’ve just been one, strange “folds” in his body, arms and legs that weren’t closed off properly that you could “look up inside of”, etc.


QUESTIONS ON EXTRUDE FUNCTION IN GENERAL

First can someone tell me if I even understand what “extrude” means?:

Webster’s defines “extrude” as, in the field of metals & plastics: To shape or form an object by forcing it through a die. I get a visual of the old Play-Dough toys where you put a flat plastic “die” with a cut-out shape of a circle, square, or star, turn the crank, and it forces the dough through that cutout hole into a 3-d object with that cross-sectional shape.

So, I’d assume the purpose of the Extrude function in Blender is to allow you to take any arbitrary-shaped 2-dimensional object (circle, square, star), and “push the dough through” to make a 3-d version which can be along a straight or curved path. I.e., a circle gets “extruded” into a cylinder or a curved garden hose.

So I’d assume there’s 2 things needed: One set of edges that describes an enclosed 2-D shape, and one path (curved or straight) going out (perpendicular) from that 2-D shape to extrude the final 3-D object along.

Are my assumptions right so far?


QUESTIONS ON “EXTRUDE” SECTION OF THE DOCUMENTATION

http://download.blender.org/documentation/htmlI/ch06s04.html

The first paragraph is followed by 6 bulleted items. I understand the first 2 of the bulleted items. The first describes how the Outside Edge-Loop is selected (i.e., your 2-D shape to extrude). The second bullet seems to describe the “side walls” getting created as you “pull” out the 2-D object into an extruded 3-D object.

Bullets 3 through 6, I totally don’t understand at all. Can anyone explain any or all of them? Maybe re-phrase them? Here are the text of the bullets, followed by my questions:

BULLET (3) TEXT: If the edges in the edge-loop belong to only one face in the complete mesh, then all of the selected faces are duplicated and linked to the newly created faces. For example, rectangles will result in cubes during this stage.

BULLET (3) QUESTIONS: Based on what was stated in Bullet (2), the “side walls” of the selected Outer Edge-Loop have already been created, so we’ve already started the extrusion. So wouldn’t it be Impossible for Any of the edges in the Outer Edge-Loop to belong to only one face? Doesn’t EVERY edge in the outer edge loop now belong to both a face in the flat 2-D surface we’re extruding, AND a “side-wall” of the 3-D extruded object mesh?

BULLET (4) TEXT: In other cases, the selected faces are linked to the newly created faces but not duplicated. This prevents undesired faces from being retained ‘inside’ the resulting mesh. This distinction is extremely important since it ensures the construction of consistently coherent, closed volumes at all times when using Extrude.

BULLET (4) QUESTIONS: What are “the selected faces” referred to in the first sentance? Are those still just the faces that make up the 2-D shape being extruded, or are those all the faces in the entire extruded 3-D object, including the “side-walls”? It says, “the selected faces are linked to the newly created faces…” What does it mean for faces to be “linked”? Does that refer to two or more faces which share one common edge, like the fold in a greeting card is the edge which “links” the front and back rectangular faces of the greeting card? It says, “…but not duplicated.” What does it mean for faces to be “duplicated”? Is that like when you select a mesh object such as a cube and make another copy of that mesh object, so you have two cubes in your scene? That doesn’t seem to make sense here…

BULLET (5) TEXT: Edges not belonging to selected faces, which form an ‘open’ edge-loop, are duplicated and a new face is created between the new edge and the original one.

BULLET (5) QUESTIONS: At this point, I’m totally lost. “Open Edge-Loop”? If it’s “open”, it’s not a “loop”, is it? So lost…

BULLET (6) TEXT: Single selected vertices which do not belong to selected edges are duplicated and a new edge is created between the two.

BULLET (6) QUESTIONS: Ditto.

Many Thanks.


(SoylentGreen) #2

Just partially. What you describe is valid for bezier curves and the like, not for mesh objects.

Extrude for mesh objects duplicates selected vertices and creates edges and faces between the old and the new vertices. Now you can move the vertices in the direction you like.

BULLET (3) TEXT: If the edges in the edge-loop belong to only one face in the complete mesh, then all of the selected faces are duplicated and linked to the newly created faces. For example, rectangles will result in cubes during this stage.

BULLET (3) QUESTIONS: Based on what was stated in Bullet (2), the “side walls” of the selected Outer Edge-Loop have already been created, so we’ve already started the extrusion. So wouldn’t it be Impossible for Any of the edges in the Outer Edge-Loop to belong to only one face? Doesn’t EVERY edge in the outer edge loop now belong to both a face in the flat 2-D surface we’re extruding, AND a “side-wall” of the 3-D extruded object mesh?

Just forget the technical details and try to understand them later. Extruding for mesh objects is totally natural to do and very easy to understand.

Select vertices, press “E”, choose Region (normally), move the vertices, you’re done.


(jaycun) #3

Although, I don’t really encourage this. . . I never really have read the manual.

I learned Blender mainly through experimentation. I saw what worked and how it worked. I also picked up a lot of tips from this forum. I use the manual as more or less a reference for when I run into a brick wall.


(Jason van Gumster) #4

Think about extrusion (when it comes to meshes) as physically linked duplicates and you’re life will get easier. You select some vertices (be they a single flat plane, or a bunch of verts on a previously created model) and when you press the EKEY, those vertices are duplicated… with edges connecting them to the originating verts.

If you select a group of verts that form a face (or a couple faces), those faces are duplicated in the extrusion, but the original face(s) get deleted to keep the overall mesh hollow (no need for an unseen face inside the mesh… unless, of course, you’re starting from single plane… then i think in that special case, the orginal face is retained).

Extruding a single vert just produces a second vert with an edge that connects to the original.

If none of this makes sense, I can post some images that may explain things better. Of course, about 5 minutes of experimenting with the EKEY and most of it should explain itself.


(Fade) #5

Place claimer: I’m making up some pictures to explain these concepts now. Will edit this post when done.

Pics done! (obviously :wink: )

question 3
http://img193.imageshack.us/img193/6094/q36sx.th.jpg

question 4
http://img193.imageshack.us/img193/1728/q48am.th.jpg

quesiton 5
http://img193.imageshack.us/img193/8545/q55hh.th.jpg

Fweeb answered question 6 in an as much detail as you need.

If I’m off in any of the pictures with my explanations, feel free to correct me.

Hope this helps