How to Use The Rail extrude Tool Effectively

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In this tutorial you'll learn the many ways in which the often misunderstood Rail Extrude Tool can be used to your advantage - and you'll even discover how it will work for you when you are sometimes suffering from lack of design ideas. We'll start with the simple stuff and then progress to something a little more advanced. Unless otherwise stated this tutorial is aimed for users of Lightwave 5.6 - the techniques are perfectly adaptable to higher versions however.


1) Previous Step | Next Step

A while back I was asked - "How do I make a pipe - I mean a real bendy pipe following loads of twists and turns?".

Now most people would probably make a cylinder with an appropriate number of divisions and then use the bend tool in a select few areas to achieve what they're looking for - this can be tedious however. Another technique is to knife a cross section or two where you'll need a bend and then use rotate on the cross section points etc.. etc.. - All perfectly acceptable - but time constraining and they don't yield brilliant smooth surface results. Now how can we do this easily?

If you haven't already - boot up Modeler and in the first layer make a disc - a simple flat one-poly disc - size doesn't matter. Now go to Layer two - with Layer one (The Disc) as the background layer. Make a spline curve with the first point near the centre of the disc in the view that you're making the curve in.

The easiest way to make a spline curve is to go to Polygon menu - Points and then right click where you want the points to be - when finished press "CTRL" & "p" and you'll make a spline curve. Then manipulate the points in all views to get a nice wiggly curve - a bit like this:

Spline Curve Created with Disc in Background

2) Previous Step | Next Step

Now before we go any further - you can if you wish download this file here - it will open in Modeler as layers - now we should pause to understand a bit about this extrude tool and the options menu that you'll get when you choose rail extrude.

Whenever you extrude -- the polygon that you're extruding must be selected (or everything in the active layer will be extruded) and it must be in the top layer with the curve in the bottom or background layer - as the extrusion will follow the curve. When you press rail extrude - you'll see this Menu:

Rail Extrude Menu

For best results the polygon to be extruded should be at the start of the curve and perpendicular to it - but this isn't written in stone. Now as for the segments: segments are basically the number of divisions or "slices" along the length of the extrusion

This will distribute the divisions at optimal spacing as per the orientation of the curve
Uniform Lengths:
This will distribute the divisions at equal distances regardless of the shape of the curve
Uniform Knots:
This will distribute the divisions evenly between the Knots {points} of the curve
This is an important one:-- when activated the divisions will orient themselves to the shape of the curve - necessary for a good pipe - when this option is off the poly's will be extruded but will stay the same as the original - they'll follow the curve but they'll stay "flat"

For Lengths and Knots when using these options you specify the number of divisions manually depending on what you require.

3) Previous Step | Next Step

Right let's look at what happens with each of these options:

The step for making the extrusion is the same for each case - you have the poly in the foreground, the curve in the background and select Rail extrude: Please bear in mind that you may have to flip the polygon to get the mesh to look correct - I find that generally the polygon should face away from the start point. You can also flip the start point of the curve.

When left on Automatic with Orientation on...

A nice smooth pipe! (smoothing angle for pipes - 60 degrees usually gives great results) - now if Polygon count isn't a problem - you're done right here! But if it is - create the original disc with fewer poly's and we can specify the divisions manually with Lengths and Knots:

Automatic - Orientation On

When using lengths - pick say 60 divisions and leave orientation on and you'll get this:

As you can see, more or less the same - but with the divisions evenly spaced along the length of the curved mesh. Now there are straight-ish bits of pipe where divisions aren't really necessary - so what can we do?

Lengths - 60 Divisions - Orientation On

Right - this time we'll use Knots with 50 divisions and orientation left on:

As you can see visually almost the same but with 240 polygons less than the other.

Knots - 50 Divisions - Orientation On

Think about the possibilities - you're not limited to just discs - for example:

Irregular shaped polygon extrusion

Right - you can see the potential here right? But - here's the good bit - you're not limited to just one curve!


4) Previous Step | Next Step

What you just followed was single rail extrusion - Now we go on to

Multiple Rail Extrude

Let's say for want of anything better - that you wanted to make a new spacecraft - an organic one - loads of weird and wonderful shapes - why don't we let Modeler do some creative thinking for us. Make a base polygon for the initial shape and then make four - YES - FOUR spline curves for the top, bottom, left and right sides of the polygon shape you made - A bit like this:

Multiple Rail Extrude Setup

You can download these layers here: All I've done is mirrored the x curve and made a separate top and bottom curve and of course the base poly. Switch back to the polygon layer and apply a surface with 60 degree smoothing if it's not already been one. These curves are going to act as shaping curves for the polygon.

Now this time when you choose rail extrude - a different requester will say hello to you -

It'll look like this:

Multiple Rail extrude Menu

Right what do these options do?

Well by now you should know the difference between Lengths and Knots from the previous steps and how Automatic or Uniform with Manually set divisions work. But what about Strength? Orientated? Scaling?

Strength: The value here determines how "aggressively" the curves fight for control of the mesh as it's extruded. There won't be a visual amount of difference unless you have scaling deselected - but the higher the value - the more the final mesh will try and "hug" the curves you've created.

Orientation: well we've covered this - the divisions will try and orient themselves to the shape of the curve - mostly in a perpendicular type fashion in relation to the curve. If deactivated the divisions stay parallel to the original polygon.

Scaling: If the curves spread apart - as they do here - the mesh will automatically stretch outwards along those axes as well This command gets a bit complicated - but let me try to explain. If we didn't have 3 separate curve shapes - like we have in the above example - you could create the two x-curves as above and then extrude with scaling and the mesh would be scaled on the y-axis in relation to the shape of the curves as well.

The effect on the original polygon will be affected by how far away the poly is from the curves at the start point - further away = less affect and vice versa.


5) Previous Step | Next Step

Right let's rail extrude this poly and see what happens - to start with leave everything as default - like the requester above - you'll get this:

Default Options

Cool eh? Imagine having to spline patch that baby!

Right now I'll run through the options and how they affect the mesh using a table with just the resultant OpenGL previews

Just click undo and redo the operation with the different settings:

  1. Lengths - Uniform - 120 divisions
  2. Strength=2.0
  3. Orientation & Scaling
  1. Knots - Automatic
    Uniform with 120 divisions produces a similar result
  2. Strength=2.0
  3. Orientation & Scaling
  1. Knots - Automatic
    Uniform with 120 divisions produces a similar result
  2. Strength=2.0
  3. Just Scaling
  1. Knots - Automatic
    Uniform with 120 divisions produces a similar result
  2. Strength=2.0
  3. NO Scaling or Orientation
  1. Lengths - Automatic
  2. Strength=1.0
  3. Scaling & Orientation
    Note how the strength's lower value lessens the curves affect?
  1. Lengths - Automatic
  2. Strength=5.0
  3. Scaling & Orientation
    Note how the strength's higher value exaggerates the curves affect?
  1. Now lets play with the original poly for a shadowy feel ;)
How the various setting affect the resultant extrude

I hope now that you can see the tremendous potential of the Rail Extrude tool - but just to make sure you get the message -

Remember that quite often with Lightwave you have to think laterally - do it the easy way - not the hard way - ESPECIALLY if the resultant mesh doesn't need to be superbly accurate.

Take this for example:

Can you Guess what it is yet!?

Just make a disc - and four curves as shown (download layers here) then rail extrude with these settings:

Lengths , Uniform, 60 divisions, Strength = 1.8, No Orientation - Scaling activated


Now, show me a quicker way of modelling a basic leg mesh - and I'm all ears! ;o)

Tutorial ©2000 F. Passaro (E-mail)


© 2000 Kier Darby and Alternate Perspective 3D Ltd.