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Modeling for Cg Viz Applications by William Alexander (wda@asapnet.net) - WDA Design Arched
Stairway TUTORIAL Part#2 The railing can be the most problematic modeling of the stair components, as discussed in the first part of the stairway tutorial. By utilizing the same methodology use for the stringers the handrail can be efficiently modeled, however more complex. Profiles vary due to having angled sections based on the rise/run of the common steps, location (inner & outer) and the progressive change for the flared area. Adding to the difficulty, we need to deal with matched profiles, common rail to match a turn or up easing stair part (fitting) and then the specific transition parts aligning with a gooseneck or volute with a set elevation (code) to match a newel post.This is where the traditional (master finish carpenter) construction techniques shine for layout & assembly. All done in place and as such matched to the stair carriage or better yet; the stair carraige is the jig for forming & fitting the handrails. Thus the approach used in the arched stair case tutorials, low mathematical calculations maximum use of geometric relationships. THE BIG PROBLEM 
The profile has been extruded on a 3d polyline oriented so the end & start profiles are parallel with the "front" view. Notice these two profiles, at the bottom of the stair is a cut and pasted section of the upper extrusion. The profiles in section show the rotation that is common and problematic. This is the big problem. Face modeling is the best solution, however to face model this entire railing would be mind numbing tedious. This is where CAD programs such as AutoCAD seem to shine for face modeling. Using the same methodology as the first Stairway tutorial for the stringers, we can efficiently model the railings and be set for a matching set of stair parts (fittings). Profiles, Sections & Fittings Determining Start / Finish and Matching The stair solid dictates the form and shape of the handrail (previous tutorial),
we will determine a few important developments and placement. Primarily are
the profiles, we need to construct our jig, for the 3d face snap points. There
is a relationship between the profiles and the related extrusion (or mesh)
solids. This relationship needs to be considered at each fitting (connection)
of stair parts.  
That mathematical explanation brings us to the simple practice used by Master
Carpenters and Stair Makers. The image above right illustrates the simplicity
in practice, no angles, no measurements - no math!!! Red annotated triangle
sides are nothing more than the rise and run rotated 90-degrees Clockwise,
or for the tradesman a cut out piece of the stringer to mark the angle of a
cut on the transition stair piece. Precision & Accuracy Importance of Checking the Model We touched upon accuracy of the software programs in the last tutorial. Now lets look at the interaction of the user & software, human error. Any time we need to array or distribute connected model geometry, the need to check / ensure, the precision and accuracy becomes important. OSNAP settings are a time saver but diligence in reading the pop up info and not having too many options selected is very critical. Just as testing a single arrayed alignment of a handrail section, very similar to a tradesman checking the fit before fastening pieces together.
A good example (above) is from one of the stair building evolutions (previous tut). The tutorial documentation is correct, but when dragging the wild polyline arc to the layout circle it snapped to the arc center and not the intersection, offset .23" or 1.3% error. Being discovered while setting up and checking the outside radius handrail, not a major problem, but a 1.3% variation ("wiggle") per step of the handrail, could show up during smoothing and rendering (specular) in a 3D program. Most often the solution is very simple. For this instance creating a negative volume solid with the correct form and subtract it from the stairs. Obviously precision and accuracy is important, catching the normal human errors as soon as possible is by far better than finding then during the final high resolution rendering!! The Handrails
• Start with a symmetric closed polygon
profile (typ cross section). It is advisable to have an endpoint
that is the midpoint of the bottom segment. (above right) The result
for this segment of the tutorial (Above Left)
• At
the stair solid and HR Center lines we need to draw a line 1/2
the height of the rise, 3.6" in the
Z direction from the mid endpoint of the treads HR center Line.
• Turn off the profile's layer, and select
the new 3dfaces Inner Handrail Profile & Snap Jig (Framework) This
will be an abbreviated explanation, using the same methodology
as before and differences shown. The centerline offset from the
tread edge is 4", the stair solid represents
the outside edge of the treads. There is now need for a mid
profile per tread segment of railing. The outer when rendered
in max showed its polygonal modeling, so we doubled the faces
and thus the profiles in Cad.
• Create the inner handrail mesh segment with 3dfaces as before. That is the inner handrail common segment.Array to form Handrail Mesh Model With the inner and outer handrail, segments done you model should
look similar to the one below right.
• Select the new blocks and start the array command. Polar settings as follows -Center= stair arc centers or in this case 0,0,0. -Select "Angle to Fill & Angle Between" - Angle to Fill= 84-degrees (total stair arc segment) - Angle Between= 6-degrees (tread arc segment) Preview and if all looks well click "accept"
• Move each block on the z-axis to match block insertion point to the mated profile mid endpoint. Alignment is critical so watch the OSNAPs and change views on the view port as needed. NOTE: a single block can be created from the Handrail segments, to minimize move operations, precision and accuracy to this point being twice as important. • Your model should resemble the image below. If it does, erase the first (4) inner handrail segments, to prepare for the flared portion on the handrail.
Using the same offsets and general methods as the common handrail, we will set up the 3 unique flared handrail segments per tread. Due to the tighter radius of the curve and changing stair pitch will be dividing the Handrail segment in half, for a smoother appearance.
• Select one of the HR center 3D Polylines, copy
with base point (leading endpoint) and erase if you like.
• Following the same sequence as used before, draw a line parallel with the y-axis, paste the HR center block at the starting point, rotate the block aligned with the y-axis line, paste the profile (typical endpoint) in the X,Z plane, then rotate the profile in the yz-plane using the stair rise as the reference change. (below left)
• Switch
UCSII to front and draw a polyline vertically 3.6" (1/2
rise), from the HR Center tread middle division.
That is the snap jig and framework for the flared portion of the handrail. On to modeling the mesh...
Modeling the Flared HR Mesh This is pure 3dface modeling at it is best in CAD. Those of you who are 3 dimensionally challenged, hang on tight. By this point, the basics of face modeling should not be a major problem. Complexity and working with the views, keeping track of face orientation, and your mental-GPS location in the model can cause problems. The orientation of the snap jig to the typical views as you will find out makes for some mental gymnastics. This part of the handrail makes the staircase what it is, so be patient and methodical when creating the faces. Start with the SW view and draw all of the backfacing 3dfaces first, per segment of railing. Then the normal (front) facing, zoom in (real close as needed) to make sure the OSNAPS are where they belong, and use only the "endpoint setting". The faces normal orientation is dependent on the current view port viewing direction beware of this when changing views. It is advisable to change views as needed. Another tip maybe to set up layers for normal and backfacing to differ between the jungle of lines, then convert the faces to a common layer. Below you can see the normal layers. Each tread segment has a similar color to the jig and layout lines for the normal facing and a somewhat contrasting color for the backfacing. Notice the brown face on the first segment, above and below are front facing due to the SW viewing direction. The last tread segment modeled in the SE viewing direction and the layers are inverted in the view shown, but the normals are correct. Take your time; it is worth the effort and using a simpler profile will help but not always with the final rendering.
A quick Render Below is the staircase to date, railings raised in max, we still need them on the stair solid to create and fit stair parts like up easing, gooseneck, turns and volute transitions.
Parting Thoughts The difficult part of this type of complex modeling is not in the methodology but the density of lines created by changing, curving meshes and not getting lost. The rigid and precise nature of Cad programs like AutoCAD can make it as easy as max especially for those who are most comfortable with the "drafting" nature of CAD. It's not instant, but the workflow provides for precise placement of all components not always being that fast or accurate in 3D programs. It can be fun; challenging and the results are at least equal to any exclusive 3D program. Cheers |
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