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Modeling Tips
Tips for Modeling with 3DS objects
General
Start your model design by identifying structure of you model: How many
objects it will have and what type of objects. Objects normally represent
different parts in the mechanism or other entity which is being modeled.
When modeling with a Generic 3D Object, start by determining orientation of
the local axis (local z-axis) of the object. Normally axis of the object points
in direction in which the object has the longest dimensions, or with respect to which the
object is the most axially symmetric.
It is also useful to position origin of the local frame at approximate center
of the model. I should be easy and natural to rotate the object around the
origin of its local frame. Relative position of an object and its local frame
can be changed at any time using "Current object">Modify submenu.
Note that these two operations are not designed for moving objects (use the
Current Object Page instead) it is rather for changing handle on the object for
more convenient rotations.
Modeling should start from lower dimension up. In a 2D view points should be
modified first, than arcs because modification of a point generally changes
curvature of adjoined arcs. Similarly Base knots, edges and patches is the preferred
sequence of modifications.
Handling validation restrictions
The most common difficulty in modeling surface of an objects is dealing with
rejection of invalid modifications. When software detects that as the result of
the last action (edit box change or mouse movement) some of the patches in the
object will become invalid it stops the change possibly with display of message
on the form.
To avoid above problem we suggest the next sequence of modeling:
- Start modeling complicated curved surfaces by using initially only flat
type of patches and straight edges.
- Set correct position of ends of 3D line segments ( edges of Patch Strips) at
each knot.
- Change 3D line segments to curves where necessary and set correct
curvature of each 3D line segment.
- Change patches which should be curved to Arc Patch type and adjust its
curvature.
- If you can not achieve desired shape of the surface on the last step due
to rejection of invalid geometry, add more patches to the object using Break
and/or New knot commands.
The main reason for invalid geometry is that some of 2D arcs in the model can become
invalid. It happens when :
- An end of the arc after the modification located too far for the current
radius of the arc
- Radius of the arc has become too small to be able to connect ends of the
arc.
Both these cases can be expressed by condition of validity of a 2D arc: Radius
of an arc must be at least half of the distance between its ends.
Sometimes it is not easy to detect which arc is being rejected. To find it, have a look at the both 2D editors first. Arcs which are full half circle normally
reject their further modifications. We call such am arc Full Arc.
If there no such an arc, move the cross-section position through the whole
range while watching Cross-section View. If in some position one of the arcs
becomes full - this is the problem. Reduce curvature of the arc or add
additional Patch Strip by using Break command.
If this is the Profile View use the New knot command instead.
If you can not find a full arc by above methods switch to wire frame view
and inspect all 3d lines.
If this does not work try to reduce curvature ( or even change it to flat )
of the most curved patch around the area which you are attempting to modify.
In a hard case, detect the most difficult patch, make it flat and modify
all other patches. Change the problem patch back to arc again, select it in
Cross-section View. Profile View will display its curvature (or radius).
Select the arc which is correspondent to the patch in Profile View and open
its Properties dialog using context menu (right mouse button). This particular
dialog does not do immediate validation of entered values. Type correct values
for curvature of this axial arc, enter also required values of curvature
(radius) at adjoined knots and press OK or Apply.
If nothing works, submit a Support Request to DInsight
with the model included, and take a break.
Folded surface problem
Some modifications of surface patches can lead to situation when a single
patch intersects itself (A flat patch screwed by 180 degrees). This is
considered invalid geometry. Software does do anything to prevent the situation.
In some cases the software will still work but sometimes it will cause holes to
appear in the surface. Correction of the geometry should remove the defect.
Another aspect of the same situation happens when edges which are supposed to
converge at the last or the first knot actually are slightly shifted against one
another creating a nearly invisible folded area. Good indicators of this
situation are either black or white spots on the surface near the convergence
point. Switch to wire frame and use zoom to examine the geometry. It is
recommended to use edit boxes from the form or dialogs to enter exact positions
of ends of edges.
Although making edges of patch strips to converge or coincide is valid
and useful technique it is recommended to avoid situation when different
edges of the same patch intersect each other in a middle point.
Smooth Patch Joins
"Smooth surf Join" check box, which appears when a common edge of
two patches is selected can create incorrect lighting (black or white spots) of
the surface around the joint if its status is significantly different from the
actual geometry of the surface. Try to change status of the check box and
see if it improves appearance of he surface in 3D.
Singular Patches
In some abnormal situations like folded surface described above the surface
can have some anomalies like white or black patches. In this case Incrementing
or decrementing geometric resolution
(Current Object menu > Options) often resolves the problem.
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