CS 294-3: RAPID PROTOTYPING
Lecture #6 -- Oct. 6, 2000.

PREVIOUS < - - - - > CS 294-3 HOME < - - - - > CURRENT < - - - - > NEXT

Entering the FDM Machine at the SSL Level

The SSL format : You DO have to understand it !.
-- The specification of the nesting of the individual contour pieces is crucial !
an example
Study the structure of simple SSL files from QuickSlice with an editor.
- - see what happens when you change a color, duplicate a contour, change its orientation ...

Operations in the Slicing-level Boolean Pipeline

Diagramatic overview
- See also the LSIF web page.
SLIDE as a design front-end
- Capabilities (and lack thereof) in SLIDE: CSG only between closed, watertight shells.
- This is a very clean conceptual model; it avoids ambiguous and erroneous model specifications.
- Cannot visualize it in SLIDE in real time -- this is a hard task for the general CAD model !
Boolean operations will be carried out on a layer by layer basis -- after slicing.
These are the necessary steps one has to go through.
-- More details on the key modules below.
The coherent B-rep slicer (by Sara McMains)
- Starts with the bottom-most vertex, tracks all upwards edges.
- Coherently morphs the contour sliced out of the B-rep by a rising z-plane.
  (Most difficult part is to determine nesting of new contours within old ones.)
- It does this individually for all components in a Boolean expression,
  and preserves the Boolean relationships between the contours from the various components.
- Output is in a machine-independent ascii format: L-SIF
Resolving Boolean expressions on each layer
- This is a harder task than one may think:
  Numerical inaccuracies may lead to small self intersections which can conuse subsequent algorithmic steps.
- Jialin Wang tried several software packages written by others and available on the net,
  and most of them crashed very quickly on not very complex test examples.
- We are currently using the Boolean operations package provided in the OpenGL pipeline.
  it may produce zero-length segments, which need to be removed with a special clean-up pass.
Generating appropriate SSL contours
- Last time we explored the implicit semantics in QuickSlice
  of how the colored contours are interpreted with respect to delineating various fill-areas.
- This was not the whole story: QuickSlice also stores some nesting information in its data structure.
  A "*" determines when a contour expressed as a list of vertices terminates,
  and this also implies its nesting within the previous contour.
  You will explore this issue in HW#6a.
Entering QuickSlice at the SSL level
- Load the generated SSL curve, and inspect the in iso-view and by stepping through them individually.
- If whether this represents your part properly,
  run "Roads" on a few critical level to see whether the semantic of the nesting of the contours has come across properly.
- If all is OK, build supports, build the base, run roads on the whole part, save SML ...
If desirable -- build part (using FDMsend).

Thorough Testing of the Slicing-level Boolean Pipeline

This is the first time we use this pipeline outside our lab.
How robust is the whole software ??
Through your HW#6b you will give this pipeline a tough shake-out
- - and hopefully also end up with some cool parts.
What features might be tough on the slicer, the Boolean resolver ?
- Slicer: maintaining contour morphs through vertices of complex branching in the z-direction
- Slicer: determining nesting of nearly coincident contours
- Boolean resolver: zero-length edges
- Boolean resolver: coinciding vertices between different contours
- SSL output module: contours with a high degree of nesting.
Put yourself into the mind-set of a QA (Quality Assurance) officer.
Ideas for parts that you might build:
- - Keep in mind that you will have to remove the supports !
- "DaVinci" polyhedral frames
- Mersenne Sponge (of low order)
- Perforated surfaces (e.g., thin spherical shell with twenty circular holes)
- Multiple nested half-shells
- A knotted hole through a disk
- Something like "Flame Beta"
If you have a really cool design from which you cannot remove the supports,
Keep it around until we have our Z-Corp 3D printer at the end of October.

Homework Assignment:

A#6a: Find out what effect the nesting and coloring information of SSL contours has on the road-fill pattern.

Start with a very simple set of three contours: an outer triangle, and a nested inner triangle which can be covered with one or two contours on top of one another.
Find out how orientation (CW or CWW), color (RED or LIGHT-MAGENTA), duplication (YES or NO) of the inner triangle contour, as well as its topological nesting (smaller loop being placed {with the closing star in the SSL file} INSIDE, or OUTSIDE, or as PART OF the outer contour) will affect the desired fill-pattern: i.e., dense fill in the outer ring region, and loose fill of the region inside the inner contour.

Write a very brief summary of what works, and give a table that details the observed fill patterns for the many different possible combinations of the four parameters mentioned above.

A#6b: Design a Complex CSG Shape with SLIDE trying to break the Boolean slicer pipeline.

Use all three Boolean set operations (U, D, I) but only clean, water tights shells for the components and make an object that you might expect to be challenging to the slicer and the Boolean operation program. Then inspect the roads generated on each layer to find out whether the pipeline was able to handle your test case -- or what feature might have broken it.

Report your results: Tough tests that did or did not pass through the pipeline.