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History-free snapshot of the private development repository at 77a82eb. Code, docs and the public catalogue only: no plant or job knowledge layers, no client, job or producer names (tools/scrub.py clean against the widened term list; a second word-level grep clean). The intro deck is left out because its slide images cannot be checked for job drawings.
247 lines
12 KiB
Markdown
247 lines
12 KiB
Markdown
# Interference checking, and what it took to make it usable
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Written down because every wrong version of this check looked right, and two
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of them reported "no interference" while being incapable of finding any.
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The code is `check_member_interference` in
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`WillTools.extension/lib/vibe/ticketchecks.py`, with the geometry in
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`ticketqc.py`. This is the reasoning, not the API.
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## The criterion
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Two bodies interfere when the material they share is **more than 1/8" thick**.
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For the *filtering* stages, which work on boxes, that is read as "the boxes
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overlap by more than the tolerance on every axis" — the same statement a
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detailer makes as "more than 1/8" of interference shows in each of the three
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views". A view is a dropped coordinate, so each view shows two axes;
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"overlaps in all three views" expands to `(0∧2) ∧ (0∧1) ∧ (1∧2)`, which is
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`0 ∧ 1 ∧ 2`. There is nothing to gain by writing it as three 2D tests; the
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gain is in not forming the pair at all.
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For the *verdict*, boxes are not good enough, and neither is volume.
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**Volume is a trap.** A bar grazing the face of a plate over a wide area has
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volume while being thousandths deep; one bar catching the corner of another
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has depth with almost no volume.
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**A bounding box of the intersection is a worse trap**, and this one survived
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several rounds of looking right. When two round bars graze, the material they
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share is a *curved lens*, and a lens is no better described by its box than a
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bent bar is — measured on a real piece, these intersections fill **10 to 15
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percent** of their own boxes. Boxing them reported **0.51" of interference for
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a sliver of 0.0166 cubic inches**: a shape a thousandth of an inch thick would
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have measured exactly the same. The box was not even mis-oriented; the
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intersection's own box was already axis-aligned. It is simply the wrong tool
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for a curved shape, twice over.
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So the verdict is a **mean thickness**. For a thin body, `Volume = Area × t`
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and `SurfaceArea = 2 × Area`, hence
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```
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t = 2 × Volume / SurfaceArea
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```
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which does not care how the sliver is oriented or how curved it is. On the
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piece where boxes claimed 44 interferences, every one measured **0.059″ to
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0.066″** thick — under half the tolerance, and the count went to zero.
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Known bias: mean thickness **under-reports a chunky intersection**, giving
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`side/3` for a cube and `diameter/3` for two bars crossing dead centre. It
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errs towards calling an interference shallower than it is, so the tolerance it
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is compared against should be set knowing that.
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## The pipeline, and why each stage exists
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Revit's own assembly clash tests every pair and solves geometry for all of it.
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Five stages instead, each one cheap enough to earn the one after it:
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| stage | rejects | cost |
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| --- | --- | --- |
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| sweep and prune along the length | pairs that never share a station | one sort |
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| **top in form**, then front, then right | pairs clear in the drawing plane | six comparisons |
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| one box per **solid** | air inside multi-part embeds | solves geometry |
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| boolean **intersection** | air inside bent bars | one boolean |
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| exemptions | what is deliberate | table lookup |
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Measured on three real pieces:
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```
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all pairs -> sweep -> top in form -> front -> solid boxes -> real
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12090 2656 1120 644 320 27
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8385 2246 980 555 187 47
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6670 1839 956 506 150 10
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```
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Under a second per piece.
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### The first view is fixed, not chosen
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Top in form — the full 299" × 143" face, where members are spread out and most
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pairs separate at once. The other two views look along a long axis at an 8"
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thickness, where nearly everything overlaps nearly everything.
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Which view is worth looking at first is a fact about how a wall panel is
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built. It does not change piece by piece, so it is not inferred from the data.
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Read off the model rather than assumed: the view named `Elevation Top` on a
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real ticket looks along the panel's local thickness axis, dot product ±1.000
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on every piece checked. The sign varies because pieces are mirrored; the axis
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does not.
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**The third view never rejects anything and cannot.** Top-in-form establishes
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length and width, front adds thickness, and right asks only about thickness
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and width — both already known. It passed 755 of 755, 596 of 596, 537 of 537.
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Kept because the code should read like the three drawings it describes; not
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load-bearing, and nobody should think it is.
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## Three ways to build this wrong
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### 1. Sampling edge vertices for a bounding box
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A cylinder's edges are **circles**, and a circle's two "end" points are the
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same point on the seam. Sampling them collapses a strand's box to a
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zero-thickness sliver in two axes — and a sliver overlaps nothing.
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The first version of this check reported **no interference anywhere**, on
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every piece, and looked entirely healthy doing it. Use
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`Solid.GetBoundingBox()`, and remember its box is stated in the solid's own
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coordinates, so its `Transform` applies before the panel's inverse.
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### 2. One box per element
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An embed drawn as a plate at one end and anchor bars at the other is **nine
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solids**, and one box around all of them claims the air between as occupied.
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Measured fill: **3%** steel by box volume. Per-solid boxes cut the reported
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count from 265 to 223 before any boolean ran.
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### 3. Trusting boxes at all, once bars are bent
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A box is the product of three intervals, so a **bent bar's box is the whole U
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including the hole**, and anything passing cleanly through that hole reads as
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an interference. How much of each box is actually steel, measured per family
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on one piece:
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```
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3.05% a 9-solid embed 6.06% a bent bar, 1 solid
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3.2% a 3-solid embed 10.16% a bent bar, 1 solid
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12.85% a bent bar, 1 solid
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--------------------------------------------------------------------
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46.32% a straight bar 50.16% a strand
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```
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The straight bar and the strand are close to what a cylinder in its own box
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should be. Everything above them is mostly air — and one of those bent bars
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is a 0.63" bar occupying a 13.63" × 6.06" box.
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Boxes claimed 265 interferences on that piece; **86 were real**. Two thirds
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was air. The boolean pass that establishes this costs about 0.1 s.
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This also produced a wrong conclusion that stood for a while: transverse bars
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were said to cross every bottom-row strand as a drafting convention, 70 such
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pairs on one piece. **Not one was real** — the bars and the strands miss each
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other through the thickness.
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Boxing the **intersection** is sound where boxing the inputs was not. A bent
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bar's own box is mostly the hole; the intersection is the small piece of steel
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actually shared, and its box describes that piece fairly.
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## What counts as deliberate
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Left alone, the check reports several hundred per piece and nobody reads it.
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Everything below is configuration, not code, and lives in the knowledge layers.
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| | where | why |
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| --- | --- | --- |
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| `same_family_ok_categories` | plant | Some hardware is placed as several overlapping instances of one family **on purpose** — the intersection is what makes Revit pick them up together when the assembly is created. |
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| `within_category_only` | plant | Reinforcing meeting an embed, or a bar running into a ledge, is how the piece goes together. Two things of one **kind** in one place is the defect. Blunt, and it does hide real cross-category problems. |
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| `insert_bars` | plant | A lifting insert is set out with a bar to either side. Those belong to the **insert**, not the reinforcing schedule — the plant places them and may move them freely, so there is nothing to hold them to. |
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| `component_groups` | either | Hardware that arrives as several families but is **one part** — see below. |
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| `exempt_families` | job | Hardware whose modelled solid sits inside other things and always will. |
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| `ignore_family_pairs` | either | A named pair, one at a time. The last resort, not the first. |
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### Component groups
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A connection is not one family. A top-of-panel-to-column connection is a
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plate, its rebar, its sleeve and a good deal else; an embed and the bar welded
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to it are one part installed together. They are modelled touching **because
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they are one thing**.
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Revit records none of this. Checked on a real model: every component's family
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name is just its own part number, and no parameter, nested family or shared
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parameter points at the assembly it belongs to. So the ownership has to be
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stated, and `component_groups` is where.
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Two strengths, and reaching for the wrong one goes blind:
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- `interferes: within_group` — the components may overlap **each other**
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freely, but one of them hitting something **outside** the group is still
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reported. Reach for this first. It explains the overlaps that are by design
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without hiding the ones that are not, and on a real piece it kept a
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`P76 × P106` finding while dismissing `P76 × its own bar`.
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- `interferes: false` — nothing in the group is reported at all. For hardware
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set out as a unit that simply is not the detailer's to check.
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### The insert-bar rule: found by symmetry, not by distance
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An insert's own bars cannot be told apart from ordinary reinforcing by
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anything local. Same family, same size, same direction, same layer.
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**Distance alone gets it backwards.** The first version of this rule exempted
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bars within 1.5" of a lifter, and on a real piece that exempted the ordinary
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#4s which happen to pass near one while leaving the actual lifter bars — #5s,
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sitting 4" away — fully checkable. It was wrong in both directions at once and
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the reported counts barely moved, so nothing gave it away.
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What does distinguish them is **symmetry**. Reinforcing comes in layers
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through the thickness and the layers mirror each other, so an ordinary bar has
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a counterpart at the same station in another layer. An insert's bar does not.
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Measured on one piece:
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```
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layer at 6.0" 20 x #4 at stations 2 4 14 32 50 68 86 104 122 140 ...
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layer at 2.5" 20 x #4 at exactly the same stations <- mirrored
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10 x #5 at 26 34 | 116 124 | 175 183 | 187 195 | 265 273
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<- unpaired
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lifter stations 30 | 120 | 179 | 191 | 269
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```
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Five pairs, 8" apart, each centred **±4" on a lifter station**. Nothing else
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sets those ten apart.
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So: a bar with no counterpart, within reach of an insert, belongs to it. A bar
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with no counterpart and **no** insert nearby is reported (`BAR-UNPAIRED`) —
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the pattern does not explain it and neither does a lifter.
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**The mirror is never assumed to be at `thickness - depth`.** The rule looks
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for a counterpart in *some* other layer rather than at a computed depth, so a
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piece whose symmetry is broken — extra clear cover on one face, say — costs
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nothing and no layer pairing has to be worked out.
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A measurement note that cost an afternoon: layer depths must be reported
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**raw**. Binning them to the nearest half inch turned 1.75 / 6.25 into
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2.0 / 6.5 and 2.25 / 5.75 into 2.5 / 6.0, which made a perfectly symmetric
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piece look like it had half an inch of extra cover on one face. The panel was
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symmetric to 0.000" all along, in every layer except the insert bars — which
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is what a wall panel should be, since the only thing that breaks the mirror is
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a lifter, and a lifter is not symmetric either.
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## Open
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- Every bar in `bar_families` is assumed to be layered reinforcing. Ledge bars
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are not, and they surface as `BAR-UNPAIRED` — correct as stated, but it is
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a category the rule does not really have.
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- If a lifter has no bars at all, nothing notices. Whether every face lifter
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must carry one either side is a *completeness* question, and no check asks
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it yet.
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- Exempting an insert's bars from **everything** is what the plant asked for,
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since it may move them freely — but it does suppress real geometry. On one
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piece it removed the deepest interference in the list, a #5 buried a full
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diameter into an embed.
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- Cross-category interference is not looked at when `within_category_only` is
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on, and some of it is certainly real.
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- AABBs remain a **conservative** approximation everywhere they survive into a
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result: they can invent an overlap but never hide one. That is the right
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direction for a filter and the wrong one for an exemption, which is why the
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insert-bar test refines geometry before measuring.
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