How plans are generated

"A plan is the building cut horizontally at a fixed height above the floor" — that is half right. Cutting a solid does not give you a plan.

This page lists the four things that cutting never produces, and explains how they shape the design of derivation. What derivation returns is catalogued in Plan form.

The cut is an input, not part of derivation

First, the cut itself. A plan is "the section at this level", and the cut plane sits at a fixed height above FL (1200 mm by default).

That height is an input to derivation, not something derivation decides. Change it and a sill window is cut or not cut. Give the same cut height and the same plan comes out — the promise in Determinism of derivation is a promise given that input.

The four things a cut cannot produce

1. Door swings

A door's swing arc is a symbol of movement; there is no thing there. Cut as precisely as you like and you have cut air, not an arc.

Counted across the bundled examples, plans carry 1,516 swing entities. Every one of them is a symbol, not a section.

2. The projection of voids above

Things above the cut plane fall onto the plan of the storey below. The outline of a void is the archetype.

This actually got dropped. The bundled examples contain 29 such projections, and for a while not one of them appeared in a viewer's plans. The instruction handed over was "cut the solid", so of course everything above the cut plane vanished.

What is projected is the derived form. Project the authored rectangles instead and a void that has been re-cut by a diagonal line comes out in its uncut shape.

3. The cut line

The line drawn where a stair crosses the cut plane. It marks the fact of being cut, not a thing.

Drawing convention puts two parallel oblique strokes there, but being two strokes is appearance. Derivation returns a single segment spanning the full width of the run; whether to draw it as two strokes or some other symbol is the drawing side's call.

4. The descending run

What is visible below the cut plane. A downward stair is under the cut, so it is not in the section. It is nonetheless drawn on the plan.

koyu produces it as what its twin upward run left unhidden, the two sharing the same frame. To be twins, the four coordinates of the rectangle must match, and direction, form, device and part count must all agree. Matching by position alone yields mirror images.

Which is why form is a classified 2D set

Leave those four to be "invented by each consumer" and you get a different plan per consumer, which contradicts Determinism of derivation.

So derivation returns the plan as a set of 2D entities with classifications, each carrying geometry, a class, and the identity of what it is the form of.

Class	What it is	Count across the bundled examples
cut	what the cut plane sliced	20,782
above	projection of what is above the cut	2,460
below	what is visible below the cut	845
swing	the arc of movement	1,516
anchor	the seat where a symbol goes	179

The essential point is that the classification lives on the form side. "This is a section, this is a projection" is decided by comparing the cut height with a z range — that is derivation's job. Leave it to the drawing side and the comparison thresholds differ per implementation, and the same source yields different drawings.

anchor occupies an interesting position. The symbol is appearance, but the seat the symbol goes in is form. "An up/down annotation is needed here" is decided by derivation; "write UP or draw an arrow" is decided by drawing.

A wall is a run of intervals with holes in it from the start

An old trick in plan drawing is to paint over the black band of a wall with the paper colour so it reads as an opening. koyu has no such operation.

A wall appears as the run of intervals into which openings divide it. Openings on the segment are ordered, full-height intervals sit between them, and at each opening a spandrel below and a head above remain.

In plan as in three dimensions, a wall is a run of intervals with holes in it from the start. With no painting-over operation, there is no failure to paint over.

And that the black bands really are the Form's cut intervals is checked: the similarity transform from world to page is solved from the bounding box, and the two are matched as sets of quadrilaterals.

Do not confuse what cannot be drawn with what cannot be written

The drawing surface does not freeze. Dimension lines, grid bubbles, door and window conventions, scale, pitched roofs — the precision can be raised a bit at a time.

With one condition. What cannot be drawn must never be mistaken for what cannot be written. Without an enumeration of what is drawn and what is not, the limits of the drawing side get imported as limits of the description side, and the source starts being dragged around by its presentation.

Because of that condition, a poor plan is never a reason to change the notation. What may change is the appearance, not the form.

Next

• Plan form — the classes and the entities
• Form — what derivation returns
• Determinism of derivation
• koyu plan