Plans are cuts
One rule generates most of the drawing conventions you would otherwise have to remember.
A floor plan is not a picture of a floor plan. It is a horizontal cut at 1.20 m above the finished floor, looking down.
Plan = keep every solid where z0 < levelElev + 1.20 < z1That is the whole rule. Drag the cut plane and watch what falls out of it.
What the slider shows
The low window is a gap — the plane passes through glass, so no wall exists there. The high window and the clerestory are solid wall.
The standard cut. The low window is now dashed: its 0.40 m sill is below the plane and its 2.10 m head is above it. The high window at 1.55 m is solid.
The clerestory opens. The door is now solid wall above its 2.10 m head.
Nobody wrote these rules
The drafting conventions on that plan are consequences, not decisions:
- A door draws as a gap because the plane passes through the opening.
- A window above the cut draws dashed because the plane passes below it.
- A lintel appears above a window because the wall piece over the head survives.
- A spandrel appears under a sill for the same reason, one piece lower.
None of this is a special case in the drawing layer. Wall cutting is the one
operation everything depends on: a wall run is a one-dimensional span, and
each opening punches [sill, head] out of it, leaving up to four pieces —
before, under-sill, over-head, after.
{"t":"wall","id":"s","level":"g","side":"south","type":"exterior","thickness":0.3}
{"t":"opening","id":"door","on":"s","kind":"door","at":"center","width":1}
{"t":"opening","id":"win","on":"s","kind":"window","at":"third","width":1.6,"sill":0.9,"head":2.3}before-window · under-sill · over-head · between · after-doorEach is a real box with its own z0/z1, and each appears — or does not —
in every view for the same reason.
The same rule, four more times
The SVG drawing contract stops at horizontal plans. Spatial presentation uses the Three.js renderer, where an orthographic camera and the same compiled solids produce whole, interior, roof-off, level and explode views.