ld

ld defines a 2D curve by number, in any phase, from straight and circular segments.

Syntax

ld number segment [segment ...]

Each segment is one of:

lp2 x z [x z ...] ;
lar x_end z_end radius ;

number is a whole number of 1 or more. A curve lives in a local plane with the coordinates x and z. A surface that revolves it, crx, cry or crz, reads x as the distance from the axis and z as the position along it.

What it does

The segments form one chain. Each segment starts where the one before it ended.

lp2 draws straight lines through its points. If its first point is not where the chain ended, James first draws a line to it.
lar draws a circular arc from where the chain ended to the point (x_end, z_end). radius is the radius of the circle. The arc is the short one. A positive radius turns counter-clockwise, with the centre on the left of the chord from start to end. A negative radius turns clockwise.

When James applies it

When it is read. ld is legal in the Control, Part and Merge phases.

Rules James adds

A number below 1 is an error (E0104). So is a curve of more than 256 segments.
An lp2 with a single point and no point before it only sets the start of the chain. It is legal when a later segment of the same ld continues from it, as lar does in the example. A chain that never gets past that one point is an error (E1002).
After a start, an lp2 of one point draws a line from where the chain ended to that point. An lp2 of one point that repeats the end point exactly draws nothing.
An lp2 list of two or more points that begins at the end point is used as written. Otherwise James puts the end point in front of it.
An lp2 with a number of coordinates that is not a multiple of 2 is an error (E1002).
A lar whose radius is too small for its chord is an error (E1003). James accepts a radius of exactly half the chord, which is a semicircle, and the sign chooses its direction. A lar with no point before it is the same error.
A segment type that James knows of but does not support is an error (E1009). A word that is no segment type at all, such as arc, is E0104 in place of the first segment. After a complete segment such a word ends the ld and is read as the next command, which James reports as not recognized (E0105).

Diagnostics

E1002: an lp2 has too few points or an odd number of coordinates.
E1003: a lar radius is too small, or it has no start.
E1009: the segment type is not supported.
E0104: the first segment type is no word James knows, or the number is below 1.
E0105: a word that is no segment type follows a complete segment, so it ends the ld and is not a command.

Example

The script tests/corpus/122-sd-crx-accept/input.tg, which make test runs:

ld 1 lp2 0 0 1 1;
sd 1 crx 1

The ld defines curve 1 as one line from (0, 0) to (1, 1). The sd then revolves it about the x axis as surface 1, a cone. The IR holds the curve with one lp2 segment and the surface with kind crx that refers to the curve.

ld