To scale

By default each drawing and 3D view is framed to fill its own picture, so a small board looks as big as a tower. To scale, machines keep their true relative size.

<hako-device key="pi-zero" scale="real" unit="2px/mm"></hako-device>
<hako-lineup keys="pi-zero,pi5,nas-2bay" unit="1px/mm" ruler labels></hako-lineup>

unit is pixels a millimetre: 2px/mm, or just 2.
At real scale the element's box is the device's own box, so its bottom edge is the ground it stands on.
<hako-lineup> stands machines side by side on one baseline, on one scale. What does not fit wraps onto another row on the same scale, so nothing is out of sight.
ruler adds a ruler; ruler="200" names its length in millimetres. labels adds names under the machines. mode="auto" on a lineup gives each machine its 3D view; by default a lineup stays drawings, since a browser allows only a few WebGL canvases at a time.

Lines stay one width

A tiny board is not hairline and a big tower is not heavy. At real scale every line carries vector-effect="non-scaling-stroke", so its width is the one its style sets, in pixels, whatever the scale. A row of dots and a drawn handle are shapes, not lines, and scale with the device.

What a drawing records

Every drawing records how it relates to millimetres, on its root element, and readMetrics reads it back:

import { readMetrics, realPlacement } from "@johnmorrisdotca/hako-devices";
import { buildDevice } from "@johnmorrisdotca/hako-devices/build";

const svg = buildDevice("pi5").svg();
const metrics = readMetrics(svg); // { mmPerUnit, box, baseline }
const place = realPlacement(metrics, 2); // its size in pixels at 2 px/mm
place.width > 0; // true

Attribute	What
`data-mm-per-unit`	Millimetres per unit of the picture, on the plane the camera looks at
`data-box`	The device's box on the picture: x0, y0, x1, y1
`data-baseline`	The y of the ground it stands on

realScaleSvg(svg, unit, style) gives a drawing's text at real scale with its lines held, for use without the element.

The picture is a perspective view, so the numbers hold exactly on the plane the camera looks at, and a corner nearer the camera is a little larger. A flat board is seen from higher up than a tall box, as in the default view.

3D at real scale

A 3D view takes the same scale from the mesh's real millimetres: its camera is the drawing's, on a canvas the size of the whole picture at that scale. It turns freely, and the canvas may reach past the element's box, so nothing is cut off.