Riding along

space.ride(target, options) on <wakusei-space> puts the camera at a moving point at its real place for the moment shown, and looks at the body it goes round. The pure maths of where the camera looks is ridePose, in @johnmorrisdotca/wakusei-planets/space, so a page or a test can ask the same question.

import "@johnmorrisdotca/wakusei-planets/space/element";

const space = document.querySelector("wakusei-space")!;
space.ride("moon", { look: "horizon" });
space.ride("io");
space.ride({ at: (ms) => ({ position: [6778, 0, 0], velocity: [0, 7.66, 0] }) }, { around: "earth" });
space.ride(null);

What can be ridden

• A body of the view ("moon", "sun", "mars", ...): it goes round the Earth. The camera is at its place in the view's scale and looks at the Earth.
• A moon ("io", "titan", "phobos", ...): it goes round its planet. The camera is at the moon's real place from its planet, in the planet's radii, and looks at the planet.
• A point of your own: { at(ms) } returns { position, velocity } in kilometres and kilometres a second from the centre of the body it goes round, in J2000 equatorial axes (x to the vernal equinox, y north), or null when it cannot be placed then. around names that body (the Earth by default). The view knows nothing of what the point is: a satellite from your own SGP4, a probe from your own table.

look is down (straight at the centre of the body: the ground goes by under the camera) or horizon (tipped forward along the way it is going until the horizon is a little above the middle). The way of going is the point's velocity less the part that points at the body; for a moon it comes from its places a second apart. view: "follow" keeps the camera a distance from the target and looks at it instead (distanceKm for a point of your own; a body or a moon is kept at the distance the zoom says).

The camera

import { ridePose } from "@johnmorrisdotca/wakusei-planets/space";

const pose = ridePose({ position: [5.9, 0, 0], velocity: [0, 1, 0], radius: 1, look: "down", yaw: 0, tilt: 0, zoom: 1 });
console.log(pose.forward.map((n) => n.toFixed(2)).join(","), pose.bodyRadiusDegrees.toFixed(1));

-1.00,0.00,0.00 9.8

position and velocity are in any unit, from the centre of the body, and radius is the body's radius in the same unit: radii of Jupiter and radius: 1, or kilometres and radius: 69911, make the same camera. The pose is the camera's place, the unit directions it looks and has up, the field of view (60 degrees at zoom 1, narrower with zoom), and the body's angular radius from there.

A drag looks about from there, as a head on a pivot does: a drag right turns the camera to its left, a drag up tips it towards the bottom of its picture, so the ground follows the pointer, about one and a half pixels for each pixel of the drag, whatever the zoom.

Changing and stopping

Another ride call glides from the old camera to the new one in about a second and a bit (smooth: false, or a reader who prefers reduced motion, jumps). ride(null) stops and gives the camera back to the body it was looking at. The body ridden, and the body it goes round, are not named on the view while you ride. A point that cannot be placed fires wakusei-ride-lost once and the ordinary camera is used. space.riding says what is ridden; stats.ride says riding, lost and gliding.

When it draws

The view draws only when something changes and at most 30 times a second. A ride changes the picture all the time, so the clock draws as often as the picture moves a pixel (the speed of the ground under the camera over its height, times the focal length in pixels, times the clock's speed), from 1 to 30 times a second. Paused: nothing is drawn. Out of sight: nothing is drawn. Reduced motion: one update a second and no glide. pnpm test:freeze and e2e/space.demo.ts test each of these.