Reusable 3D Earth globe widget (pure OCaml + WebGL)
feat(ocaml-globe): add Satellite abstraction with cached Kepler elements
New module Globe.Satellite:
- v: create from J2000 state vector, precomputes orbital elements
and ghost orbit points (once, at creation)
- position_at: current GL position from precomputed elements (~20 FLOPs)
- trail_positions: per-frame trail array for Orbit.add_trail
- ghost_points: cached full-orbit for Orbit.add_ghost (never recomputed)
- dot: position + color pair for Orbit.draw dots
Eliminates manual caching of elements/ghosts in app code.
Typical usage cuts ~80 lines from main.ml:
let sat = Globe.Satellite.v ~pos ~vel ~color () in
(* once: *)
Orbit.add_ghost gl orbit (Satellite.ghost_points sat) ~color:Color.ghost;
(* each frame: *)
Orbit.clear_trails orbit;
Orbit.add_trail gl orbit (Satellite.trail_positions sat ~dt) ...
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-1
lib/dune
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-1
lib/dune
+51
lib/satellite.ml
+51
lib/satellite.ml
···
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(*---------------------------------------------------------------------------
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Copyright (c) 2025 Thomas Gazagnaire. All rights reserved.
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SPDX-License-Identifier: ISC
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---------------------------------------------------------------------------*)
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(** Satellite state management for globe rendering.
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Caches precomputed Kepler elements, generates ghost orbits and
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per-frame trail positions. Pure OCaml, no WebGL dependency. *)
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type t = {
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elements : Kepler.Analytic.elements;
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pos : Kepler.Vec3.t;
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vel : Kepler.Vec3.t;
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color : Color.t;
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period : float;
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ghost_points : Math.Vec3.t option array;
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trail_length : int;
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}
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let v ~pos ~vel ~color ?(trail_length = 50) () =
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let elements = Kepler.Analytic.precompute ~pos ~vel in
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let period = Kepler.Analytic.period elements in
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let ghost_points =
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let n = 120 in
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Array.init n (fun i ->
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let t = Float.of_int i *. period /. Float.of_int n in
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let p = Kepler.Analytic.at_precomputed elements ~dt:t in
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Some (Gl_coord.of_astro p.x p.y p.z))
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in
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{ elements; pos; vel; color; period; ghost_points; trail_length }
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let color t = t.color
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let period t = t.period
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let ghost_points t = t.ghost_points
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let position_at t ~dt =
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let p = Kepler.Analytic.at_precomputed t.elements ~dt in
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Gl_coord.of_astro p.x p.y p.z
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let trail_positions t ~dt =
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let n = t.trail_length in
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let step = t.period /. Float.of_int (n * 3) in
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Array.init n (fun i ->
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let t_offset = dt -. (Float.of_int (n - 1 - i) *. step) in
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let p = Kepler.Analytic.at_precomputed t.elements ~dt:t_offset in
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Some (Gl_coord.of_astro p.x p.y p.z))
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let dot t ~dt =
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let pos = position_at t ~dt in
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(pos, t.color)
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lib/satellite.mli
+40
lib/satellite.mli
···
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(** Satellite state management for globe rendering.
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Caches precomputed Kepler elements, generates ghost orbits and
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per-frame trail positions. Pure OCaml, no WebGL dependency. *)
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type t
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(** Satellite state with cached orbital elements and ghost orbit. *)
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val v :
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pos:Kepler.Vec3.t ->
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vel:Kepler.Vec3.t ->
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color:Color.t ->
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?trail_length:int ->
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unit ->
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t
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(** [v ~pos ~vel ~color ?trail_length ()] creates a satellite from a
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J2000 state vector. Precomputes orbital elements and ghost orbit.
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[trail_length] defaults to 50 points. *)
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val color : t -> Color.t
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(** [color t] is the satellite's display color. *)
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val period : t -> float
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(** [period t] is the orbital period in seconds. *)
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val ghost_points : t -> Math.Vec3.t option array
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(** [ghost_points t] is the precomputed full-orbit ghost (GL coords).
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Computed once at creation, reusable across frames. *)
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val position_at : t -> dt:float -> Math.Vec3.t
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(** [position_at t ~dt] is the satellite's GL position at [dt] seconds
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from epoch. Very fast (~20 FLOPs). *)
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val trail_positions : t -> dt:float -> Math.Vec3.t option array
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(** [trail_positions t ~dt] generates trail points up to [dt], fading
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into the past. For use with {!Globe_webgl.Orbit.add_trail}. *)
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val dot : t -> dt:float -> Math.Vec3.t * Color.t
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(** [dot t ~dt] is the satellite's current position and color,
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for building {!Globe_webgl.Orbit.dot} values. *)