Mavis
49 lines
2.1 KiB
TypeScript
49 lines
2.1 KiB
TypeScript
/**
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* Geometric occultation: given a position (relative to a body's center)
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* and the radii of the occluder (R1) and the body the observer is on (R2),
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* is the observer currently in shadow?
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*
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* Used for both:
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* - "is this vessel in the planet's shadow?" (R1 = planet radius, R2 ≈ 0)
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* - "is this ground station blocked by the local terrain?" (R1 = planet, R2 = earth station)
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*
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* Returns the fraction (0..1) of the line of sight to the sun that is
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* occluded. 0 = full sun, 1 = total eclipse.
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*
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* Note: the canonical way to do this is to compute the half-angle between
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* the sun and the occluding body as seen by the observer. We treat the
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* sun as effectively at infinity (parallel rays) which is fine for KSP
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* since Kerbol is the system root and we're never going to need parallax
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* precision at this scale.
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*/
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export function shadowFraction(
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observerToSun: { x: number; y: number; z: number },
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occluderToObserver: { x: number; y: number; z: number },
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occluderRadius: number,
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): number {
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// Vector from observer to sun, normalized
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const sunDist = Math.hypot(observerToSun.x, observerToSun.y, observerToSun.z);
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if (sunDist === 0) return 0;
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const sx = observerToSun.x / sunDist;
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const sy = observerToSun.y / sunDist;
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const sz = observerToSun.z / sunDist;
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// Project occluder center onto the sun-direction line
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const proj = occluderToObserver.x * sx + occluderToObserver.y * sy + occluderToObserver.z * sz;
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if (proj >= 0) {
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// Occluder is behind the observer relative to the sun → no eclipse
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return 0;
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}
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// Perpendicular distance from occluder center to sun ray
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const px = occluderToObserver.x - proj * sx;
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const py = occluderToObserver.y - proj * sy;
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const pz = occluderToObserver.z - proj * sz;
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const perpDist = Math.hypot(px, py, pz);
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if (perpDist >= occluderRadius) return 0;
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// Approximate the angular size of the sun as seen from the occluder
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// vs the angular size of the occluder; we use 1.0 for the sun
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// (i.e. effectively point source) — good enough for visualization.
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return Math.min(1, 1 - perpDist / occluderRadius);
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}
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