cup.c

#include <math.h>
#include <stdlib.h>
#include <raylib.h>
#include <raymath.h>

typedef struct Hermite {
	float x;
	float y;
	float d;
} Hermite;

// 1 dimensional cubic hermite spline interpolation
float hermite(float x, Hermite* p, int n) {
	if (x < p[0].x || x > p[n - 1].x) {
		return NAN;
	}

	int i;
	for (i = 0; i < n - 1; i++) {
		if (x >= p[i].x && x <= p[i + 1].x) {
			break;
		}
	}

	float x0 = p[i].x;
	float y0 = p[i].y;
	float d0 = p[i].d;
	float x1 = p[i + 1].x;
	float y1 = p[i + 1].y;
	float d1 = p[i + 1].d;
	float X = (x - x0) / (x1 - x0);
	float a = d0 + d1 + 2 * y0 - 2 * y1;
	float b = 3 * y1 - 3 * y0 - 2 * d0 - d1;
	float c = d0;
	float d = y0;
  
	return d + X * (c + X * (b + X * a));
}

Hermite C[3] = {
	{0, 1, 0},
	{1, 0.28f, 0},
	{4, 1.73, 0},
};

typedef struct Point {
	Vector3 pos;
	Color c;
} Point;

int cmp_point(const void* a, const void* b) {
	Point* pa = a;
	Point* pb = b;

	return pa->pos.z < pb->pos.z ? 1 : -1;
}

int main() {
	int w = 640;
	InitWindow(w, w, "Cup");

	SetTargetFPS(60);

	const int n = 40;
	const int n2 = 40;
	Point points[n * n2];

	while (!WindowShouldClose()) {
		BeginDrawing();
		ClearBackground(BLACK);

		for (int i = 0; i < n; i++) {
			float x = Remap(i, 0, n - 1, 0, 4);
			float y = hermite(x, C, 3);

			for (int j = 0; j < n2; j++) {
				float t = Remap(j, 0, n2 - 1, 0, 2 * PI);
				float phase = Remap(i, 0, n - 1, 0, PI) + GetTime();

				float X = cosf(t + phase) * y * 50 + w / 2;
				float Y = (4 - x) * 50 + w / 2;
				float Z = sinf(t + phase) * y * 50 + w / 2;

				float camY = 80;
				float camT = 30 * DEG2RAD;
				Y = Y * cosf(camT) - Z * sinf(camT);
				Z = Y * sinf(camT) + Z * cosf(camT);
				Y += camY;

				Color c = {(j % 2 + 1) * 127, (i % 2) * 127, 0, 255};
				
				points[i * n + j] = (Point){(Vector3){X, Y, Z}, c};
			}
		}

		qsort(points, n * n2, sizeof(Point), cmp_point);

		for (int i = 0; i < n * n2; i++) {
			Point p = points[i];
			DrawCircleV((Vector2){p.pos.x, p.pos.y}, 5, p.c);
		}

		EndDrawing();
	}

	CloseWindow();

	return 0;
}

// https://twitter.com/2DArray/status/1670644975338418176