// ============================================================
//  Parachute Canopy — 16 curved panels, hollow shell
//  Each panel spans 22.5° of azimuth, curved over a dome.
//
//  Design notes:
//    • The canopy surface is derived from a sphere of radius R.
//    • The dome covers polar angles 0° (apex) → 70° (rim).
//    • Shell thickness is controlled by THICKNESS.
//    • 8 red panels + 8 white panels alternate around the 360°.
//    • Each panel is built by intersecting the spherical shell
//      with a wedge-shaped "pie slice" volume, giving true
//      curved gores — not flat triangles or pizza slices.
//    • Suspension line anchors are added at the rim.
// ============================================================

// ── Global parameters ───────────────────────────────────────
R          = 100;    // Outer sphere radius (mm)
THICKNESS  =   2.5;  // Shell wall thickness (mm)
N_PANELS   =  16;    // Must be even for colour alternation
PANEL_DEG  = 360 / N_PANELS;   // 22.5°

// Dome extent in polar angle (0 = apex, 90 = equator)
APEX_ANGLE =   0;    // degrees from top (the tip)
RIM_ANGLE  =  70;    // degrees from top (open bottom edge)

// Detail quality
$fn = 120;

// ── Colours ─────────────────────────────────────────────────
RED_COL   = [0.85, 0.10, 0.10];
WHITE_COL = [0.96, 0.96, 0.96];

// ── Helper: one spherical shell gore ────────────────────────
//
//  Strategy:
//   1. Build a full spherical shell (outer sphere minus inner sphere).
//   2. Intersect it with a wedge that spans PANEL_DEG in azimuth
//      and is bounded in Z to keep only the dome cap
//      (from the apex down to the rim latitude).
//
//  The wedge is a simple rotated half-space pair (two half-planes)
//  implemented as a very tall, thin box rotated to ±PANEL_DEG/2.

module spherical_shell() {
    difference() {
        sphere(r = R);
        sphere(r = R - THICKNESS);
    }
}

// Azimuthal wedge spanning angle_start → angle_start + PANEL_DEG
// centred on Z axis, unlimited height (clipped by dome_cap later)
module azimuth_wedge(angle_start) {
    // Two half-plane cuts leave a pie-slice prism
    // Tall enough to enclose the full sphere
    H = R * 2.5;
    rotate([0, 0, angle_start])
        intersection() {
            // Half-space 1: keep what's on the +X side of the
            //               plane rotated by 0°
            rotate([0, 0, 0])
                translate([0, -H/2, -H/2])
                    cube([H, H, H]);
            // Half-space 2: keep what's on the -X side of the
            //               plane rotated by PANEL_DEG
            rotate([0, 0, PANEL_DEG])
                translate([-H, -H/2, -H/2])
                    cube([H, H, H]);
        }
}

// Dome cap: keep only the polar-angle band [APEX_ANGLE, RIM_ANGLE]
// This is a cone frustum aligned with +Z (apex at top).
module dome_cap() {
    // Upper cut: everything above the apex polar angle
    //   z_apex = R * cos(APEX_ANGLE)  — but apex=0 means z=R (the very top)
    //   We keep z ≤ R (always true) → no upper cut needed when APEX_ANGLE=0.
    //
    // Lower cut: remove everything below rim latitude
    //   z_rim = R * cos(RIM_ANGLE)
    z_rim = R * cos(RIM_ANGLE);

    // A large cube from z_rim upward covers the dome region
    W = R * 3;
    translate([-W/2, -W/2, z_rim])
        cube([W, W, R * 2]);
}

// One curved panel gore
module panel_gore(angle_start) {
    intersection() {
        spherical_shell();
        azimuth_wedge(angle_start);
        dome_cap();
    }
}

// ── Suspension line anchor (small cylinder at rim) ───────────
//  Placed at the midpoint azimuth of each panel, at the rim latitude.
module line_anchor(angle_mid) {
    // Position on the rim circle
    x = R * sin(RIM_ANGLE) * cos(angle_mid);
    y = R * sin(RIM_ANGLE) * sin(angle_mid);
    z = R * cos(RIM_ANGLE);

    translate([x, y, z])
        rotate([0, 0, angle_mid])   // orient outward
            rotate([RIM_ANGLE, 0, 0]) // tilt with sphere surface
                cylinder(h = 6, r = 2, center = true, $fn = 16);
}

// ── Assemble the canopy ──────────────────────────────────────
module parachute_canopy() {
    for (i = [0 : N_PANELS - 1]) {
        angle_start = i * PANEL_DEG;
        angle_mid   = angle_start + PANEL_DEG / 2;

        col = (i % 2 == 0) ? RED_COL : WHITE_COL;

        color(col)
            panel_gore(angle_start);

        // Anchor at rim midpoint of each panel
        color([0.3, 0.3, 0.3])
            line_anchor(angle_mid);
    }
}

// ── Render ───────────────────────────────────────────────────
parachute_canopy();


// ============================================================
//  RENDER TIPS
//  • Open in OpenSCAD ≥ 2021.01.
//  • Press F5 for preview (fast), F6 for full render (slower).
//  • To export STL: Render (F6) → Export → Export as STL.
//  • To print multi-colour, export each colour set separately
//    by commenting out the alternate panels.
//
//  CUSTOMISATION
//  • Change R to scale the canopy.
//  • Change RIM_ANGLE (e.g. 65–80°) to make the opening
//    wider or narrower.
//  • Change THICKNESS for a thicker/thinner shell.
//  • Change N_PANELS to 8 or 32 for different panel counts
//    (keep it even).
// ============================================================