From 033797264ff58a1994765d7b99cf07be657a226e Mon Sep 17 00:00:00 2001
From: Dan Goodliffe <dan@randomdan.homeip.net>
Date: Sat, 6 Feb 2021 16:54:54 +0000
Subject: Fix up the maths in creating curved rails

---
 application/main.cpp  | 23 ++++++++++++++++-------
 game/network/rail.cpp | 22 ++++++++++++++++------
 2 files changed, 32 insertions(+), 13 deletions(-)

diff --git a/application/main.cpp b/application/main.cpp
index a723b20..73d4440 100644
--- a/application/main.cpp
+++ b/application/main.cpp
@@ -1,8 +1,6 @@
 #include <SDL2/SDL.h>
-#include <array>
 #include <chrono>
 #include <collection.hpp>
-#include <game/network/link.h>
 #include <game/network/rail.h>
 #include <game/terrain.h>
 #include <game/world.h>
@@ -14,9 +12,9 @@
 #include <gfx/manualCameraController.h>
 #include <gfx/renderable.h>
 #include <gfx/window.h>
+#include <glm/glm.hpp>
 #include <memory>
 #include <special_members.hpp>
-#include <utility>
 #include <vector>
 #include <worker.h>
 
@@ -66,11 +64,22 @@ public:
 		Worker w;
 
 		world.create<Terrain>();
+
 		auto rl = world.create<RailLinks>();
-		auto a = rl->addLink<RailLinkStraight>({-1190, 5, -1190}, {-1190, 6, -1180});
-		auto b = rl->addLink<RailLinkCurve>(a->ends.back().first->pos, {-1180, 5, -1180}, {-1185, -1180});
-		auto c = rl->addLink<RailLinkStraight>(b->ends.back().first->pos, {-1180, 4, -1190});
-		rl->addLink<RailLinkCurve>(c->ends.back().first->pos, a->ends.front().first->pos, {-1185, -1190});
+		{
+			const glm::vec3 j {-1100, 5, -1100}, k {-1100, 5, -1000}, l {-1150, 10, -1050}, m {-1050, 10, -1050};
+			rl->addLink<RailLinkStraight>(j, k);
+			rl->addLink<RailLinkCurve>(k, l, {l.x, k.z});
+			rl->addLink<RailLinkStraight>(l, m);
+			rl->addLink<RailLinkCurve>(j, m, {m.x, j.z});
+		}
+		{
+			const glm::vec3 j {-1190, 5, -1190}, k {-1190, 6, -1180}, l {-1180, 4, -1180}, m {-1180, 4, -1190};
+			rl->addLink<RailLinkStraight>(j, k);
+			rl->addLink<RailLinkCurve>(l, k, {-1185, k.z});
+			rl->addLink<RailLinkStraight>(l, m);
+			rl->addLink<RailLinkCurve>(j, m, {-1185, m.z});
+		}
 
 		Shader shader;
 		Camera camera({-1250.0F, 35.0F, -1250.0F}, 70.0F, (float)DISPLAY_WIDTH / (float)DISPLAY_HEIGHT, 0.1F, 10000.0F);
diff --git a/game/network/rail.cpp b/game/network/rail.cpp
index 2ae65f7..93fc9f6 100644
--- a/game/network/rail.cpp
+++ b/game/network/rail.cpp
@@ -81,6 +81,14 @@ flat_orientation(const V & diff)
 	return (std::isnan(e[0][0])) ? oneeighty : e;
 }
 
+template<typename V>
+auto
+flat_angle(const V & diff)
+{
+	const auto flatdiff {glm::normalize(glm::vec3 {diff.x, 0, diff.z})};
+	return glm::orientedAngle(flatdiff, north, up);
+}
+
 template<typename T>
 constexpr auto
 round_frac(const T & v, const T & frac)
@@ -123,18 +131,20 @@ RailLinkCurve::RailLinkCurve(End a, End b, glm::vec2 c) : RailLink(std::move(a),
 	const glm::vec3 centre3 {centre.x, e0p.y, centre.y};
 	const auto diffa = centre3 - e0p;
 	const auto diffb = centre3 - e1p;
-	const auto anga = glm::orientedAngle(glm::normalize(diffa), north, up) - half_pi;
+	const auto anga = flat_angle(diffa);
 	const auto angb = [&diffb, &anga]() {
-		const auto angb = glm::orientedAngle(glm::normalize(diffb), north, up) - half_pi;
-		return (angb < anga) ? angb + glm::radians(360.f) : angb;
+		const auto angb = flat_angle(diffb);
+		return (angb < anga) ? angb + glm::two_pi<float>() : angb;
 	}();
 	const auto radius = glm::length(e0p - centre3);
 	const auto length = round_sleepers(radius * (angb - anga) / 2.F);
-	const auto step {glm::vec3 {std::abs(angb - anga), e1p.y - e0p.y, length} / std::round(angb - anga) / 5.F};
+	const auto segs = std::round(5.F * length / std::pow(radius, 0.7F));
+	const auto step {glm::vec3 {angb - anga, e1p.y - e0p.y, length} / segs};
 	const auto trans {glm::translate(centre3)};
 
 	auto addRcs = [this, trans, radius](auto arc) {
-		const auto t {trans * glm::rotate(arc.x, up) * glm::translate(glm::vec3 {radius, arc.y, 0.F})};
+		const auto t {trans * glm::rotate(glm::half_pi<float>() - arc.x, up)
+				* glm::translate(glm::vec3 {radius, arc.y, 0.F})};
 		for (const auto & rcs : railCrossSection) {
 			const glm::vec3 m {(t * glm::vec4 {rcs.first, 1})};
 			vertices.emplace_back(m, glm::vec2 {rcs.second, arc.z}, up);
@@ -146,8 +156,8 @@ RailLinkCurve::RailLinkCurve(End a, End b, glm::vec2 c) : RailLink(std::move(a),
 	addRcs(glm::vec3 {angb, e1p.y - e0p.y, length});
 
 	for (auto n = 4U; n < vertices.size(); n += 1) {
-		indices.push_back(n);
 		indices.push_back(n - 4);
+		indices.push_back(n);
 	}
 	meshes.create<Mesh>(vertices, indices, GL_TRIANGLE_STRIP);
 }
-- 
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