From 8391c0c23e0c2a25411169f21c717bbfef3985bb Mon Sep 17 00:00:00 2001
From: ktyl <me@ktyl.dev>
Date: Mon, 14 Aug 2023 01:46:29 +0200
Subject: [PATCH] feat: position orbiter along orbit

chore: move pi to header
---
 src/main.cpp  |  7 ++-----
 src/orbit.cpp | 48 ++++++++++++++++++++++++++++--------------------
 src/orbit.hpp |  6 ++++++
 3 files changed, 36 insertions(+), 25 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index 1334c61..9b035d9 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -62,11 +62,8 @@ int main()
         glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
         float time = glfwGetTime();
-        float r = .7;
-        float x = cos(time);
-        float y = sin(time);
-        glm::vec3 pos(x, 0, y);
-        pos *= r;
+
+        glm::vec3 pos = orbit.getPosition(time);
         orbiter.setPosition(pos);
 
         // Render lit objects
diff --git a/src/orbit.cpp b/src/orbit.cpp
index 1ed4758..316f581 100644
--- a/src/orbit.cpp
+++ b/src/orbit.cpp
@@ -3,31 +3,23 @@
 #include "astro/stateVectorIndices.hpp"
 #include "astro/orbitalElementConversions.hpp"
 
-Orbit::Orbit(int vertexCount)
+Orbit::Orbit(int vertexCount) :
+    _keplerianElements(std::vector<float>(6))
 {
-    const float pi = 3.14159265359;
-
-    std::vector<float> keplerianElements(6);
-    keplerianElements[astro::semiMajorAxisIndex] = .75;
-    keplerianElements[astro::eccentricityIndex] = .1;
-    keplerianElements[astro::inclinationIndex] = pi / 2.0 + 0.1;
-    keplerianElements[astro::argumentOfPeriapsisIndex] = 0;
-    keplerianElements[astro::longitudeOfAscendingNodeIndex] = 0;
+    _keplerianElements[astro::semiMajorAxisIndex] = .75;
+    _keplerianElements[astro::eccentricityIndex] = .1;
+    _keplerianElements[astro::inclinationIndex] = _pi / 2.0 + 0.1;
+    _keplerianElements[astro::argumentOfPeriapsisIndex] = 0;
+    _keplerianElements[astro::longitudeOfAscendingNodeIndex] = 0;
 
     for (int i = 0; i < vertexCount; i++)
     {
-        float a = (float)i / (float)vertexCount;
-        a *= 2.0 * pi;
+        float t = (float)i / (float)vertexCount;
+        glm::vec3 pos = getPosition(t);
 
-        keplerianElements[astro::trueAnomalyIndex] = a;
-
-        std::vector<float> cartesian = astro::convertKeplerianToCartesianElements(
-            keplerianElements,
-            1.0);
-
-        _vertices.push_back(cartesian[astro::xPositionIndex]);
-        _vertices.push_back(cartesian[astro::yPositionIndex]);
-        _vertices.push_back(cartesian[astro::zPositionIndex]);
+        _vertices.push_back(pos.x);
+        _vertices.push_back(pos.y);
+        _vertices.push_back(pos.z);
     }
 
     glGenVertexArrays(1, &_vao);
@@ -46,6 +38,22 @@ Orbit::Orbit(int vertexCount)
     glBindVertexArray(0);
 }
 
+// Interpolate a position around the orbit.
+// t is in range 0..1 and wraps.
+glm::vec3 Orbit::getPosition(float t)
+{
+    float a = t * 2.0 * _pi;
+
+    std::vector<float> kepler(_keplerianElements);
+    kepler[astro::trueAnomalyIndex] = a;
+
+    std::vector<float> cartesian = astro::convertKeplerianToCartesianElements(kepler, 1.0);
+    return glm::vec3(
+        cartesian[astro::xPositionIndex],
+        cartesian[astro::yPositionIndex],
+        cartesian[astro::zPositionIndex]);
+}
+
 void Orbit::render()
 {
     glBindVertexArray(_vao);
diff --git a/src/orbit.hpp b/src/orbit.hpp
index a9773e9..5a79f21 100644
--- a/src/orbit.hpp
+++ b/src/orbit.hpp
@@ -10,10 +10,16 @@ class Orbit
 public:
     Orbit(int vertexCount);
     void render();
+
+    glm::vec3 getPosition(float t);
+
     ~Orbit(); 
 private:
+    const float _pi = 3.14159265359;
+
     GLuint _vbo;
     GLuint _vao;
 
     std::vector<float> _vertices;
+    std::vector<float> _keplerianElements;
 };