From 4d4830f82f6aa802bad527fa675ebeb6882b956d Mon Sep 17 00:00:00 2001
From: ktyl <me@ktyl.dev>
Date: Sat, 7 Oct 2023 23:53:01 +0100
Subject: [PATCH] feat(orbit): update orbit vertices over time

---
 src/main.cpp  | 14 +++++++++++++-
 src/orbit.cpp | 31 +++++++++++++++++++------------
 src/orbit.hpp | 12 ++++++++++--
 3 files changed, 42 insertions(+), 15 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index 5e9d478..ef40713 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -37,6 +37,7 @@
 
 #include <GL/glew.h>
 #include <GLFW/glfw3.h>
+#include <astro/stateVectorIndices.hpp>
 
 #include "gfx.hpp"
 #include "icosphere.hpp"
@@ -54,7 +55,14 @@ int main()
 
     Icosphere planet(0.2, 3, litProgram);
     Icosphere orbiter(0.07, 2, litProgram);
-    Orbit orbit(100);
+
+    std::vector<float> keplerianElements(6);
+    keplerianElements[astro::semiMajorAxisIndex] = .75;
+    keplerianElements[astro::eccentricityIndex] = .5;
+    keplerianElements[astro::inclinationIndex] = 3.142 / 2.0 + 1;
+    keplerianElements[astro::argumentOfPeriapsisIndex] = 2.0;
+    keplerianElements[astro::longitudeOfAscendingNodeIndex] = 0;
+    Orbit orbit(keplerianElements);
 
     Widget widget(orbit, unlitProgram);
 
@@ -66,6 +74,10 @@ int main()
 
         float time = glfwGetTime();
 
+        float e = .25 + .2 * sin(time);
+        keplerianElements[astro::eccentricityIndex] = e;
+        orbit.setElements(keplerianElements);
+
         glm::vec3 pos = orbit.getPosition(time);
         orbiter.setPosition(pos);
 
diff --git a/src/orbit.cpp b/src/orbit.cpp
index f820af3..a6b10ef 100644
--- a/src/orbit.cpp
+++ b/src/orbit.cpp
@@ -3,18 +3,22 @@
 #include "astro/stateVectorIndices.hpp"
 #include "astro/orbitalElementConversions.hpp"
 
-Orbit::Orbit(int vertexCount) :
-    _keplerianElements(std::vector<float>(6))
+Orbit::Orbit(Vector6 keplerianElements) :
+    _keplerianElements(keplerianElements)
 {
-    _keplerianElements[astro::semiMajorAxisIndex] = .75;
-    _keplerianElements[astro::eccentricityIndex] = .5;
-    _keplerianElements[astro::inclinationIndex] = _pi / 2.0 + 1;
-    _keplerianElements[astro::argumentOfPeriapsisIndex] = 2.0;
-    _keplerianElements[astro::longitudeOfAscendingNodeIndex] = 0;
+    glGenVertexArrays(1, &_vao);
+    glGenBuffers(1, &_vbo);
 
-    for (int i = 0; i < vertexCount; i++)
+    regenerateVertices();
+}
+
+void Orbit::regenerateVertices()
+{
+    _vertices.clear();
+
+    for (int i = 0; i < _vertexCount; i++)
     {
-        float t = (float)i / (float)vertexCount * 2.0 * _pi;
+        float t = (float)i / (float)_vertexCount * 2.0 * _pi;
         glm::vec3 pos = getPosition(t);
 
         _vertices.push_back(pos.x);
@@ -22,9 +26,6 @@ Orbit::Orbit(int vertexCount) :
         _vertices.push_back(pos.z);
     }
 
-    glGenVertexArrays(1, &_vao);
-    glGenBuffers(1, &_vbo);
-
     glBindVertexArray(_vao);
 
     glBindBuffer(GL_ARRAY_BUFFER, _vbo);
@@ -38,6 +39,12 @@ Orbit::Orbit(int vertexCount) :
     glBindVertexArray(0);
 }
 
+void Orbit::setElements(Vector6 keplerianElements)
+{
+    _keplerianElements = keplerianElements;
+    regenerateVertices();
+}
+
 // Interpolate a position around the orbit.
 // t is in range 0..1 and wraps.
 glm::vec3 Orbit::getPosition(const float meanAnomaly)
diff --git a/src/orbit.hpp b/src/orbit.hpp
index e917d89..f717a61 100644
--- a/src/orbit.hpp
+++ b/src/orbit.hpp
@@ -5,22 +5,30 @@
 
 #include "glm/glm.hpp"
 
+typedef std::vector<float> Vector6;
+
 class Orbit
 {
 public:
-    Orbit(int vertexCount);
+    Orbit(Vector6 keplerianElements);
     void render();
 
     glm::vec3 getPosition(const float meanAnomaly);
+    glm::vec3 getVelocity(const float meanAnomaly);
     glm::vec3 getTangent(const float meanAnomaly);
 
+    void setElements(Vector6 keplerianElements);
+
     ~Orbit(); 
 private:
     const float _pi = 3.14159265359;
+    const int _vertexCount = 100;
 
     GLuint _vbo;
     GLuint _vao;
 
     std::vector<float> _vertices;
-    std::vector<float> _keplerianElements;
+    Vector6 _keplerianElements;
+
+    void regenerateVertices();
 };