skein/src/orbit.cpp

#include "gfx.hpp"
#include "orbit.hpp"

#include "astro/stateVectorIndices.hpp"
#include "astro/orbitalElementConversions.hpp"

#include <glm/gtc/matrix_transform.hpp>

Orbit::Orbit(Vector6 keplerianElements, GLuint shaderProgram) :
    _keplerianElements(keplerianElements),
    _shaderProgram(shaderProgram)
{
    glGenVertexArrays(1, &_vao);
    glGenBuffers(1, &_vbo);

    regenerateVertices();
}

void Orbit::regenerateVertices()
{
    _vertices.clear();

    for (int i = 0; i < _vertexCount; i++)
    {
        float t = (float)i / (float)_vertexCount * 2.0 * _pi;
        glm::vec3 pos = getPosition(t);

        _vertices.push_back(pos.x);
        _vertices.push_back(pos.y);
        _vertices.push_back(pos.z);
    }

    glBindVertexArray(_vao);

    glBindBuffer(GL_ARRAY_BUFFER, _vbo);
    size_t vboBufferSize = _vertices.size() * sizeof(float);
    glBufferData(GL_ARRAY_BUFFER, vboBufferSize, &_vertices[0], GL_STATIC_DRAW);

    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);

    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);
}

void Orbit::setElements(Vector6 keplerianElements)
{
    _keplerianElements = keplerianElements;
    regenerateVertices();
}

void Orbit::getElements(Vector6& keplerianElements) const
{
    keplerianElements = _keplerianElements;
}

glm::mat4 Orbit::getLookAlongMatrix(const float meanAnomaly)
{
    glm::vec3 position = getPosition(meanAnomaly);

    // get the tangent of the orbital ellipse
    glm::vec3 tan = getTangent(meanAnomaly);
    // we want to point along the orbit
    glm::vec3 target = position + tan;
    // TODO: this is not 'up' with respect to the orbited body!
    // 'up' is just the normalized position vector because the orbit is centred at the origin
    glm::vec3 up = glm::normalize(position);
    // easy peasy lookAt matrix
    glm::mat4 look = glm::lookAt(position, target, up);

    // invert the lookat matrix because it's meant for cameras, cameras work backwards and
    // we are not a camera
    glm::mat4 lookAlong = glm::inverse(look);

    return lookAlong;
}

float Orbit::getEccentricAnomaly(const float meanAnomaly)
{
    const float eccentricity = _keplerianElements[astro::eccentricityIndex];
    float eccentricAnomaly = astro::convertEllipticalMeanAnomalyToEccentricAnomaly(
        eccentricity,
        meanAnomaly,
        (float)10e-3,
        100);

    return eccentricAnomaly;
}

// Interpolate a position around the orbit.
// t is in range 0..1 and wraps.
glm::vec3 Orbit::getPosition(const float meanAnomaly)
{
    Vector6 cartesian = getCartesianCoordinates(meanAnomaly);
    return glm::vec3(
        cartesian[astro::xPositionIndex],
        cartesian[astro::yPositionIndex],
        cartesian[astro::zPositionIndex]);
}

float Orbit::getTrueAnomaly(const float meanAnomaly)
{
    const float eccentricAnomaly = getEccentricAnomaly(meanAnomaly);
    const float eccentricity = _keplerianElements[astro::eccentricityIndex];
    return astro::convertEccentricAnomalyToTrueAnomaly(
        eccentricAnomaly,
        eccentricity);
}

Vector6 Orbit::getCartesianCoordinates(const float meanAnomaly)
{
    Vector6 kepler(_keplerianElements);
    kepler[astro::trueAnomalyIndex] = getTrueAnomaly(meanAnomaly);
    return astro::convertKeplerianToCartesianElements(kepler, 1.0);
}

glm::vec3 Orbit::getTangent(const float meanAnomaly)
{
    float epsilon = 0.01;
    glm::vec3 ahead = getPosition(meanAnomaly + epsilon);
    glm::vec3 behind = getPosition(meanAnomaly - epsilon);
    return glm::normalize(ahead - behind);
}

void Orbit::render(const float time)
{
    glUseProgram(_shaderProgram);
    updateModelViewProjectionMatrix(_shaderProgram, time);

    glBindVertexArray(_vao);
    glBindBuffer(GL_ARRAY_BUFFER, _vbo);

    glDrawArrays(GL_LINE_LOOP, 0, _vertices.size() / 3);
}

Orbit::~Orbit()
{
    glDeleteVertexArrays(1, &_vao);
    glDeleteBuffers(1, &_vbo);
}