OpenGL: Orbiting with Orthogonal projection - c++

Question

I'm modelling the solar system and testing the orbit of the Earth around the Sun, using orthogonal projection.
The starting point of the Earth is to the right of the Sun, direction of moving is to the left hand side. But the Earth is not completely rendered when it is in front of the Sun and when go further behind the Sun, as you can see from 3 images below:
Starting point:
Next frame:
Final frame:
Can some one explain what exactly happening here? I think this is the cull face problem, I tried:
glEnable(GL_CULL_FACES);
glCullFace(GL_FRONT);
It doesn't work actually.
This is my code:
// global vars
int width = 1820, height = 960;
#define CENTRE_X static_cast<float>(width/2)
#define CENTRE_Z static_cast<float>(-height/2)
#define EARTH_SIZE glm::vec3(30.0f, 40.0f, 30.0f)
#define SUN_SIZE EARTH_SIZE*3.0f
void renderSun(int i){
glPushMatrix();
glLoadIdentity();
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture_ID[0]);
glUniform1i(texture_Location, 0);
glm::mat4 Projection = glm::ortho(0.0f, static_cast<float>(width), 0.0f, static_cast<float>(height), 0.0f, 100.0f);
glm::mat4 View = glm::lookAt(
glm::vec3(0, 120, 1),
glm::vec3(0, 0, 0),
glm::vec3(0, 1, 0)
);
/* Animations */
GLfloat angle = (GLfloat) (i);
View = glm::translate(View, glm::vec3(static_cast<float>(width/2), 0.0f, static_cast<float>(-height/2)));
View = glm::scale(View, SUN_SIZE);
View = glm::rotate(View, angle * 0.5f, glm::vec3(0.0f, 0.0f, 1.0f));
/* ******* */
glm::mat4 Model = glm::mat4(1.0f);
glm::mat4 MVP = Projection * View * Model;
glUniformMatrix4fv(glGetUniformLocation(shaderProgram, "mvpMatrix"), 1, GL_FALSE, glm::value_ptr(MVP));
glDrawElements(GL_TRIANGLES, numsToDraw, GL_UNSIGNED_INT, NULL);
glPopMatrix();
}
void renderEarth(int i){
glPushMatrix();
glLoadIdentity();
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture_ID[3]);
glUniform1i(texture_Location, 0);
glm::mat4 Projection = glm::ortho(0.0f, static_cast<float>(width), 0.0f, static_cast<float>(height), 0.0f, 100.0f);
glm::mat4 View = glm::lookAt(
glm::vec3(0, 150, 1),
glm::vec3(0, 0, 0),
glm::vec3(0, 1, 0)
);
/* Animations */
GLfloat angle = (GLfloat) (i);
View = glm::translate(View, glm::vec3(cos(orbitPos)*150, sin(orbitPos)*150, 0.0f));
View = glm::translate(View, glm::vec3(CENTRE_X, 0.0f, CENTRE_Z));
View = glm::scale(View, EARTH_SIZE);
View = glm::rotate(View, angle * 0.5f, glm::vec3(0.0f, 0.0f, 1.0f));
/* ******* */
glm::mat4 Model = glm::mat4(1.0f);
orbitPos += 0.005;
glm::mat4 MVP = Projection * View * Model;
glUniformMatrix4fv(glGetUniformLocation(shaderProgram, "mvpMatrix"), 1, GL_FALSE, glm::value_ptr(MVP));
glDrawElements(GL_TRIANGLES, numsToDraw, GL_UNSIGNED_INT, NULL);
glPopMatrix();
}

Answer 1

You are using inconsistent View matrices, your code is essentially rotating the view of the earth, which causes it to move relative to the sun. This is very unusual way to do things, and is probably the cause of your problems. It is (presumably) causing the models to collide in clip space, and intersect with one another. You should instead use the same View matrix for both, and modify the Model matrix for the earth model. Keeping the renderSun method the same, you could do this by modifying renderEarth:
void renderEarth(int i){
//...
/* Animations */
GLfloat angle = (GLfloat) (i);
glm::mat4 M0 = glm::translate(View, glm::vec3(cos(orbitPos)*150, sin(orbitPos)*150, 0.0f));
glm::mat4 M1 = glm::translate(View, glm::vec3(CENTRE_X, 0.0f, CENTRE_Z));
glm::mat4 M2 = glm::scale(View, EARTH_SIZE);
glm::mat4 M3 = glm::rotate(View, angle * 0.5f, glm::vec3(0.0f, 0.0f, 1.0f));
/* ******* */
orbitPos += 0.005;
glm::mat4 MVP = Projection * View * M3 * M2 * M1 * M0;
// ...