# C++ FreeGLUT - Making Planets Orbit

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I wasn't sure if this would be the right place for a topic like this so sorry if it isn't.

I'm currently working on a project for Uni using FreeGLUT to make a simple solar system simulation. I've got to the point where I've implemented all the planets and have used a Scene Graph to link them all together. The issue I'm having with now though is basically the planets and moons orbit correctly at their own orbit speeds.

I'm not really experienced with using matrices for stuff like this so It's likely why I can't figure out how exactly to get it working. This is where I'm applying the transformation matrices, as well as pushing and popping them. This is within the Render function that every planet including the sun and moons will have and run.

if (tag != "Sun")
{
glRotatef(orbitAngle, orbitRotation.X, orbitRotation.Y, orbitRotation.Z);
}

glPushMatrix();

glTranslatef(position.X, position.Y, position.Z);
glRotatef(rotationAngle, rotation.X, rotation.Y, rotation.Z);
glScalef(scale.X, scale.Y, scale.Z);

glDrawElements(GL_TRIANGLES, mesh->indiceCount, GL_UNSIGNED_SHORT, mesh->indices);

if (tag != "Sun")
{
glPopMatrix();
}

The "If(tag != "Sun")" parts are my attempts are getting the planets to orbit correctly though it likely isn't the way I'm meant to be doing it. So I was wondering if someone would be able to help me? As I really don't have an idea on what I would do to get it working. Using the if statement is truthfully the closest I've got to it working but there are still weird effects like the planets orbiting faster then they should depending on the number of planets actually be updated/rendered.

Edited by codelyoko373

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13 hours ago, codelyoko373 said:

weird effects like the planets orbiting faster then they should depending on the number of planets actually be updated/rendered

I see some problems in your code:

1. The first is that your rotation depends on your frame rate so on a faster computer you will render more frames and rotate faster. Use some kind of time delta here to fix the update for every PC you run your code at
2. You shoudldn't make a difference for "sun" an other objects in your scene. If it isnt a sun, you first rotate the planets last matrix, add a new matrix to the stack and then translate and rotate again, this couldnt work at all!

You should first clean up your object class. An object has an origin from its parent that may or may not be in a linar hirachy. So you need first to load a clean matrix for your root object in the scene (your sun) and do the transformation for that object. Translate the object first to where it belongs to as the objects origin, scale the object because you would get mismatches in translation either and then rotate the object for each axis on its own so you wont get any rotation mismatches (like a planet is spinning arround a certain axis). The you should iterate any child object that is in this objects hirachy with the same matrix transformation as base so the origin of any child object is the origin and transform of the root object + the child objects relative coords and transform. This way you build the scene graph.

In code it would look like this

void Render()
{
glPushMatrix();

glTranslatef(position.X, position.Y, position.Z);
glScalef(scale.X, scale.Y, scale.Z);
glRotatef(rotation.X, 1, 0, 0);
glRotatef(rotation.Y, 0, 1, 0);
glRotatef(rotation.Z, 0, 0, 1);

glDrawElements(GL_TRIANGLES, mesh->indiceCount, GL_UNSIGNED_SHORT, mesh->indices);

for(int i = 0; i < childCount; i++)
childs[i]->Render();

glPopMatrix();
}

This is untested but should work in theory

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6 hours ago, Shaarigan said:

I see some problems in your code:

1. The first is that your rotation depends on your frame rate so on a faster computer you will render more frames and rotate faster. Use some kind of time delta here to fix the update for every PC you run your code at
2. You shoudldn't make a difference for "sun" an other objects in your scene. If it isnt a sun, you first rotate the planets last matrix, add a new matrix to the stack and then translate and rotate again, this couldnt work at all!

You should first clean up your object class. An object has an origin from its parent that may or may not be in a linar hirachy. So you need first to load a clean matrix for your root object in the scene (your sun) and do the transformation for that object. Translate the object first to where it belongs to as the objects origin, scale the object because you would get mismatches in translation either and then rotate the object for each axis on its own so you wont get any rotation mismatches (like a planet is spinning arround a certain axis). The you should iterate any child object that is in this objects hirachy with the same matrix transformation as base so the origin of any child object is the origin and transform of the root object + the child objects relative coords and transform. This way you build the scene graph.

In code it would look like this


void Render()
{
glPushMatrix();

glTranslatef(position.X, position.Y, position.Z);
glScalef(scale.X, scale.Y, scale.Z);
glRotatef(rotation.X, 1, 0, 0);
glRotatef(rotation.Y, 0, 1, 0);
glRotatef(rotation.Z, 0, 0, 1);

glDrawElements(GL_TRIANGLES, mesh->indiceCount, GL_UNSIGNED_SHORT, mesh->indices);

for(int i = 0; i < childCount; i++)
childs[i]->Render();

glPopMatrix();
}

This is untested but should work in theory

Thanks a lot, I've not tested it yet (currently am) though does this mean that each "entity" should have a Scene Graph of it's own? Since currently I've just got one main Scene graph and added all the planets to it with the sun being the parent. But having it that way, I have no way to loop over the children within the parent like you've shown, I can only loop over them one by one

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The scene graph is simply a tree structure of entity references where each node is itself a leaf of its parent node. In AAA games and engines like Unity or Unreal, the scene graph simply holds some transform matrices and reference pointers to the entities. Then when an entity changes, the scene graph will update its matrix and update the matrices of all of its children.

Then when rendering happens, the scene graph is traversed from root node to its leafs (leaving all the frustum culling and hidden object detection away) sets the transform to the pre-calculated matrix and then pushes the VBO onto the hardware pipeline

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12 hours ago, Shaarigan said:

The scene graph is simply a tree structure of entity references where each node is itself a leaf of its parent node. In AAA games and engines like Unity or Unreal, the scene graph simply holds some transform matrices and reference pointers to the entities. Then when an entity changes, the scene graph will update its matrix and update the matrices of all of its children.

Then when rendering happens, the scene graph is traversed from root node to its leafs (leaving all the frustum culling and hidden object detection away) sets the transform to the pre-calculated matrix and then pushes the VBO onto the hardware pipeline

So I've implemented your code and so far it seems to work fine though how exactly could I get it so the planets orbit at their own speeds since currently they orbit based on the rotation of the sun instead of their own orbit speeds.

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You should imagine that what you actually have is a solar system model as something like this

Anything is connected to each other and to rotate one means rotating all its children, so when you rotate the sun, any planet connected to the sun also rotates and any moon connected to the planets also rotates based on the planets rotation. What you need is something that looks like this

Here each planet is connected to its own axis (so dont has a parent child relation to the sun. What you can do is to add proxy objects to your scene graph that themselfs hold the planets. Instead of rotating the sund, you rotate the proxy object at certain angle increase and so get 'individual' orbital movement

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8 hours ago, Shaarigan said:

You should imagine that what you actually have is a solar system model as something like this

Anything is connected to each other and to rotate one means rotating all its children, so when you rotate the sun, any planet connected to the sun also rotates and any moon connected to the planets also rotates based on the planets rotation. What you need is something that looks like this

Here each planet is connected to its own axis (so dont has a parent child relation to the sun. What you can do is to add proxy objects to your scene graph that themselfs hold the planets. Instead of rotating the sund, you rotate the proxy object at certain angle increase and so get 'individual' orbital movement

That makes sense . So What I've done is created, basically a blank object with it's own scene graph and attached all the planets to it, and then attached it to the Sun.

What I'm not sure how to do now though is have the planets affected by the orbit speed since currently the render function still looks like this:

glPushMatrix();

glTranslatef(position.X, position.Y, position.Z);
glRotatef(rotationAngle, rotation.X, rotation.Y, rotation.Z);
glScalef(scale.X, scale.Y, scale.Z);

if (mesh != nullptr)
{
glDrawElements(GL_TRIANGLES, mesh->indiceCount, GL_UNSIGNED_SHORT, mesh->indices);
}

if (sceneGraph != nullptr)
{
}

glPopMatrix();

Which is applied to every object including the sun and the empty object I just made. So would I need to have some if statement to check what entity is currently rendering and then change the rotation matrix accordingly or?

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As I wrote, you need a proxy object for each different orbit speed. Look at the second image

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1 hour ago, Shaarigan said:

As I wrote, you need a proxy object for each different orbit speed. Look at the second image

But how exactly do I go about applying that orbit speed?

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You alter the position and velocity of each planet by integrating (adding). Your position is updated by the velocity, and the velocity is updated by the acceleration.

Here is some math related to calculating a position and velocity for the orbiting body:

Edited by cowcow

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