Firstly, planet rotation. The first thing I had to do was work out a way to animate the rotation - this was fairly simple, and required a glRotatef call before drawing the planet in myDrawGLScene. For example, the Earth would rotate by "earthSpin" degrees every frame as the program runs. The variable earthSpin (there is a unique Spin variable for every planet) is updated in the method planetRot, which assigned the appropriate rotation value to each variable.
In order to calculate the correct rotation values, I needed a few things - firstly, how long it takes each planet to rotate 360 degrees, which I found at this website. Secondly, I needed to calculate how many frames it took the Julian day to increase by 1 as the program ran. Since the Julian day is currently set in the program to increase by 0.05 every frame, that means that it takes 20 frames for the day to increase by 1. Using this information I was able to work out the correct rotation values for each planet - the math is explained in more detail in the comments beside the code in the following screenshots of the planetRot function (click to expand):
each value has to be multipled by speedFactor, so that the rotation increases as the speed of orbit increases.
The next major problem was rotation around the axis of the planet. Each planet had already been rotated slightly by it's axis value - these rotations were all done around the Z axis of each planet, basically rotating around a vertical axis instead of the off centre one it needed to go around. This gave me a major headache, as opengl only allows for transformations around the X, Y and Z axes. I spent many hours reading information on matrix tranformations and aligning an axis to a vector but to be honest, I didn't really understand it too well. From the information and code examples I had come across, I developed the following function:
...which didn't work. It is meant to align the axis to a given vector, but I realised that the vector I was aligning it to (the vector position of the planet) would not be at the correct axis tilt anyway, and I have no idea how to get that vector. I have kept the method in the program for now, and it is something I will need to discuss with Rob tomorrow.
The solution I have been working on since that method failed is rotating by the axis value before the planet is drawn, then correcting that afterwards, like so:
This does seem to improve the look of the planets rotation, but I'm not sure if it's correct.
Now, onto opengl menus. The creation of the menu is all handled by a method called CreateMenus() in the main.cpp file. The handling of what each menu option does is taken care of by a method called myMenuResponder(int selection) in the DrawSolarSystem class. The menu handles:
- Listing the keyboard controls (submenu)
- Controlling the scale of the distance between the planets and the Sun (submenu)
- Controlling the scale of the planets (submenu)
- Zooming into a view for each planet (submenu)
- Changing the view between horitzontal and vertical
- Switching to full screen mode
- Resetting the scene to the load up config
- Exiting the program
Here is the CreateMenus method that shows how all these options are programmed:
The very last line tells the program to call this menu whenever the right button of the mouse is clicked - this is how the user accesses it. Here is a small section of the myMenuResponder method. Each option in the above code is given a number, which is how these two methods are linked.
And finally, a couple of screenshots that demonstrate the program in vertical / horizontal view mode, and with the pop up menu up:
Note that the planets have been increased to their maximum scales, with the outer four planet's distances also decreased to their minimum value (as close as they can get to the Sun).
That's all for this week. Hopefully I'll be able to address the axis rotation and lighting/shadow issues for next week.
