• Announcements

    • khawk

      Download the Game Design and Indie Game Marketing Freebook   07/19/17

      GameDev.net and CRC Press have teamed up to bring a free ebook of content curated from top titles published by CRC Press. The freebook, Practices of Game Design & Indie Game Marketing, includes chapters from The Art of Game Design: A Book of Lenses, A Practical Guide to Indie Game Marketing, and An Architectural Approach to Level Design. The GameDev.net FreeBook is relevant to game designers, developers, and those interested in learning more about the challenges in game development. We know game development can be a tough discipline and business, so we picked several chapters from CRC Press titles that we thought would be of interest to you, the GameDev.net audience, in your journey to design, develop, and market your next game. The free ebook is available through CRC Press by clicking here. The Curated Books The Art of Game Design: A Book of Lenses, Second Edition, by Jesse Schell Presents 100+ sets of questions, or different lenses, for viewing a game’s design, encompassing diverse fields such as psychology, architecture, music, film, software engineering, theme park design, mathematics, anthropology, and more. Written by one of the world's top game designers, this book describes the deepest and most fundamental principles of game design, demonstrating how tactics used in board, card, and athletic games also work in video games. It provides practical instruction on creating world-class games that will be played again and again. View it here. A Practical Guide to Indie Game Marketing, by Joel Dreskin Marketing is an essential but too frequently overlooked or minimized component of the release plan for indie games. A Practical Guide to Indie Game Marketing provides you with the tools needed to build visibility and sell your indie games. With special focus on those developers with small budgets and limited staff and resources, this book is packed with tangible recommendations and techniques that you can put to use immediately. As a seasoned professional of the indie game arena, author Joel Dreskin gives you insight into practical, real-world experiences of marketing numerous successful games and also provides stories of the failures. View it here. An Architectural Approach to Level Design This is one of the first books to integrate architectural and spatial design theory with the field of level design. The book presents architectural techniques and theories for level designers to use in their own work. It connects architecture and level design in different ways that address the practical elements of how designers construct space and the experiential elements of how and why humans interact with this space. Throughout the text, readers learn skills for spatial layout, evoking emotion through gamespaces, and creating better levels through architectural theory. View it here. Learn more and download the ebook by clicking here. Did you know? GameDev.net and CRC Press also recently teamed up to bring GDNet+ Members up to a 20% discount on all CRC Press books. Learn more about this and other benefits here.
Sign in to follow this  
Followers 0
Shervanator

Spherical coordinates for orbital motion

5 posts in this topic

Hi, I've been working on getting some object to orbit around another object by using a spherical coordinate system. This has worked fine when the object is orbiting horizontally and vertically, however if you try to get the object orbiting diagonally it runs into issues with converting the spherical coordinates back to Cartesian coordinates.

All orbiting objects start with an initial theta, phi, and radius values and also a 2D heading vector to specify the orbit.

Then every frame this information is updated in the following way:

[CODE]
direction = Vector2.Normalize(direction);
theta += timeDelta * direction.X * orbitSpeed;
phi -= timeDelta * direction.Y * orbitSpeed;


pos.X = radius * (float)Math.Cos(theta) * (float)Math.Sin(phi);
pos.Y = radius * (float)Math.Cos(phi);
pos.Z = radius * (float)Math.Sin(theta) * (float)Math.Sin(phi);

pos.Normalize();
pos = radius * pos;
[/CODE]

Where direction is the heading direction, radius is the distance away from the centre, timeDelta is the time since the last frame, orbit speed is a speed constant for the orbit, theta is the angle about the y-axis, phi is the angle about the x-axis, and pos is the final position vector for the object.

This works fine when direction = (1, 0) or (0, 1), however when it is made to equal (1, 1) then the object does weird orbits and it seems to be due pos.z not being calculated correctly.

Below is an image of the problem:

[attachment=11652:bad orbit.jpg]

The white repeating boxes show the motion of the bad orbit.

The expected orbit can be seen below:

[attachment=11653:expected orbit.jpg]

I will be very happy with any help I could get,

Thanks! Edited by Shervanator
0

Share this post


Link to post
Share on other sites
The problem is your setup for manipulating the values of [font=courier new,courier,monospace]phi[/font] and [font=courier new,courier,monospace]theta[/font]. The way you have it, if [font=courier new,courier,monospace]direction.y[/font] is ever not zero, then the satellite will always pass through the north and south poles (that is, the positions (0, 1, 0) and (0, -1, 0) assuming radius = 1 orbit) because [font=courier new,courier,monospace]phi[/font] will always exercise its full range of [0, pi], which if you look at the second picture, it's easy to see that [font=courier new,courier,monospace]phi[/font] never reaches 0 or pi (instead, [font=courier new,courier,monospace]phi[/font] looks like it's about in the range [pi / 4, 3 * pi / 4]). To properly model a circular orbit, most of the time [font=courier new,courier,monospace]phi[/font] doesn't exercise the full range [0, pi].

In other words, the way to properly manipulate [font=courier new,courier,monospace]phi[/font] and [font=courier new,courier,monospace]theta[/font] is much more complex than that. I don't have time to sit down and work it out though, as I have a test I'm supposed to be studying for. I just wanted to point out the problem so you/others can start investigating.
1

Share this post


Link to post
Share on other sites
So any suggestions as to how I should go about manipulating phi and theta correctly to achieve the orbit in the second picture?
0

Share this post


Link to post
Share on other sites
Really, it's going to be WAY easier to just not use spherical coordinates.

For ideal circular orbit (and 'very' easily expandable to elliptical orbits), in a circle defined by an axis of rotation (y) through the planet, you can describe the position of the orbiting body with:

r * (x * cos(theta) + z * sin(theta)) where (x, y, z) is a basis determined by y, and a radius r.
0

Share this post


Link to post
Share on other sites
I wouldn't use the spherical coordinates, but store the radius and the rotation mapping the object in (radius, 0, 0) and tangent space basis (0, 1, 0), (0, 0, 1) to the current position and orientation (where I suppose that the first basis vector is the direction the object is heading). To move the object you either define how the current rotation change in time or simply compose the current rotation with the new local one.

If all you want is an object orbiting around a point/planet maintaining some fixed perpendicular vector, you can simply define the current rotation as rotations around that vector with increasing angles.
0

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0